Strengthening teaching and learning of particle theory

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1 Introductory section 1 Strengthening teaching and learning of particle theory Introductory section How to use this study guide This study guide is one of a suite designed to support the development of aspects of subject knowledge with which you are less secure. All teachers recognise that some parts of science can be difficult to teach and this can be even more problematic if it is outside your area of expertise. It has been produced for teachers who are planning to teach pupils in the secondary phase using contexts taken from Chemical and material behaviour. It assumes a general scientific background and an understanding of effective pedagogy. There will be aspects of the guide where you will need to consult other reference materials on chemistry, however no detailed knowledge of the area is assumed. This study guide offers background information and practical suggestions to support classroom practice when teaching about particles and particle theory. All the strategies suggested have been tried and tested by teachers and draw upon academic research. Equally, many of the activities could be used with pupils who need to develop similar knowledge. Your science consultant can help you work through this unit or you could team up with a colleague/s who also wish/es to enhance the teaching of this aspect of science. The unit contains tasks for you to undertake which will help you consider the advice or try out new techniques in the classroom. It also contains reflections and next steps which may encourage you to revise an idea or change your own practice. You can work through the materials in a number of ways: Work with your science consultant on developing and planning the teaching of an aspect of particle theory. After three weeks, meet together to review progress. Discuss which strategies have been the most effective with one class and plan to use these with other classes. Find another science teacher to pair with and team-teach. Design the activities together and divide the teacher s role between you. Work with a group of teachers in the department. Use the study guide as a focus for joint working, meet regularly to share ideas and then review progress after a few weeks. Identify the sections of the guide that are the most appropriate for you and focus on those. You may find it helpful to keep a learning log as you work through the tasks. You could add this to your personal continuing professional development (CPD) portfolio. Ask a chemistry specialist to help by providing a sounding board for your ideas. Crown copyright PDF-EN-12

2 2 The National Strategies Secondary Introductory section Important considerations when teaching particles Particle theory is an important explanatory idea in science and it is explicitly taught for the first time at Key Stage 3. Pupils need to be taught explicitly about consensus models of particle theory and, importantly, how these can be used to explain phenomena. Understanding particle theory doesn t happen in one step. Throughout the science curriculum, from secondary school to university, ever more sophisticated views of particle theory develop to explain wider ranges of phenomena. At Key Stage 3 teachers should seek to develop models of particle theory that are good enough in a step-by-step fashion. The yearly learning objectives set out clearly the progression expected. Although many pupils will not be capable of assimilating fully abstract ideas about particles early in their Key Stage 3 career, it is important that these ideas are carefully introduced from Year 7 if the pupils are to have developed sufficient understanding by the end of Year 9 and then into Key Stage 4. Remember that many pupils start Key Stage 3 unable to handle abstract concepts and this cognitive ability develops during these three years. It is necessary for teachers, particularly in Year 7, to be sensitive to this and to use supportive visual and physical models. Many pupils will not be able to form a fully abstract model of particles immediately, but the yearly learning objectives are based on an evolving understanding of particles across the five years. Teachers will need to adapt and differentiate their teaching according to the cognitive abilities of the pupils in their classes. There is an emphasis in this study guide upon Key Stage 3; this is not because this is seen as being more important, but because it was considered more likely that a teacher preparing work in this area at Key Stage 3 will be looking for support and guidance. Furthermore, courses at Key Stage 4 are likely to be following a unitised structure and to have support from awarding bodies in terms of the structuring of teaching and learning. However, the learning pathways are strongly based upon progression to Key Stage 4 and recognition is given to this. The learning pathway The Framework for science includes a set of learning pathways that provide guidance on the relationship between the concepts in a topic; this helps to place these within the context of the pupils wider understanding of science. The pathways suggest a relationship between concepts in the secondary phase. It is important to remember that: they show one possible arrangement and are not intended as definitive models the development of concepts starts prior to this phase and for some students will continue beyond this they are designed to show how contexts that are used with students in Key Stage 3 will form foundations for other learning activities in Key Stage 4. The pathway attempts to present a big picture of the teaching of Chemical and material behaviour. Within the learning pathway there are three main journeys highlighted in the three diagrams below. It is important to make the links between these journeys, and to other aspects of science, explicit to pupils, particularly the use and development of the particle model. Too often pupils experience the teaching of particles and particle models in a disjointed way and they fail to see how and why the model evolves PDF-EN-12 Crown copyright 2009

3 Introductory section 3 The three journeys The following sequence shows how the three journeys merge and link and the importance of the particle model. Formulate a simple particle model to explain the physical characteristics of solids, liquids and gases. Use the simple model of particles to explain the effect of temperature on states of matter. Develop the simple model of particles to include elements, mixtures and compounds. Use a particle model to explain the outcomes of simple chemical reactions. Use a particle model to explain the outcomes of a range of chemical reactions. Use a particle model to predict the outcomes from a range of chemical reactions. Use a particle model to include the effect of external factors on chemical reactions e.g. temperature, pressure, concentration and catalysts. Use particle models to produce word equations of chemical reactions. Use particle models to produce symbol equations of chemical reactions. Use particle models to produce ionic equations of chemical reactions. Use particle models to explain the structure of an atom. Use particle models to explain dynamic equilibrium. Use particle models to exemplify different types of chemical bonding. Use particle models to explain endo- and exothermic reactions. Predict whether a reaction will be endo- or exothermic using particle models and bond enthalpy. Use particle models to describe and explain trends within groups of the periodic table. Use particle models to describe and explain trends within periods of the periodic table. Use particle models to describe and explain trends in the reactivity series. Manipulate models in a range of contexts including industrial contexts. Crown copyright PDF-EN-12

4 4 The National Strategies Secondary Introductory section PDF-EN-12 Crown copyright 2009

5 Introductory section 5 The learning demand One way of thinking about why some science topics are more difficult to teach and learn than others relates to the concept of learning demand. The learning demand focuses on the differences between everyday ways of talking and thinking about phenomena and the scientific way of doing so. For example, take the case of smelling fish and chips as you pass a chip shop: Everyday way of explaining You smell fish and chips as you walk past the shop because the smell moves to your nose. Scientific way of explaining The cooking of the chips results in fast-moving gas particles because the average speed of particles increases with temperature. These particles diffuse quickly into the air. Chemical receptors in the nasal passages detect these gas particles. Here there is a big difference between the everyday or common sense way of explaining and the scientific way. Rather than thinking in terms of smell as an entity the pupil must get hold of the idea of particles and diffusion and the relationship between the two. For other science topics, the everyday view is similar to the scientific view and the learning demand is small. For example, everyday understandings of speed are likely to be the same as the scientific definition: pupils know that high speed involves covering a certain distance in a short time. Some areas of science have a high learning demand because: they are counter-intuitive of their tiny scale of their big scale they rely on interlinked variables they depend on being able to integrate other concepts they are abstract concepts they involve mathematical concepts they are not taught at Key Stage 2. The everyday ways of explaining phenomena are what are referred to as alternative frameworks or misconceptions. See Appendix 1 for more information on the learning demand. Key Stage 2 experience It is likely that at Key Stage 2 pupils: will have been introduced to changes of state and will have been taught the simple definitions of words such as melting, evaporation, freezing, boiling, dissolving, reversible and non-reversible will have learned about some irreversible changes, including baking, burning, vinegar reacting with bicarbonate of soda and the effect of mixing water with plaster of Paris will have learned to distinguish irreversible change in terms of not being able to get the original material back or new substances being formed may have investigated the formation of crystals, e.g. of sugar and may have incidentally heard about particles, but the particle explanation is not formally introduced until the Key Stage 3 programme of study. Crown copyright PDF-EN-12

6 6 The National Strategies Secondary Introductory section Teaching needs to be planned to build on prior knowledge and experiences that pupils bring with them in order to progress their learning. Similarly, it is important to provide pupils with the opportunity to consolidate and develop ideas that may previously have not been secure without considerable repetition. Barriers to learning There are common barriers that could prevent learning in this area. They need to be identified in your scheme of learning and addressed through teaching. In particles you might find that pupils think that: substances contain particles rather than consist of particles, e.g. pupils think water has particles in it, with water or air between the particles; pupils think that air has oxygen particles in it and there is air between the particles particles are comparable in size to cells, dust specks, etc. and they can be seen with an optical microscope particles of the same substance have different properties in solid, liquid or gas state, e.g. some pupils think solid ice particles are cold and hard, liquid water particles have expanded and so they are larger and softer, while water vapour particles expand even more and are very large and squashy all liquids contain water air is good (breathing) and gas is bad (flammable or poisonous) gases have no weight, or even have negative weight, and that is why things filled with gas float when water evaporates it splits up into atoms of hydrogen and oxygen the bubbles in a boiling liquid are bubbles of air melting and dissolving are the same boiling points are not fixed and the temperature will continue to go up as more energy is transferred condensation forms as a result of the cold causing oxygen and hydrogen in the air to make water atoms are a small bit of the parent material with all the same properties as that material the proportions of combining elements in a compound are not fixed metallic properties are due to properties of the atom rather than the atomic arrangement non-metals are substances such as sugar or wood rather than non-metallic elements chemical change is what is observed during the reaction, e.g. fizzing, not the production of a new substance. Pupils are often confused or uncertain about: the function of the air, although they know that air is needed for burning the use of scientific words, such as material, matter, substance and pure lead, where there is also a different everyday meaning, e.g. pure means it doesn t contain anything harmful conservation of mass if they still think gases are weightless or substances disappear, e.g. by evaporation or burning PDF-EN-12 Crown copyright 2009

7 Introductory section 7 How science works How science works is more than just scientific enquiry. It provides a wonderful opportunity for pupils to develop as critical and creative thinkers and to become flexible problem-solvers. As well as developing a range of practical enquiry skills pupils need to be able to: process and evaluate evidence from secondary sources use evidence to produce and test explanations and arguments present and share explanations to a variety of audiences understand how the scientific communities function to strengthen the quality of explanations. In the science Framework How science works has been split into two substrands: Explanations, argument and decisions Practical and enquiry skills Scientific thinking: developing explanations using ideas and models Scientific thinking: challenge and collaboration in the development of explanations Scientific thinking: developing argument Applications, implications and cultural understanding Communication for audience and with purpose Using investigative approaches: planning an approach Using investigative approaches: selecting and managing variables Using investigative approaches: assessing risk and working safely Using investigative approaches: obtaining and presenting primary evidence Working critically with primary evidence Working critically with secondary evidence Often work is repeated because it is felt that pupils have not understood an aspect of range and content. Too often this understanding is demonstrated by how well pupils recall and apply scientific knowledge. This lack of understanding is not because pupils need repetition, i.e. the same lesson again, but because they need to engage with the science in a way that promotes their thinking and challenges their understanding. How science works is sometimes considered that bit of science that is added on to a range and content lesson or even worse taught as a separate lesson. In fact, How science works should provide the teaching approach in science lessons and it provides the route into consolidating particle theory without tedious repetition. Good pedagogy in science consists of two interwoven strands, generally referred to as range and content and How science works. The former consists of the big ideas in science, such as the gene theory of inheritance or the particulate model of matter; these are cherished as they provide ways of making sense of a variety of phenomena. The latter consists of the processes and skills that scientists use, ranging from the use of equipment, through to the manipulation of data, to an understanding of how scientific ideas are challenged and tested. These two strands are sometimes seen as theory and practical work, but the relationship is much closer than that. For pupils to interact with a concept in a meaningful way, they need to be able to deploy a range of process skills; they may need to be able to apply it to a new context, to communicate it to a particular audience, or to find evidence to support or oppose it. This is enshrined in the level descriptors, and Assessing Pupils Progress (APP), and gives us a very powerful and flexible way of assessing progress. Crown copyright PDF-EN-12

8 8 The National Strategies Secondary Introductory section Task 0: What is the difference? Consider the difference between the first activity and the two that follow it. 1) Pupils recognise the differences between solids, liquids and gases. 2) Support pupils in exploring possible misconceptions they might have about particles. 3) Create opportunities for pupils to compare and contrast different particle models used to explain changes in state. In the first example, pupils could classify a range of materials, be told the scientific definition of solids, liquids and gases and then copy diagrams to show the arrangement of the particles. A teacher might feel confident that pupils have understood about solids, liquids and gases when they can recall the scientific definitions and answer questions correctly on a worksheet or from the text book. Examples two and three have How science works integrated and provide a way of teaching particles that will prompt discussion and exploration of ideas. This can be daunting because it is a different style of teaching and teachers might be concerned about how to cope with the ideas pupils have. But this approach supports better learning and the pupils conceptual development. Below is a small selection of the teaching strategies taken from the science Framework section on Particle models. 1. Identify where the How science works aspect has been integrated. 2. Range and content is being taught but through an approach that will develop pupils thinking and understanding. Compare these teaching strategies to those in your scheme of learning or that you currently use. Explore with pupils how the particle model can start to explain changes in matter and some of the limitations of the model. Explore with pupils how the use of the particle model can support an explanation of the behaviour of solids, liquids and gases. Provide and support opportunities to explore and compare the strengths and weaknesses of the particle model, e.g. explaining what is between the particles. Explore with pupils to what extent materials can be classified by identifying their particular properties. Provide opportunities for pupils to start to form links between the energy-transfer model and the particle model to explain changes in state. Provide opportunities for pupils to experience changes of state and the reverse, including the associated energy changes. Provide opportunities for pupils to select an appropriate model to explain separation techniques. If you have not seen the progression in How science works, you can download a copy from the Framework site ( scienceframework) or go to Appendix PDF-EN-12 Crown copyright 2009

9 Introductory section 9 Assessing Pupils Progress (APP) in science A fundamental aspect of planning effective teaching and learning is the role of assessment. A good teacher will want to know how pupils are progressing in terms of their command of an area of study. This enables the teacher to evaluate the strategies that are being used, to support the pupil and to identify the next steps in the pupil s learning journey. Instead of trying to gauge what a pupil knows (never a good test of a scientist) we can assess the process skills they have mastered which are generic to the whole of science. This approach is used by APP and is based around five Assessment Focuses (AFs): AF1: Thinking scientifically AF2: Understanding the applications and implications of science AF3: Communicating and collaborating in science AF4: Using investigative approaches AF5: Working critically with evidence Each AF consists of a series of outcomes between levels 3 and 8 at Key Stage 3 that represent what pupils will be able to do at that level. By using Assessment for Learning (AfL) and looking across a range of evidence it is possible to recognise the progress that has been made against a range of AFs and to identify the next steps in the development of that process. APP and the study of particles If How science works is built into a range of teaching strategies when pupils are studying particle models, this presents a variety of opportunities to assess their progress. Depending upon the activity it will be possible for the teacher to assess using criteria from different assessment focuses. APP is periodic assessment and is designed to be used approximately once a term to inform a synoptic view of progress. The following two examples of lesson episodes exemplify how assessment should be an integral part of teaching and learning not additional assessment activities, devised purely for the purposes of testing pupils, but learning activities that are valid in their own right. Example 1: Year 7 mixed ability, working at levels 4 to 6: Particles. In this sequence of lessons pupils had been exploring the differences between solids, liquids and gases and what caused a change of state. The teacher had not yet introduced pupils to the particle model. Pupils were asked to work in small groups to create their own models to try to explain the differences between solids, liquids and gases. The teacher listened to the pupils discussions, only prompting if absolutely necessary. Each group presented their model to the whole class where questions could be asked or clarification sought. Once all groups had presented, pupils returned to their small groups to review their models and make changes. The teacher was able to assess a number of pupils and use this to contribute to a periodic assessment against AF1 (Thinking scientifically), AF3 (Communicating and collaborating in science) and AF5 (Working critically with evidence). Example 2: Year 9 Set 1, working at levels 6 and 7: Making new materials In this sequence of lessons pupils had been studying a range of chemical reactions when the teacher realised that there was some confusion about conservation of mass. The teacher then presented the class with information about the phlogiston theory, Priestley s work on oxygen and Lavoisier s experiments on burning. Crown copyright PDF-EN-12

10 10 The National Strategies Secondary Introductory section The pupils were challenged to use the particle model, and other evidence, to explain to what extent the findings agreed or differed with the ideas we have today: some pupils realised that their ideas matched those of earlier scientists! The teacher discussed their ideas with them and used this as evidence for periodic assessment against AF2 (Understanding the applications and implications of science), AF3 (Communicating and collaborating in science) and AF5 (Working critically with evidence). More information can be found on the science Framework at the National Strategies website ( PDF-EN-12 Crown copyright 2009

11 Developing an understanding of particle theory Developing an understanding of particle theory 1 For pupils to understand the particle theory properly we need to: teach a simple model challenge pupils to use the model to explain what they observe. The yearly learning objectives require that pupils are explicitly taught a simple model of particle theory in Year 7. They are then expected to use this model to explain a range of phenomena. Year 7 pupils should be challenged to use their developing understanding to explain things they observe. This may require a change in the way we teach and question pupils. When pupils encounter the phenomenon of expansion we need to be asking questions such as what do you know about particles that can help us explain why heating has led to expansion? Reflection Look at this extract from the yearly learning objectives for How science works. This extract is from the substrand 1.1a1, Scientific thinking: developing explanations using ideas and models. 1.1a1 Scientific thinking: developing explanations using ideas and models Year 7 Year 8 Year 9 Year 10 Year 11 Extension use an existing model or analogy to explain a phenomenon recognise and explain the value of using models and analogies to clarify explanations describe more than one model to explain the same phenomenon and discuss the strengths and weaknesses of each model describe how the use of a particular model or analogy supports an explanation describe the strengths and weaknesses of a range of available models and select the most appropriate explain why the manipulation of a model or analogy might be needed to clarify an explanation justify the selection of a particular model as the most appropriate devise own simple models or analogies to explain observations, data or scientific ideas evaluate the effectiveness of using models and analogies in their explanations evaluate the strengths and weaknesses of their own models and analogies recognise that it is possible to have and to use different, and sometimes conflicting, models in their explanation explain how devising and using alternative models could help to make a creative leap in an explanation Crown copyright PDF-EN-13

12 2 The National Strategies Secondary Developing an understanding of particle theory You should be able to identify: that the learning about models begins specifically in Year 7 where the learning develops progressively across the five years how the learner is expected to use, apply and evaluate models, not just remember, draw or represent them that the learner may need to adapt or replace a model with a more appropriate one. Now look at these yearly learning objectives from Chemical and material behaviour Chemical and material behaviour Year 7 Year 8 Year 9 Year 10 Year 11 Extension 3.1 Particle models describe matter using a simple model and use it to explain changes of state recognise the link between heating and cooling and changes of state use the simple particle model to explain the physical characteristics of solids, liquids and gases apply and use the particle model to describe a range of physical observations apply and use the particle model to describe a range of separation techniques evaluate and refine the particle model to explain a range of physical observations evaluate and refine the particle model to explain a range of separation techniques apply particle models in unfamiliar contexts, and begin to evaluate the strengths and weaknesses of the model refine the particle model to explore the structure of atoms, including protons, neutrons and electrons use the particle model and ideas from science and across disciplines to explain phenomena and evaluate the use of the model use the particle model and ideas from science and across disciplines to explain complex phenomena and make critical evaluations to justify the use of a good enough model PDF-EN-13 Crown copyright 2009

13 Developing an understanding of particle theory 3 Chemical and material behaviour Year 7 Year 8 Year 9 Year 10 Year 11 Extension 3.2 Chemical reactions sort some reactions into reversible and irreversible recognise that new materials are made during chemical reactions recognise that materials can be made up of one or more kinds of particles describe the type and arrangement of atoms in elements, compounds and mixtures describe and develop a particle model to explain the differences between the terms atoms, elements, compounds and mixtures use a particle model to construct predictions for simple chemical reactions and to produce word equations use a particle model to construct predictions for chemical reactions and to produce symbol equations explain the evidence that a chemical reaction has taken place in terms of energy transfer and rearrangements of bonds between atoms use a particle model to predict the outcome of chemical reactions and to produce balanced symbol equations explain the evidence that a chemical reaction has taken place in terms of rearrangements of bonds between atoms, using the model of the differences of electron structure between elements use a particle model to predict the outcome of complex chemical reactions and to produce balanced symbol equations and ionic half-equations when appropriate explain the evidence that a chemical reaction has taken place (in a system at equilibrium) in terms of energy transfer and rearrangements of bonds between atoms Chemical and material behaviour Year 7 Year 8 Year 9 Year 10 Year 11 Extension 3.3 Patterns in chemical reactions describe, record and group observations from chemical reactions describe patterns in a range of chemical reactions link experimental and numerical data to illustrate a range of patterns in chemical reactions explain properties and patterns in reactivity in terms of particle model for atomic structure apply knowledge of patterns of reactivity in the periodic table to predict the outcomes of reactions from a range of familiar contexts apply knowledge of patterns of reactivity in the periodic table to evaluate critically a range of domestic and industrial processes including systems at equilibrium Crown copyright PDF-EN-13

14 4 The National Strategies Secondary Developing an understanding of particle theory You should be able to identify: how the principles from the Scientific thinking: developing explanations using ideas and models substrand translates into learning about particles and chemical behaviour how pupils are taught a simple model in Year 7, with only one kind of particle that pupils use a more refined model with more than one kind of particle in Year 8 to explain elements, mixtures and compounds by Year 10 pupils are expected to use a more refined model which includes the structure of atoms several examples of where a model that has been used in previous teaching has to be changed or made more sophisticated because the simpler model can no longer explain events. In other words it is no longer a good enough model at some point, of course, the simple model becomes not good enough and a more sophisticated model is needed. Task 1: School-based assignment This task will be more effective if you are working with a colleague or colleagues from your own department, but it can be carried out individually. Using particle models to explain phenomena Year 7 Year 8 Year 9 Matter made of particles that are moving, very small and broadly of similar size. Explains Matter made of particles that are moving, very small and can be different sizes and mass. Explains When particles interact in a chemical change they swap places in a limited set of ways. Explains changes of state: melting evaporating solidifying condensing diffusion expansion When particles interact there are forces acting between them. Explains density osmosis Brownian motion Particles are of different types. There is a set number of kinds of atoms and all other particles are made from these. Explains patterns of chemical change mass is conserved in reactions dissolving crystal formation elements, compounds and chemical change PDF-EN-13 Crown copyright 2009

15 Developing an understanding of particle theory 5 This is an example of an analysis of where the particle model is used in one science department s scheme of learning (scheme of work) for Key Stage 3. You will see that two main particle model ideas are listed for Year 7, two for Year 8 and one for Year 9, together with the phenomena these are used to explain. Compare this with your department s scheme of learning for Key Stage 3. Which of these main particle model ideas are taught explicitly in your scheme of learning? Add any other phenomena you think pupils could attempt to explain with the model they are taught in each year group. For example, you may demonstrate the collapsing can experiment which gives pupils a good opportunity to apply the model of moving particles causing air pressure. Exploring pupils misconceptions about particles Many pupils have misconceptions about particles which tend to inhibit their learning. Even at the start of Year 7 misconceptions about material changes are likely to be present: there are many accepted ways of revealing pupils misconceptions. Among these are concept mapping and related cognitive mapping methods. For example, this is the response from a high-attaining pupil who had studied material changes in the first term of Year 7 when asked to show what they knew about the changes to materials when a candle burns. What misconceptions can you see? Alternative frameworks or misconceptions are views held by pupils (and adults) that do not fully coincide with scientific views. They can be held by a large proportion of the population or just by an individual based on personal experience and often they are developed through everyday talk. Crown copyright PDF-EN-13

16 6 The National Strategies Secondary Developing an understanding of particle theory Misconceptions may be: linked to everyday use of language constructed from everyday experience and are usually adequate for everyday life personal or shared with others used to explain how the world works in simple terms similar to earlier scientific models (e.g. the earth is flat) inconsistent with science taught in schools resistant to change. Many ideas pupils hold about the world around them come from sensory experience. Pupils construct a framework from these events that is coherent and fits their experiences but which may be very different from the scientific view. The scientific explanation can then seem counter-intuitive; for example gases have mass. Task 2: Where might misconceptions come from? The root of many misconceptions is often language. What is your response to the questions below? A) Expressions such as throwing a glance at someone could be why some people think that light travels from the eye to the object. Or turn the light off to save electricity could be why some people think that a bulb uses up electricity. Can you think of any other expressions or old wives tales that could cause misconceptions? B) Scientific words that also have an everyday meaning. The word force in everyday usage could be why pupils think a force is being applied during any movement. Switching between scientific and everyday usage can be confusing unless it is clearly explained and reinforced. For example, physics teachers will talk about measuring weight in Newtons and mass in grams, but chemistry teachers will talk about weight in grams. Outside lessons, physics teachers will also talk about weight in grams! Can you think of other science words where the everyday meaning might cause a misconception? C) Other causes of misconceptions can be our own observations. For example we see heavy objects falling faster than lighter ones; we see parachutes go upwards when the cord is pulled; we feel cold going into our feet when we stand bare foot on the floor. This often means that the scientific explanation feels counter-intuitive. Can you think of everyday observations that might account for pupils thinking that gas is bad but air is good ; boiling points are not fixed, i.e. temperature continues to rise if a liquid is heated; all liquids contain water? D) Sometimes models or diagrams used in text books can lead to misconceptions. Look through textbooks to find pictures or representations of: solid, liquid and gas models change of state structure of the atom processes like dissolving, diffusion, osmosis and Brownian motion. Think about how these could lead to misconceptions if the weaknesses in them were not made explicit PDF-EN-13 Crown copyright 2009

17 Using particle models to explain phenomena Using particle models to explain phenomena 1 Remember that: Creating models is a major way in which scientific understanding advances. When teaching pupils about particle models it is important to help them develop an understanding that this is so. You need to be specific with pupils that it is a model you are using. You should not make assumptions that pupils see things as you do. You need to help pupils visualise ideas. You need to build pupils pictures of the world step by step. One model cannot explain everything; models sometimes break down. You need to help pupils develop their understanding of how the particle model can help them explain what they see. Pupils need opportunities to explore models, to create their own models and to appreciate how models are used in science. They need plenty of opportunities to use and apply these models in constructing their explanations of the world. Crown copyright PDF-EN-14

18 2 The National Strategies Secondary Using particle models to explain phenomena A strategy for using models There is a four-stage approach to teaching the use of models, summarised as: Stage 1 Teach the model explicitly: show which part relates to which, making sure pupils understand it and picture it; or ask pupils to develop their own model to explain a particular phenomenon. Stage 2 Test the model by applying it: pupils practise using the model by attempting to explain a limited range of simple phenomena, so exploring the model s strengths and limitations. Stage 3 Challenge the model: provide some phenomena that the model cannot explain, so that pupils see the limitations of the model. Evaluate the model, exploring what it can explain and what it cannot. Stage 4 Increase sophistication if necessary: explore with pupils the development of a better model, or provide a more sophisticated model. Test it out on the new phenomena, exploring what it can explain and suggest its likely limitations. These stages may take place in one lesson or more likely over a period of time. A lesson may focus on just one stage or part of a stage. If you are working with a colleague from your department, discuss your experiences of how this strategy compares with any others in use in your department. In using this four-stage teaching strategy, developing pupils abilities to evaluate the strengths and limitations of models is implicit in Stages 2 and 3. There are some generally accepted ways of helping pupils to evaluate the strengths and weaknesses of models PDF-EN-14 Crown copyright 2009

19 Using particle models to explain phenomena 3 Encouraging pupils to identify the strengths and weaknesses in a model Discuss the model and encourage pupils to: identify what each part represents think about the strengths and weaknesses; what it can explain, what it cannot explain suggest improvements for the model, or analogy. Provide models created by others that are problematic and encourage pupils to: identify limitations of the model consider what misconceptions it might generate. Remember that: Constructive criticism of their own and other people s models can be motivating. An important element of teaching using models and analogies in this way is the role pupils play in developing their own understanding. Discussing with pupils the merits of a model, or an analogy, helps them to realise that models and analogies are the ways we often visualise science. However, they are only models and one model cannot give the whole picture. Encouraging critical thought develops pupils ability to reason and helps them to appreciate that modelling is a useful way of thinking. Discussing strengths and limitations of models or analogies helps pupils make good progress in developing their understanding. Reflection It may help to illustrate this methodology by thinking of the use of a physical model in the classroom. Think about the use of a popcorn maker to model the particles in a solid, a liquid and a gas to Year 7 pupils. Try to identify some of the strengths and limitations of this model, as might be done with a Year 7 class. Some suggestions can be found in the Answers section. Crown copyright PDF-EN-14

20 4 The National Strategies Secondary Using particle models to explain phenomena Task 3: Building this into your teaching In order to apply some of the ideas from this section, identify a lesson or series of lessons in the near future where you will be using the key idea of particles or scientific models. Decide how you will modify the teaching to incorporate ideas from this section and what the outcome will be of this enhanced teaching and learning approach. It might be useful to use Appendix 2 and Appendix 3 to support this You might be able to plan the lesson jointly with a colleague, teach and then review it together. Decide the type of evidence your colleague should look for in the lesson that will demonstrate improved learning. Developing ideas in Year 7 These are some of the ideas about particles typically developed in Year 7: The substance and size of the particles in a solid, a liquid and a gas of one material are the same. Heating a material makes its particles move faster as the temperature rises. The arrangement and motion of the particles change with a physical change of state. Particle theory can explain melting, evaporation, freezing, condensing, diffusion, pressure and the conservation of mass in dissolving. Solid and liquid substances dissolve more quickly with increasing temperature. Notice that the main emphasis in Year 7 is on using the idea of one kind of particle to explain observed physical phenomena such as: Solids and liquids are much less compressible than gases. Heating causes expansion in solids, liquids and gases. Air exerts a pressure. Why there are changes of state. Why mass is conserved when substances dissolve to form solutions. Why saturated solutions form. Why temperature increases are likely to result in substances dissolving more quickly. Each of these phenomena provides an opportunity for pupils to use and apply the simple particle model to explain what is happening, but too easily this opportunity for pupils to apply the model is lost PDF-EN-14 Crown copyright 2009

21 Using particle models to explain phenomena 5 Task 4: Using particle theory to explain practical demonstrations Work with another teacher or lab technician and try the three activities below. A Collapsing plastic bottle 1. Use a two-litre fizzy drink bottle with clear plastic walls. 2. Keep the lid off the bottle and plunge it into very hot water contained in a bowl so that the opening is not submerged. 3. After about three minutes place the lid on the bottle and leave in the hot water for a further five seconds. Make sure there is no risk of spilling the very hot water. 4. Take the bottle out of the hot water and immediately plunge into very cold (preferably icy) water in a large container such as a bowl. 5. When the bottle sides have collapsed take the bottle out of the cold water. The top can then be unscrewed and there is an audible rush of air into the bottle. The bottle lid can then be replaced and the complete bottle replaced in very hot water. B Egg in a bottle Wear eye protection for this demonstration. 1. Boil a small or medium hen s egg until it is hard-boiled. Peel it. 2. Use a glass milk bottle or equivalent sized bottle with an opening a few millimetres less than the maximum diameter of the egg. Place upright into water at about 50 C. 3. Place the boiled egg on top with the pointed end put into the mouth of the bottle. 4. Remove the complete system (bottle with the egg on top) out of the hot water. 5. Leave to stand in cold water for five minutes. The egg will move into the bottle. 6. As a variant, before the egg moves fully into the bottle, replace the bottle in hot water and the egg will rise back up C Chromatography of water-soluble felt pen ink 1. Use a strip of filter paper 10cm long and 1cm wide. 2. Place a central spot of black ink about 2cm from one end. Allow the ink spot to dry. 3. Repeat a couple of times to get a darker ink spot. 4. Holding the filter paper vertically, place the very bottom of the filter paper nearest the spot into a shallow container of water. Do not let the standing water overlap the ink spot. 5. The water (the solvent) will rise up the filter paper and the black ink will separate into its constituent colours. Explain what happened using the Year 7 model of particles. Identify any strengths or limitations of using this model and decide if the model needs amending. Some suggestions can be found in the Answers section. Crown copyright PDF-EN-14

22 6 The National Strategies Secondary Using particle models to explain phenomena Description of model Collapsing plastic bottle Egg in a bottle Chromatography Strengths Limitations Amendments to the model Many pupils (and adults) may confuse the idea of temperature with that of heat. The word heating refers to the process of transferring energy, whereas temperature is related to the average kinetic energy of the particles. Heating transfers energy to the particles and there is a rise in temperature as a result. Avoiding the word heat as a noun and focusing on heating as a process in this context helps to avoid confusion between heat and temperature. Reflection Models are an important aspect of the development of scientific ideas outside the school laboratory, what is often called science in the real world. Sir Harry Kroto is an internationally renowned scientist, Research Professor at the University of Sussex and a Nobel laureate for his part in the discovery of C60 Buckminsterfullerene. An audio clip is available with this document of him talking about using models in his scientific research and in science generally. This clip helps us to appreciate the importance of models in developing scientific understanding. How well do your pupils understand about the role of models in science? PDF-EN-14 Crown copyright 2009

23 Using particle models to explain phenomena 7 Task 5: Using role-play to teach particle theory Discuss with a colleague which aspects of the particle model of the three states of matter pupils find hardest to understand. Plan with a colleague how to use role-play with a group of pupils to help overcome these misunderstandings. Then both try the approach with a group of pupils. Jointly review the effectiveness of this approach on their learning. Further guidance Getting pupils to role-play the arrangement and movement of particles in a solid, liquid and gas can be an effective way of helping them to better understand and remember the three states of matter. Many teachers already use the strategy but in a limited way. There are several aspects of these arrangements and movements that pupils still find difficult to distinguish. If you have not used this method of teaching with pupils, follow the ideas below: 1. Each pupil represents a single particle of a substance, e.g. a molecule of water. 2. Each particle (pupil) should vibrate (shake) and become more vigorous as kinetic energy and therefore temperature increases. 3. Particles should be: 3.1. close together and in a pattern for a solid 3.2. moving about but still frequently in contact for a liquid 3.3. able to move freely and quickly anywhere, but in a defined space, for a gas. 4. Particles in solids are closely packed, held by strong forces. They cannot move from a fixed point, except to vibrate, and have very small spaces between them. 5. Particles in a liquid are loosely packed in a random arrangement with very small spaces between them. The forces between particles in a liquid are weaker than in solids and the particles can move around each other. 6. Particles in a gas have, on average, larger spaces between them than in liquids or solids. The particles in a gas move in straight lines and the forces between the particles are very weak except when they collide. Crown copyright PDF-EN-14

24 8 The National Strategies Secondary Using particle models to explain phenomena Task 6: Using particle theory to explain observed physical phenomena These images are produced as a result of monitoring electric current across the surface of the sample. How real are the images? Although these images are how atoms appear under the STM, they are not optical images. Scanning tunnelling microscope (STM) image of platinum particles Source: There is little direct evidence for the existence of particles that is available for use in school. One piece of evidence that pupils can observe is Brownian motion. It is instructive to reflect on the fact that the scientific community has accepted the existence of particles for many years, yet direct microscope evidence, such as the STM images above, only became available at the very end of the twentieth century. The development of ideas about atoms and molecules is developed later. Brownian motion Either watch the video clip provided with this document or set up a smoke cell. Can you suggest explanations for why the tiny fat globules in the milk or the smoke particles move without using the particle theory? How many aspects of this movement can be explained by these suggestions? As part of How science works pupils are required to learn about evidence and the degree to which it supports or disproves theories. Concept Cartoons are a useful way of helping pupils to engage with this type of thinking. Try this Concept Cartoon TM with pupils. How did their explanations compare with your ideas? If you want to see what some other teachers thought refer to the Answers section PDF-EN-14 Crown copyright 2009

25 Using particle models to explain phenomena 9 How to use a Concept Cartoon TM approach Each cartoon offers four different viewpoints about an aspect of science to provide a stimulus for discussion and promote thinking. The views do not usually have a right answer but could contain part of the answer. They are designed to find out what pupils think in a non-threatening way by presenting the viewpoints through a third party. The cartoons can be used in different ways. For example: to promote a general discussion to introduce a topic as a small group discussion, to find out the level of agreement or next steps in a discussion about why certain viewpoints might be held for a class vote on the alternatives to devise additional alternatives. More information and further Concept Cartoons TM can be found at Reflection You should now feel more confident about explaining many phenomena using the particle model. These diagrams represent the five types of solid, liquid or gas as indicated under each one. For each diagram identify any mistakes in the representation. Crown copyright PDF-EN-14

26 10 The National Strategies Secondary Using particle models to explain phenomena If you would like to compare your diagnosis with those mistakes that were intended please consult the Answers section. Here is a list of questions whose answers may be explained by particle theory. Work with a colleague to agree your answers. 1. Why are some substances more soluble than others? 2. What is the state of matter called plasma? 3. Are there any substances that change directly from a solid to a gas? How do they do this? 4. Why do some granite rocks have big crystals but others have only small crystals of the same material? 5. Some people think that the Earth had denser air in the past. How would that have affected dinosaurs? 6. Why does a cricket ball swerve if one side of the ball is rougher than the other? 7. Why can a blue marlin swim much more quickly than a basking shark? 8. Why do Olympic cyclists cycle more quickly behind each other than if they cycle alongside each other? 9. What happens to the water underneath a frozen ice-sheet in a pond or lake during a cold winter? Can you think of an explanation for your observations? 10. Using particle theory, can you explain up thrust on an object floating in mid-water, for example a pear in a tank of water PDF-EN-14 Crown copyright 2009

27 Using particle models to explain phenomena Why do deep-sea fish pulled rapidly to the surface explode? 12. Can you suggest, using role-play, why some volcanoes are explosive but others are not? 13. Are there any similarities between the electrical conductivity and thermal conductivity of metals? 14. Can you explain simply how an aerofoil works? 15. Can you think of an alternative to the particle theory to explain the features of solids, liquids and gases? 16. Glass is often described as a super cooled liquid. What does this mean? Why would some people find the term misleading? Acknowledgements Image of Platinum Particles: Atomic Resolution on Pt(100) E.Bergene, Trondheim, Norway; Published in Surf. Sci /2 (1994) Used with kind permission. Crown copyright PDF-EN-14

28 Using particle theory to improve the understanding of properties of elements and compounds Using particle theory to improve the understanding of properties of elements and compounds 1 The six main ideas about particles that are usually developed in Year 8 are: there are different types and sizes of particles the atom as the basic building block molecules as groups of atoms elements and compounds representation by symbols and formulae interaction between atoms or molecules in chemical reactions. You will see that this represents progression from Year 7, where the main emphasis is on using the idea of one kind of particle to explain physical phenomena. These are also an expansion of the Year 8 yearly learning objectives which can be found in Appendix 3: apply and use the particle model to describe a range of physical observations apply and use the particle model to describe a range of separation techniques recognise that materials can be made up of one or more kinds of particles describe the type and arrangement of atoms in elements, compounds and mixtures describe and develop a particle model to explain the differences between the terms atoms, elements, compounds and mixtures describe patterns in a range of chemical reactions. Reflection The following misconceptions are very common at this stage: Particles exhibit the macro properties of the material. For example, the particles expand when a material is heated; a solid melts when heated because the particles melt; copper metal is ductile because copper atoms are ductile. The particles are destroyed when a substance is burned, so it loses mass. Compounds are mixtures. The elements can be mixed in any proportions. The name is not systematically related to the constituent elements. Reactions where gases are formed result in a loss of mass. When two elements react together the atoms from the reactants are transmuted into new atoms (of the products). The reaction is a magical change. If any of these misconceptions is apparent with your pupils you need to think about how you would treat it. Decide how you would plan to overcome it in your teaching. Discuss the teaching sequence, the practical work and/or the model/s you might use with another teacher. Crown copyright PDF-EN-15

29 2 The National Strategies Secondary Using particle theory to improve the understanding of properties of elements and compounds Task 7: Some possible models and analogies for teaching about elements and compounds Remember that using ideas of particles to clarify the differences between elements, compounds and mixtures is an essential part of the Year 8 yearly learning objectives. Therefore it is helpful to have several possible models to help represent this for pupils. Which of these images represents an element and which a mixture? Why does this picture represent a compound rather than a mixture? Based on a model from Particle Models for Key Stage 3 Science published by University of Southampton. Both compounds and mixtures contain more than one element (in this case represented by mint and toffee), but there are two significant differences between compounds and mixtures. In a compound the elements are combined and cannot be separated by physical methods. The constituent parts of a mixture can be separated more easily. (In this case the toffees and mints can be picked out.) In a compound the elements are combined in a fixed ratio (not very clearly illustrated by this model). In a mixture the constituents can be mixed in any proportions. This paper clip analogy is a model that may help you explain to pupils the ideas of elements and atoms. What are the strengths and weaknesses of this analogy? PDF-EN-15 Crown copyright 2009

30 Using particle theory to improve the understanding of properties of elements and compounds 3 paper clip models iron metal Element iron nail a pile of 20 paper clips Sub-divide 10 paper clips 10 paper clips Sub-divide iron filings individual paper clips Sub-divide iron powder heap single paper clip Fe single iron atom Break into fragments paper clip fragments are not a paper clip atomic fragments are not iron Source: Adapted from Teaching chemistry to KS4, by Elaine Wilson, Hodder and Stoughton, 1999, ISBN Do you have a particular model used in your department to help pupils understand the concepts of elements, mixtures and compounds, and the differences between them? How does it compare to the two offered above? Crown copyright PDF-EN-15

31 4 The National Strategies Secondary Using particle theory to improve the understanding of properties of elements and compounds Task 8: Examining some physical particle models for elements and compounds You will now examine and compare three physical models that pupils can use as they learn about elements and compounds: commercial molecular models plastic bricks element cards (see template below). Use each of these systems to create models to represent as many of these compounds as you have time for: aluminium chloride (AlCl 3 ); aluminium oxide (Al 2 O 3 ); aluminium nitride (AlN); magnesium chloride (MgCl 2 ); magnesium oxide (MgO); magnesium nitride (Mg 3 N 2 ); hydrogen chloride or hydrochloric acid (HCl); water (H 2 O); ammonia (NH 3 ); methane (CH 4 ); carbon dioxide (CO 2 ); tetrachloromethane (CCl 4 ) As you try out any of the models identify any strengths or limitations you see in their use for pupils learning and keep a note of these. Molecular models You need the following model atoms (or ones similar): carbon (black, 4 holes) magnesium (silver, 2 holes) aluminium (silver, 3 holes) chlorine (green, 1 hole) nitrogen (blue, 3 holes) hydrogen (white, 1 hole) oxygen (red, 2 holes). You link the models together using the grey plastic links, which represent bonds. Plastic bricks Use these standard toy building bricks to represent the following atoms; yellow for aluminium, blue for nitrogen tan for hydrogen, brown for chlorine grey for magnesium, red for oxygen black for carbon PDF-EN-15 Crown copyright 2009

32 oxygen O The National Strategies Secondary Using particle theory to improve the understanding of properties of elements and compounds 5 Cards Each card represents the atom of an element. The name of the element is printed on each card. Fit the cards together to represent the compounds like this: This handout also includes a photocopy original if you wish to make similar cards for use at school. A card game, Atom@tak, has previously been provided free to secondary schools by the Royal Society of Chemistry and could be used to extend this model. hydrogen H hydrogen H Crown copyright PDF-EN-15

33 6 The National Strategies Secondary Using particle theory to improve the understanding of properties of elements and compounds Template for element cards copy and cut up carbon C carbon C carbon C aluminium Al aluminium Al carbon C aluminium Al nitrogen N nitrogen N nitrogen N nitrogen N aluminium Al magnesium Mg oxygen O magnesium Mg magnesium Mg oxygen O oxygen O oxygen O oxygen O oxygen O magnesium Mg hydrogen H oxygen O oxygen O hydrogen H hydrogen H hydrogen H hydrogen H hydrogen H magnesium Mg magnesium Mg chlorine Cl chlorine Cl chlorine Cl chlorine Cl chlorine Cl chlorine Cl chlorine Cl chlorine Cl chlorine Cl chlorine Cl hydrogen H hydrogen H hydrogen H hydrogen H PDF-EN-15 Crown copyright 2009

34 Using particle theory to improve the understanding of properties of elements and compounds 7 If you use any other models add them to the list and consider the questions below. If you are collaborating with a colleague this would be an effective way to work and compare findings. What strengths and limitations of these models did you find? Compare your findings with your colleague if you are working with one. Is there a standard set of physical models used in your department to teach about elements and compounds? If so, how does it compare with these models? If you would like to compare your findings about these types of model, including the use of toffees, with that of a group of experienced science teachers refer to the Answers section. Does this suggest any implications for the way you use models to teach about elements and compounds in your department? It is important for pupils to realise that these are only physical models and that our mental pictures of particles are also models. They need to realise that these models have limits and weaknesses. How can you help pupils to understand the limitations of these models? Further guidance While these three models can be used to represent atoms of elements and molecules of many compounds, not all compounds are composed of molecules. Nearly all compounds of a non-metal with a non-metal are composed of discrete molecules, e.g. carbon dioxide, but silicon dioxide (SiO 2 ) in sand is an exception. Many compounds of metals with non-metals do not form discrete molecules but form a giant structure of millions of particles. Examples are magnesium oxide, aluminium oxide and magnesium chloride. Therefore, do not talk about molecules of these compounds. Aluminium chloride is an exception and exists as molecules, often paired into an Al 2 Cl 6 structure. However, it is still possible to use physical models to represent the ratios in which the elements combine in these compounds. At Key Stage 3, there is no need to introduce pupils to ions, valency or giant structures. Task 9: Using appropriate teaching and models to address learning barriers There are a number of misconceptions that pupils may have about particles in the context of elements and compounds. Research suggests that several of these misconceptions are quite common. To find out about misconceptions held by some pupils try this exercise with a small group of pupils (about six). Demonstrate the combustion of iron wool using the instructions on the folowing page or set light to a large tuft of iron wool that rests on a top pan balance. Protect the pan of the balance with a piece of aluminium foil. The increase in mass can be seen as the iron burns. Crown copyright PDF-EN-15

35 8 The National Strategies Secondary Using particle theory to improve the understanding of properties of elements and compounds This activity is derived from the Royal Society of Chemistry 1995 publication Classic Chemistry Demonstrations, which has previously been supplied free of charge to secondary school science departments. Ask pupils to explain what s happening to the particles during this reaction. Give them a hint that it involves iron and oxygen particles if necessary. Listen carefully to their responses. If possible video or audio record their ideas and explanations. Review their talk and discussion with a colleague. What did you say or do to develop their learning or help overcome any barriers to learning evident? Further guidance PDF-EN-15 Crown copyright 2009

36 Using particle theory to improve the understanding of properties of elements and compounds 9 Case study Here is an example of an evaluation of particle models carried out by Year 9 pupils. Pupils models for cell membranes Pupils were asked to picture what was happening as particles passed through the cell membrane. They generated four alternative models: Source: Matthew Newberry, as part of a continuing models and modelling project involving Hampshire LEA and the University of Reading This was part of a teaching programme for Year 9. Pupils were being taught that the simple visking tubing model was insufficient to explain how particles pass through a cell membrane. Pupils were challenged to develop their own models that explained how this might occur. The ideas generated fell into four main camps. The outcomes were used to discuss the merits of each model. This helped move pupils to a consensus view of a preferred model, but the teacher made it explicit that no one model gave the whole picture. Acknowledgements Sweets Model based on a model from Particle Models for Key Stage 3 Science published by University of Southampton. Paper Clip Model of an Element, adapted from Wilson, E. (1999) Teaching chemistry to KS4, Hodder and Stoughton. Used with kind permission. The Combustion of Iron Wool Activity derived from Classic Chemistry Demonstrations. Reproduced by permission of the Royal Society of Chemistry. Image of Particles Matthew Newberry, Cams Hill Science Consortium ( This work arose from a models and modelling project involving Hampshire LA and the University of Reading. Crown copyright PDF-EN-15

37 Using particle models to understand digestion and absorption Using particle models to understand digestion and absorption 1 During Key Stage 3 pupils will learn about the structure and function of the digestive system, the names of organs and their function. This is generally well understood. However, their understanding of how and why food is broken down and absorbed is less well understood. The visking tubing model of the gut can confuse pupils about the process of digestion and absorption as the model has limitations. The teaching of digestion provides a good mechanism for pupils to apply their knowledge of particles. At the same time emphasising the role of particle size can improve their understanding of digestion. Pupils could be asked to apply their knowledge to explain how energy drinks work. Most such drinks supply glucose at an appropriate concentration to be absorbed through the stomach wall almost immediately. This is because glucose is a small carbohydrate molecule and does not have to be broken down into smaller molecules before it can be absorbed into the blood. Other more complex carbohydrates do require digestion before absorption and are better for longer-term energy supply. More sophisticated sports drinks contain complex mixtures of different-sized carbohydrate molecules, designed to match the energy requirements of the athlete and nature of the sport. Crown copyright PDF-EN-16

38 2 The National Strategies Secondary Using particle models to understand digestion and absorption Task 10: Try this as an alternative! You can look quickly at items 1 to 4 below, which may represent a fairly typical start to this topic, but you may want to spend more time on items 5 to 9, which focus on the role of molecule size within digestion. The true/false cards and the Predicting the digestion of starch task are intended for pupil use. 1. The topic can be introduced in the usual way. For example: drawing a life-size body on paper (e.g. wallpaper) and adding internal organs and labels check size and position of organs with textbook teach names of the parts (oesophagus, stomach, small intestine, etc.) and other key words, with careful pronunciation and other vocabulary strategies. 2. Ask pupils to think about the possible process of digestion and to suggest how food is digested. Scaffold their answers suggesting the framework: first, then, next, etc. 3. Ask pupils to find out the role of enzymes in the digestion of food using standard textbooks as a source of reference. 4. Use the Digestion true or false? cards as a card sort activity. Then take just the true cards and sort them into an approximate sequence to represent the process of digestion. There is more than one possible sequence. These could be attached to appropriate places on a wall poster if one is displayed. Invite faster workers to explain the mistakes in the false cards. Cards 1, 6, 9, 10, 11, 13 and 16 are the deliberately false statements. 5. Ask pupils to explain why enzymes are needed for digestion. Scaffold their answers by giving them the root Enzymes are needed in the process of digestion because Review their answers. How will you test to show that this has happened? 6. Once pupils are clear about their explanation, introduce the visking tubing experiment. Get them to predict what will happen and why. What do you know about the connection between glucose and starch? What do you know about the action of enzymes (amylase) on starch? What do you know about the visking tubing walls? What do you expect to happen? 7. Alternatively, you could use the Predicting the digestion of starch diagram labelling exercise as a group work activity. 8. Pupils can now carry out the visking tubing practical with a greater certainty of understanding the purpose and details of the practical work and how this is a model of digestion. 9. It is possible for pupils to model the process of digestion on a larger scale (see following page) PDF-EN-16 Crown copyright 2009

39 Using particle models to understand digestion and absorption 3 How to model the process of digestion on a larger scale Crown copyright PDF-EN-16

40 4 The National Strategies Secondary Using particle models to understand digestion and absorption Digestion true or false? 1 The food molecules that get through the gut wall get passed out of your body when you go to the toilet. 2 Enzymes help break down large food molecules into smaller molecules. 3 Enzymes such as amylase in saliva start breaking down the starch in potato as you chew it. 4 Many food molecules are large and need to be broken down into smaller molecules. 5 In the large intestine water is extracted from the soup to leave a more solid waste. 6 Starch molecules are much smaller than molecules of sugars such as glucose. 7 The food and liquid is churned up to make a liquid like soup. 8 As the mashed-up food goes along the small intestine small molecules can get through the intestine wall. 9 In the stomach all the goodness is removed from the food to leave just waste. 10 Chewing protein molecules breaks them up into small molecules. 11 Enzymes in the human body work best at a temperature of 87 C. 12 The food molecules that get through the gut wall are picked up by the blood supply and carried round the body. 13 As the mashed-up food goes along the small intestine big molecules of starch can get through the intestine wall. 14 More digestive juices are added to the food in the stomach. 15 Only some molecules are able to go through the gut wall. 16 Enzymes are powerful acids that break down other chemicals PDF-EN-16 Crown copyright 2009

41 Using particle models to understand digestion and absorption 5 Predicting the digestion of starch Draw arrows from each label to the correct part of the diagram Tap water Starch, water and amylase (enzyme) This will turn dark blue with iodine This is like the food inside the intestine This is like a piece of intestine At 37 o C, like body temperature The small molecules can get through the wall of the tubing This represents the blood supply This now contains water and sugars The enzyme is breaking down the starch into sugars This will go brown with Benedict s test This won t change colour with iodine Now consider the three suggested models to show digestion on a larger scale drawing; bead model; wet model. Are any of these three models suitable for use when you teach this topic? Be sure to consider the strengths and limitations of any model you might think of adopting (or you currently use). Are there any other advantages to this suggested approach which you might adopt? Crown copyright PDF-EN-16

42 Changing evidence and ideas about particles Changing evidence and ideas about particles 1 Teaching about the interplay between evidence and scientific ideas, and how this has led to the development of scientific thinking, is fundamental to How science works throughout Key Stages 3 and 4. It is interesting to reflect on why the idea that materials can be categorised into elements, compounds and mixtures and are composed of atoms and molecules has been held as a consensus model by generations of scientists, when direct evidence from microscopes only became available recently; 2003 was the two hundredth anniversary of Dalton s atomic theory. Further information can be found in Chemistry in a social and historic context, published by the Royal Society of Chemistry ( and previously distributed free to British secondary schools. Crown copyright PDF-EN-17

43 2 The National Strategies Secondary Changing evidence and ideas about particles Task 11: Matching evidence Try this activity yourself and then decide on an appropriate class to try it with. Match the evidence cards to the idea it supports. Timescale Idea/theory Evidence about 400 BC Some Ancient Greek thinkers (philosophers) suggested that materials are made from particles. These ideas were not widely accepted for about 2000 years Robert Boyle wrote a book containing his idea that there are just a few simple chemicals called elements. about 1800 John Dalton of Manchester was studying the air and weather. He thought that gases must be made of tiny particles, called atoms, mixed together Dalton thought each element only contains one type of atom that is special to that one element. The atoms cannot be broken up into anything smaller Dalton decided that atoms must join together to make molecules. (These were called compound atoms.) 1806 Dalton thought that each type of atom had a particular weight (its atomic weight). He drew a table of symbols and weights for atoms of different elements to 1825 Books by Dalton and the French chemist Gay-Lussac did not agree about how hydrogen and oxygen combine to make water. More scientists began to believe Gay-Lussac s idea The Italian scientist Avogadro suggested that the volume of a gas is directly connected to the number of molecules it contains Cannizzaro, an Italian, realised that this earlier work on atomic weights and volumes of gases explained that two hydrogen molecules react with one oxygen molecule to form two water molecules. It became the accepted idea PDF-EN-17 Crown copyright 2009

44 Changing evidence and ideas about particles 3 A Although oxygen is heavier than nitrogen it stays mixed up in the air. The oxygen doesn t sink to the ground and the nitrogen doesn t float away. D Elements have the same properties no matter how big or how small an amount you investigate. Different elements have different properties from each other. G One volume of hydrogen reacts with half its own volume of oxygen to make water. Dalton said that you can t get half an oxygen atom to combine with one hydrogen atom. B He had no direct evidence about the number of molecules in a gas (he had no way of measuring this). It was a hypothesis (unproven idea). E Certain volumes of a gas have a particular mass. Dalton thought this mass of the gas gives a clue about the mass of the individual atoms in it. H Experiments on breaking down materials showed that some can be broken down into simpler materials. Some other chemicals can t be broken down into anything simpler. C New evidence and ideas about the molecules in gases and the way they react together led to a better understanding about the formula of water molecules. F They had no evidence, only their ideas. They could not test their ideas. They could not convince others about their theory. I Chemical reactions between elements make new materials that are more complicated than the elements. The elements join together in a simple ratio. Can you see how the ideas and thinking about atoms and molecules developed over many centuries? Can you identify where new evidence led to the modification of models and ideas? At what point in this process did direct evidence for the existence of particles emerge? Identify the yearly learning objectives for How science works to which the examples in this timeline are most appropriate. Crown copyright PDF-EN-17

45 The formation of compounds and chemical reactions The formation of compounds and chemical reactions 1 Published schemes generally have a limited range of practical work to illustrate the formation of compounds from elements. The combination of iron and sulphur into iron sulphide is a commonly cited example. In this practical work the differences between the following are stressed: iron sulphide iron sulphur an iron and sulphur mixture. Practical work and related teaching about the formation of compounds from elements should be used to emphasise a number of important differences between compounds and their constituent elements and mixtures of elements. The elements in a compound are combined in a fixed proportion (usually a simple ratio) whereas in a mixture the proportions can be varied at will. (This ratio is shown in the chemical formula of the compound.) Compounds do not usually resemble their constituent elements in either chemical or physical properties. The component elements cannot easily be separated from the compound by physical means, e.g. magnetism, filtration, distillation. Although a new substance has been formed, no new matter has been created. The same atoms are still present, but have just been rearranged. The name of the compound signifies the constituent elements and reflects the fact that new matter has not been magically created. The -ate suffix signifies the presence of oxygen in the compound and the -ide suffix signifies the absence of additional oxygen; copper sulphate and copper sulphide for example. At this level the -ite suffix can denote a smaller proportion of the oxygen than -ate, but at more advanced levels of chemistry a more precise nomenclature is used. Crown copyright PDF-EN-18

46 2 The National Strategies Secondary The formation of compounds and chemical reactions Reflection There is a considerable range of possible additional practical work that could be used to illustrate the formation of compounds. Are there some alternatives pieces of practical work to those that you usually use? Ask a colleague who is experienced in chemistry to demonstrate some of these possible pieces of practical work to you. Points to emphasise when teaching this: compounds need not resemble their constituent elements elements are not easily separated again no new matter is created names of the compounds usually signify the constituent elements elements combine in fixed proportions (unlike mixtures). Please note that the reference sections or Hazcards numbers may change slightly as new editions are produced. Those who are not experienced with chemical reactions must rehearse these procedures. Teachers should not do a demonstration for the first time in front of a class. See CLEAPSS Hazcards and Laboratory handbook section 13.1 (only available to members of CLEAPSS) on accepted practice when handling chemicals and consult the science department s risk assessment. Eye protection is required for all these activities. Useful advice on health and safety in school science is available from several other sources, including: Safety in science education (DfEE, 1996; ISBN ); Hazardous chemicals for science education (SSERC, 1997 but updated regularly); Safeguards in the school laboratory (ASE, 1996; ISBN ) and other ASE publications. Compound Outline details References Aluminium chloride Aluminium iodine Calcium oxide Chlorine is passed over very hot aluminium and yellow aluminium chloride is formed. Use a fume cupboard. A weighed mixture of aluminium powder and iodine is placed on a heat-proof mat in a fume cupboard. The vigorous reaction is initiated by adding a few drops of water. Purple fumes of iodine are produced, leaving behind a white dust containing aluminium iodine. (However, much of the product is aluminium oxide from combustion in air.) A demonstration. Place a granule of calcium on four layers of heat-proof paper. At arm s length, direct a flame from a kitchen torch. A very bright red light is formed. White calcium oxide powder is formed. CLEAPSS Laboratory handbook section 13.2 Chlorine is toxic and irritant Hazcards 2 and 22 Classic chemistry demonstrations page 207 Iodine is harmful Hazcard 54 CLEAPSS guide L195 Wear a face shield and thermal glove PDF-EN-18 Crown copyright 2009

47 The formation of compounds and chemical reactions 3 Compound Outline details References Carbon dioxide Copper chloride Copper chloride decomposition Copper oxide Copper sulphide Iron iodide Heat charcoal on a deflagrating spoon to red heat and place it in a gas jar of oxygen. Carbon dioxide is formed but there is little visible sign of this. When a sheet of Dutch metal foil (copper/ zinc alloy) is introduced into a gas jar of chlorine it spontaneously ignites forming clouds of product, including copper chloride. A demonstration. The electrolysis of 0.1 mol dm -3 copper chloride solution with carbon electrodes to produce copper and chlorine can be carried out as a class practical. If copper foil is heated in a Bunsen flame a layer of black copper oxide is formed. If the foil is folded into an envelope before heating, the difference between the copper oxide layer and the unchanged copper inside can be seen clearly when the envelope is unfolded. This can be carried out on a small scale as a class practical. Prepared quantities of a copper and sulphur mixture are heated in an ignition tube with a mineral wool plug. 0.5g of iodine in a boiling tube is gently vaporised and iron wool above it is strongly heated to initiate the reaction. Brown iron(iii) iodide is formed. A fume cupboard is needed for this demonstration. Hazcard 69 Chlorine is toxic and irritant. Must be done in a fume cupboard. Hazcards 22 and 27 Hazcards 22 and 27. Once the chlorine has been detected using starch/iodide paper or bleaching moist litmus paper, the reaction should be stopped. Do not use currents in excess of 0.5A. Classic chemistry experiments page 59 QCA scheme of work unit 8E Hazcard 96 Classic chemistry demonstrations page 204 Iodine is harmful. Hazcard 54 Crown copyright PDF-EN-18

48 4 The National Strategies Secondary The formation of compounds and chemical reactions Compound Outline details References Iron oxide Iron sulphide Iron (III) bromide Iron (III) chloride Small tufts of iron wool can be burned in air as a class practical. A larger tuft can be burned in a gas jar of oxygen as a demonstration. Finely powdered iron can be prepared (from iron(ii) ethanedioate) which can be demonstrated to ignite spontaneously when it is sprinkled out onto a heat-proof mat and forms brown iron (III) oxide. Iron filings and sulphur can be mixed and separated again with a magnet. When strongly heated they combine to form iron sulphide. (The product is still magnetic unfortunately.) Class practical if appropriate procedure is used. About 0.2ml of bromide is placed in a borosilicate test-tube. Iron wool is placed in the centre of the test-tube. The iron wool is heated. It glows red hot as the bromide fumes react with the iron. A demonstration. Using tongs, place hot iron wool in a gas jar of chlorine. Brown fumes of iron (III) chloride are produced. A demonstration. Classic chemistry demonstrations page 155 QCA scheme of work unit 8F Classic chemistry experiments page 35 CLEAPSS guide L195. This procedure must not be carried out on open bottle tops, heat-proof paper, etc. Use a small test-tube fitted with a mineral wool plug. Hazcard 96 A fume cupboard must be used. CLEAPSS Laboratory handbook section 13.2 Bromine is very toxic and corrosive. Dealing with bromine must be done very carefully and practised. Hazcard 15 CLEAPSS Laboratory handbook section 13.2 Chlorine is toxic and irritant. Hazcard PDF-EN-18 Crown copyright 2009

49 The formation of compounds and chemical reactions 5 Compound Outline details References Lead bromide decomposition Magnesium oxide Phosphorus (V) oxide Silver oxide decomposition Sodium chloride Electrolyse a saturated solution of lead bromide with carbon electrodes. Lead is produced at the cathode and red bromine appears at the anode. A demonstration. Pupils can burn magnesium ribbon in the oxygen of the air and compare the resulting white magnesium oxide with the original elements. A class practical if eye protection is worn and pupils do not look directly at the flame. Red phosphorus can be heated on a deflagrating spoon and placed into a jar of oxygen. Clouds of phosphorus (V) oxide are formed. A demonstration. If silver oxide is heated, silver is produced and oxygen is driven off. A gas jar of chlorine can be inverted over a piece of sodium that has been heated on a fire brick. White sodium chloride is formed and collects on the wall of a gas jar. A demonstration. Stop the electrolysis once the chemical changes have been demonstrated. The solubility of lead bromide is 0.84g in 100ml of water. Hazcards 15 and 57 QCA scheme of work unit 8E Hazcard 59 CLEAPSS Laboratory handbook section 13.2 Red phosphorus is highly flammable. Hazcards 69 and 73 Silver oxide is made by adding small drops of 2mol dm -3 sodium hydroxide solution (corrosive) to 0.1 mol dm -3 silver nitrate solution and filtering. Hazcards 87 and 91 QCA scheme of work unit 8E CLEAPSS guide L195 Chlorine is toxic and irritant. Sodium is highly flammable and corrosive. Hazcards 88 and 22 Crown copyright PDF-EN-18

50 6 The National Strategies Secondary The formation of compounds and chemical reactions Compound Outline details References Sodium oxide Sulphur dioxide Tin (II) bromide Water Water decomposition Sodium is heated in a deflagrating spoon so it burns and is placed in a gas jar of oxygen. White sodium, oxide is formed. A demonstration. Place sulphur in a deflagrating spoon. Heat until it burns and plunge it into a jar of oxygen. It burns with a bright blue flame to form a gas, sulphur dioxide. A demonstration. Place 0.5ml of bromine in a borosilicate test-tube. Add a piece of tin. An exothermic reaction starts. A fume cupboard must be used. Small quantities of hydrogen and oxygen mixture can be exploded as a demonstration. Hydrogen from a cylinder can be burned in air and water collected from the flame. Water, containing sodium sulphate, is separated into hydrogen and oxygen by electrolysis. Use platinum or nickel electrodes if you wish to collect a pure sample of oxygen at the anode. CLEAPSS Laboratory handbook section 13.2 Sodium is highly flammable and corrosive. Hazcards 69 and 88 Sulphur dioxide is toxic and corrosive. Hazcards 69 and 96 CLEAPSS Laboratory handbook section 13.2 Bromine is very toxic and corrosive. dealing with bromine must be done very carefully and practised. Wear protective gloves. Hazcard 15 QCA scheme of work unit 8E Classic chemistry demonstrations pages 88 and 164 Hazcard 48 CLEAPSS guide L195 Classic chemistry experiments page 177, gives an interesting variation. CLEAPSS guide L PDF-EN-18 Crown copyright 2009

51 The formation of compounds and chemical reactions 7 Compound Outline details References Zinc chloride composition Zinc iodide Zinc sulphate Anhydrous zinc chloride has a sufficiently low melting point for it to be decomposed into zinc chlorine by electrolysis as a demonstration. Alternatively, 0.1 mol dm -3 zinc chloride solution decomposes using carbon electrodes and produces zinc metal on the cathode and chlorine at the anode. Zinc powder mixed with iodine is used and the reaction is initiated with a few drops of ethanol. It can be adapted to a class practical. The vigorous reaction between a freshly prepared mixture of zic powder and sulphur makes a lively demonstration when appropriate safety precautions are taken. Do not carry out in fume cupboard, but at the end of the lesson in a well-ventilated laboratory. CLEAPSS guide L195 Hazcards 22 and 108 Classic chemistry experiments page 44 Iodine is harmful. Zinc is flammable. Hazcards 54 and 107 Zinc is flammable. Hazcards 96 and 107 In the latter part of Key Stage 3 teaching about particles often includes: chemical reactions and particle rearrangements conservation of mass using word equations and predicting simple reactions types of chemical reactions, e.g. neutralisation; reactions of metals. This is reflected in the yearly learning objectives for Year 9 of 3.2 Chemical reactions in the science Framework. In Key Stage 4 this is usually extended to include: moving from word equations to symbol equations developing the model of an atom to one that comprises protons, neutrons and electrons rearrangement of bonds between atoms using patterns of reactivity using the particle model to predict outcomes of reactions. There are also a number of misconceptions linked to this aspect of chemistry. Particles are destroyed when a substance is burned, so it loses mass. Compounds are mixtures. The elements can be mixed in any proportions. The name isn t systematically related to the constituent elements. Reactions where gases are formed result in a loss of mass. When two elements react together the atoms from the reactants are transmuted into new atoms (of the products). The reaction is a magical change. Crown copyright PDF-EN-18

52 8 The National Strategies Secondary The formation of compounds and chemical reactions Conservation of mass is an essential concept at this stage in pupils learning. It underlies all the subsequent development of ideas of chemical reactions by rearrangement of particles and chemical equations. Task 12: Illustrating the conservation of mass If you are working with a colleague, jointly try these two demonstrations in a laboratory or prep room. Or ask a colleague experienced in this subject to help you try out these demonstrations. 1. Reaction of effervescent tablets Materials Eye protection Large plastic fizzy drink bottle 50cm 3 water Effervescent tablets Top pan balance with large display Computer with data logging and display software, if possible Method Assemble the apparatus as shown. Add the tablet to the water in the bottle and replace the cap securely to give a gas-tight fit. It may be necessary to break the tablet into two in order to get it through the neck of the bottle. Observe the mass of the bottle as the reaction proceeds. If the top pan balance is interfaced to the computer the mass can be graphed or displayed for participants to see clearly. Although the reading of the top pan balance will be marginally affected as the bottle inflates and receives slightly more upthrust from the surrounding air, the displayed mass will remain almost the same. Pupils frequently believe that reactions involving the production of a gas result in a decrease in mass. Repeating this demonstration with the bottle cap loose, to allow the escape of gas, will show the difference between closed and open systems. The point should be made that in the closed system no matter is destroyed or lost. The atoms have been rearranged, and some are now in the form of a gas (carbon dioxide), but there is the same number of atoms present in the bottle at the end as at the start. Matter has been conserved PDF-EN-18 Crown copyright 2009

53 The formation of compounds and chemical reactions 9 2. The blue bottle experiment Description A colourless solution in a flask is shaken. It turns blue and then gradually back to colourless but the mass remains constant. The cycle can be repeated many times. Apparatus Eye protection One 1dm 3 conical flask with stopper Top pan balance, preferably with large digital display 8g of potassium hydroxide or 6g of sodium hydroxide (corrosive) 10g of glucose (dextrose) 0.05g of methylene blue 50cm 3 of ethanol (highly flammable) Method Before the demonstration make a solution of 0.05g of methylene blue in 50 cm 3 of ethanol (0.1% solution). Weigh 8g of potassium hydroxide or 6g of sodium hydroxide into a 1dm 3 conical flask. Add 300cm 3 of water and 10g of glucose and swirl until the solids are dissolved. Wear eye protection. Add 5cm 3 of the methylene blue solution. None of the quantities is critical. The resulting blue solution will turn colourless after about one minute. Stopper the flask. This solution is an irritant. Shake the flask vigorously so that air dissolves in the solution. The colour will change to blue. Place the flask on the top pan balance and observe the mass. The colour will fade back to colourless over about 30 seconds. Observe the mass of the flask and contents again. The more shaking, the longer the blue colour will take to fade. The process can be repeated for over 20 cycles. After some hours, the solution will turn yellow and the colour changes will fail to occur. A white background is helpful. On a cold day, it may be necessary to warm the solution to C or the colour changes will be very slow. Health and safety Ethanol is highly flammable. Extinguish any naked flames. Potassium hydroxide and sodium hydroxide are corrosive. Wear eye protection (see CLEAPSS Hazcards 40 and 91). Consult your department s risk assessment. What practical work is included in your department s scheme of learning to help pupils understand about conservation of mass? How could you use these experiments to provide an opportunity in class to teach pupils about the reliability and reproducibility of results? Crown copyright PDF-EN-18

54 10 The National Strategies Secondary The formation of compounds and chemical reactions Further guidance The blue bottle experiment: glucose is a reducing agent and in alkaline solution will reduce methylene blue to a colourless form. Shaking the solution admits oxygen which will re-oxidise the methylene blue back to the blue form. Within the limits of experimental error mass is conserved during these reactions. Because they are closed systems no matter is lost to the surroundings, so we can track the mass of the system before and after the reaction. There are the same numbers of atoms at the end of the reaction as at the start. The atoms have just been rearranged during the reaction. The activity is adapted from the Royal Society of Chemistry 1995 publication Classic chemistry demonstrations. A class practical based on the vivid colour change in the reaction between potassium iodide and lead nitrate solutions is described on page 152 of the Royal Society of Chemistry 2000 publication Classic chemistry experiments. Both of these publications have previously been supplied free of charge to secondary school science departments and can be found at Some possible models to help develop understanding of conservation of matter are: molecular models plastic bricks cards computer animations drawings flicker books. The first three of these models are the same as those that were used in Using particle theory to improve understanding of properties of elements and compounds to represent formation of compounds. Here they can be used to represent the particles of reactants before a reaction and the products at the end of the reaction. Computer animations will depend upon which software you have available within your department. The fifth model requires a drawing of the particles of the reactants before the reaction and the products at the end of the reaction PDF-EN-18 Crown copyright 2009

55 The formation of compounds and chemical reactions 11 Draw pictures to represent the particles before and after the chemical reaction. Before After The sixth model is a flicker book, to give a crude moving image representation of the rearrangement of the particles during a chemical reaction. Print off a paper or thin card copy, cut it up and staple together to create a flicker book model of the reaction of methane combining with oxygen. You can use the blank flicker book framework to create flicker books for other chemical reactions. Crown copyright PDF-EN-18

56 12 The National Strategies Secondary The formation of compounds and chemical reactions Flicker book model These six modelling systems, as well as many others, can also be used to help pupils to: understand more about chemical reactions realise that these reactions are caused by the rearrangement of particles reinforce the idea of conservation of matter during reactions begin to name some of the products of reactions write simple word equations pave the way for writing symbol equations later in pupils education. Using only one model to teach about chemical reactions is likely to be too limiting. It will be to pupils advantage to encounter a range of models and analogies over a period of time PDF-EN-18 Crown copyright 2009

57 The formation of compounds and chemical reactions 13 Task 13: Evaluating the models Work with a colleague. Individually consider how well each of the six models can be used to support the teaching and pupils learning of: conservation of matter rearrangement of particles writing equations naming compounds formed. Now use the model(s) to represent one or more of these reactions: C + O 2 CO 2 burning charcoal [carbon], 2H 2 + O 2 2H 2 O burning hydrogen, CH 4 + 2O 2 CO 2 + 2H 2 O Using a Bunsen burner flame [methane]. Share and discuss your ideas with your colleague. Reflection As pupils progress into Year 10 they will usually study displacement reactions between metals and solutions of salts. A more sophisticated particle model is needed. How does the particle model need to change to help explain displacement? Are the illustrations used in text books helpful or could they cause confusion? Can this model be used to represent atoms and ions? Displacement occurs where a more reactive metal is added to a solution containing a salt of a less reactive metal. The more reactive metal tends to dissolve, forming a solution of its own salt, and the less reactive metal is pushed out of the salt solution to appear as grains of the metal. This can be demonstrated to pupils using the reaction between copper and very dilute silver nitrate solution. It can be shown effectively to a class by using a digital microscope linked to a computer with the associated software and a data projector. In 2002 the Department for Education and Skills Year of Science initiative provided all secondary schools in England with an Intel digital microscope. The reaction is also available as a movie clip. Crown copyright PDF-EN-18

58 14 The National Strategies Secondary The formation of compounds and chemical reactions Practical tip It is important to rehearse this small-scale displacement reaction using the microscope before demonstrating it in the classroom with pupils. Use a single piece of copper foil about 0.5cm long on a small watch-glass. Add about 2cm 3 of 0.05mol dm 3 silver nitrate solution; the concentration is not critical and a more dilute solution will give you more time to manoeuvre the reactants and equipment. Setting up and manoeuvring the copper foil so that the edge of the foil is in the field of view is best done at x60 magnification, rather than x200. Keeping the growing crystals of silver in focus can be a little tricky. It is helpful to have previously recorded a sequence of crystal growth at x200 magnification on time lapse so that you have a good-quality sequence ready to show. The appearance of attractive crystals of silver can be seen as the product of the reaction. 0.05mol dm 3 silver nitrate solution is low hazard but will stain clothes and skin. Wear eye protection when using silver nitrate solution. See CLEAPSS Hazcard 87. Displacement reactions can also be demonstrated by: placing a strip of zinc foil into a test-tube of lead nitrate solution placing a strip of iron or steel into a test-tube of copper sulphate solution suspending a coil of copper wire in a test-tube of silver nitrate solution. The suspended crystals that are produced will collapse if jolted, so they will not survive being passed round a class. Task 14: Modelling displacement reactions Try the displacement reaction above for yourself or refer to the time lapse video of silver crystals growing, made using this method. Write the formula for the reaction. Identify the particles on the diagram below. Annotate the diagram to explain what is happening. If you would like to see what other teachers thought please refer to the Answers section PDF-EN-18 Crown copyright 2009