Introducing the mole. David Paterson. Lesson Sequence. Lesson 1:

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1 Geological Survey NZ Journal of Geology and Geophysics, 8, Reid, L.R. (2008). Promoting science: New Zealand Institute. In S. Nathan & M. Varnham (eds), The Amazing World of James Hector (pp ). Wellington: Awa. Rollinson, H. (1993). Using geochemical data: Evaluation, presentation, interpretation. UK, Harlow: Longman. Searle, E.J. (1958). A note on the occurrence of native iron and other effects associated with contact of basalt and carbonised wood at Auckland, New Zealand. NZ Journal of Geology and Geophysics, 1, Steiner, A. (1958). Petrogenetic implications of the 1954 Ngauruhoe lava and its xenoliths. NZ Journal of Geology and Geophysics, 1, Washington, H.S. (1917). Chemical analysis of igneous rocks. US Geological Survey Professional Paper, 99. Williams, P.P. (Ed.). (1981). Chemistry in a young country. Christchurch: New Zealand Institute of Chemistry. Yardley, B. (1991). The successful alchemist. New Scientist, 131, 1781, Introducing the mole David Paterson Head of Science, Cashmere High School ( pt@cashmere.school.nz) The International System (SI) of Units defines seven standard units of measurements from which all other units can be derived. The mole is the SI base unit for amount of substance and is defined as: The amount of substance that contains as many entities as there are in exactly kg of carbon-12 (12 g of C-12 atoms). When the mole is used the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles (CGPM, 1971). Teachers and students have grappled for many years trying to understand the concept of the mole and apply it to chemical reactions. Furio et al. (2000) reports that a large number of chemistry teachers and a large proportion of chemistry text books have misconceptions about the mole compared with the accepted view held by the scientific community. In discussing students difficulties when problem solving using moles Lumpe (1995) points out that many students do not understand the chemical concept of the mole, and therefore struggle to apply it to chemical reactions. In writing about constructivist theory von Glasersfeld (1995) develops this idea more generally and suggests that abstract concepts cannot simply be transferred from teachers to students; they must be conceived by the students themselves. I decided to produce a series of lessons that draw on constructivist theory to help students build their own concept of the mole. One of the basic tenants of constructivism is to find out what students already know and link new knowledge to their existing experiences (Driver et al. 1994, Duit & Treagust 1998, Novak 1978). Another important aspect of constructivism is that students are motivated to adopt a new concept when it is seen as being fruitful. Or as Bodner (1986, p.185) states, Knowledge is good if and when it works. The problem with teaching about the mole is that there is little personal experience for students to draw on when trying to connect with this new concept, and they have limited knowledge of situations in which the mole might be useful, that is, when it might work for them. Lesson Sequence Lesson 1: Objectives: to find the mass of oxygen that combines with magnesium when a piece of magnesium ribbon is heated, to work out the formula of magnesium oxide. Introduction/instructions (10 mins): Teacher introduces the new topic of quantitative chemistry, explaining it involves measuring quantities of matter, for example, masses and volumes. Emphasis is on careful, accurate measurements, in this experiment through the use of high quality electronic balances. The instructions for the experiment are given along with two questions for them to think about while doing the experiment: 1. Predict-Observe-Explain. Predict what happens to the mass of the magnesium after the reaction. Observe carefully what happens. Explain what happened in terms of the substances/ particles involved, and compare with your prediction. 16

2 Chemistry Education in New Zealand February Can you work out the formula of the new compound, magnesium oxide. Is it MgO, MgO 2, Mg 2 O, or some other combination? If you can, how did you do it? If you can t, what other information might you need? Practical work (40 mins): Students work in groups of 3 or 4 to heat magnesium ribbon in a crucible. Careful measurements are made to determine the mass of magnesium used and the mass of the compound formed. Teacher circulates round the groups, questioning to help build understanding. Conclusion (5 mins): Each group briefly shares their weighing results; it is established that a gain in mass is observed. Lesson 2: to understand how substances combine Hook (5 mins): Students are asked to imagine they are one of the early chemists. Teacher links previous lesson s experiment to the humorous video to come. Data processing (15 mins): The students own results are analysed along with a set of results provided by the teacher. First they complete a table requiring mass subtractions to calculate the mass of magnesium and oxygen which reacted. Students graph the mass of magnesium used against the mass of oxygen used, the latter being calculated from the masses recorded in the experiment. Teacher circulates, helping with mathematical and graphing tasks and questioning students about the relationship emerging from the graph. Key questions (15 mins): Students discuss the two key questions posed in Lesson 1 and write answers in their books. Teacher circulates and questions the groups. Quick summary session with whole class where ideas are shared. Video (15 mins): Approximately half of the video The History of the Atom (Davis & Whittle, 2002) is shown with discussion and questions. Conclusion (5 mins): Return to the idea raised at the start of the lesson, having seen the video and done the experiment can they imagine what it was like for the early chemists trying to fathom what exactly is happening in chemical reactions? Lesson 3: to find the mass of magnesium ribbon that will produce exactly 40 ml of hydrogen gas when reacted with excess hydrochloric acid. Intro (5 mins): The work of the early chemists investigating the properties and nature of matter is linked to today s experiment. Practical work (40 mins): Students are shown the experimental set-up, given a piece of magnesium ribbon that will produce less than 40 ml hydrogen gas, and work in groups to solve the problem. Two questions are put on the board for them to answer during and after the experiments. Explain how you worked out the mass of magnesium required? Explain in terms of atoms/molecules what actually happens during the reaction, and how the gas displaces the water in the collection tube. Teacher circulates round the groups asking questions and giving feedback. Class discussion (10 mins): Sharing of results and answers to the questions. Lesson 4: to introduce the term mole, explain why it is needed and how the current definition came about. Hook: All will be revealed! Today we will bring together experiments and theory to see how experiment and theory combine to deliver useful concepts. Class discussion (10 mins): Re-visiting the previous experiment with students sharing their explanations for the two questions posed. Video (20 mins): The remaining half of The History of the Atom is shown. PowerPoint presentation (25 mins): Quantitative Chemistry: the history of the mole is shown with accompanying explanations and questioning by the teacher. Conclusion: Attractive picture of a mole (the animal) shown with the caption, Moles rule OK! Participants The class consisted of 29 Year 12 students at a large co-educational school in Christchurch, New Zealand. The students have completed a General Science course in Year 11 and represent a range of abilities. The students receive four periods of chemistry per week each of 55 minutes. Reflection on Lessons The first experiment was chosen because the materials and reaction were familiar to students. They have burnt magnesium ribbon many times in school science classes. I have even used this experiment in 17

3 the past when teaching the mole topic. However, this time I was cognisant of constructivist theory and tried to probe student understanding more deeply by posing questions to individuals and groups during the experiment, and in class discussions. A new strategy I used was predict-observe-explain (POE) as described by Gunstone (1995) to encourage students to think more explicitly about experiments and examine their existing beliefs. I asked them to predict what would happen to the mass of the magnesium as it turned into ash, and come up with the formula for the ash. Another question posed was: explain what is happening within the reaction crucible in terms of the particles involved. This question had two functions, the first being to get students to think about the particle nature of matter, a topic that should be familiar to them from science lessons throughout their schooling; the second being to establish the conceptually difficult link between atomic scale events and macroscopic properties such as weight, which as Lumpe (1995) states, is the principal value of the mole. The students responses to questioning were interesting with most prepared to say that the magnesium would gain weight but not by how much. Their understanding of particle theory was reasonable but took considerable probing and encouragement to elicit. For example, after an initial conversation about the changes in the crucible and her predictions, one discussion went like this: Teacher: So what is happening in the crucible in terms of particles? Sarah: Well, the oxygen molecules sort of bang into the magnesium. Teacher: So what is the actual reaction? Sarah: Then something gets transferred electrons? This shows that Sarah did have a mental picture of molecules moving in the gas phase, and could link the experiment to prior knowledge of new substances forming through electron transfer. In the second lesson the students own and given data were analysed through the familiar task of graphing. Students plotted points and most could see that a straight line offered the best fit, clearly indicating a direct relationship between the mass of magnesium and the mass of oxygen. These concrete activities of physically manipulating apparatus, weighing objects and drawing graphs were designed to build links between the real world and the abstract concepts of atoms, molecules and their reactions. This type of linking is promoted by many constructivist researchers (Gunstone 1995, Venville 2003). In thinking about how to work out the formula of magnesium oxide some students were already seeing that more information was needed: Liz: Don t we need to know the mass of magnesium atoms compared to oxygen atoms? This question clearly shows that Liz was developing the conceptual framework that would make the mole a useful idea when finally introduced. Then I presented to students some of the history of chemistry in order to explore the origins of the term mole. Why did the early chemists need to develop this concept connecting number of particles to chemical reactions? If students could put themselves in the position of these historical figures, perhaps they would see a purpose for the mole. Social-constructivists argue that learning requires the novice student being led into the cultural world of the expert. Driver et al. (1994, p.7) state that: Learners need to be given access not only to physical experiences but also to the concepts and models of conventional science. Duit & Treagust (1998, p.18, p.19) talk about a cognitive apprenticeship, and the crossing over from the culture of everyday life into the foreign culture of science. Students move from one culture to the other by being exposed to the language and symbolism of science, and by appreciating the way in which scientific ideas progress. The video The History of the Atom presents the language and symbolism of science in a humorous way, which engages the students. It follows developing ideas about atomic structure from the Greek philosophers, through the alchemists to Dalton, and highlights some of the debates and controversies scientists had along the way. In the next period the students returned to a practical task in which they were challenged to produce exactly 40 ml of hydrogen using magnesium ribbon and hydrochloric acid. Here, students were motivated to complete the task through personal challenge and by competition with other groups. The underlying purpose was to make the connection between the mass of magnesium that reacts and the volume of gas produced, volume being a property dependant on the number of gas particles present. Again, the experimental procedure was very familiar to students, having made and tested hydrogen gas by this method many times in science classes. However, questions were set at the beginning to ensure they were thinking more deeply about concepts and connections: Explain how you worked out the mass of magnesium required. 18

4 Chemistry Education in New Zealand February 2011 Explain in terms of atoms/molecules what actually happens during the reaction, and how the gas displaces the water in the collection tube. The students were keen to solve this problem and as I circulated round the groups I kept asking them the two questions. This is my normal practice with laboratory work, but with my constructivist lens in place I found I was more focussed on the students thinking and asked many questions to establish their current views and to guide them towards the accepted scientific explanations. Sfard (1998) describes this type of learning as using a participation metaphor. Here the student is learning to become part of a particular community (scientists) through participation in suitable activities and by discourse with the teacher acting as the expert participant. The students adopted a variety of strategies to solve the problem, most through trial and error. They quickly recognised that changing the mass of magnesium caused a change in volume of gas collected in a predictable manner. Some groups looked for a mathematical relationship, and busily punched numbers into their calculators, working out the mass of magnesium required based on their first experiment. When they then weighed out the calculated mass and found that almost exactly 40 ml of gas was collected there was obvious satisfaction with the result. These students were well on their way to constructing relationships between matter and molecules, which should enable them to see the value of the mole concept when introduced in the next lesson. In the class discussion that followed this experiment I was surprised to find that students had difficulty in explaining why or how the hydrogen gas was able to push the water out of the collection tube. Responses such as the gas is taking up space were typical and despite much probing I could not get a clear view of gas molecules as particles colliding with surfaces to produce pressure. Fortunately this concept was addressed when showing the second part of the DVD The History of the Atom in the next period. In a scene where Boyle and Newton are discussing their ideas of particles in motion an analogy is presented where marbles bounce around in a wooden box. I immediately asked the class if this was the same mental model as they held in their heads? The response was a set of fairly blank looks! Once again, with my thinking and practice sharpened by reading in constructivist theory I was able to make more of these situations than perhaps I would have done in the past. Driver et al. (1994) give examples of teacher questioning to build common knowledge in the classroom, which was my intention in these sessions. The relationship between number of particles and volume was then explored in more detail through the historical context of the early chemists. A more formal PowerPoint presentation was shown which highlighted the work of Avagadro, the first time his name is mentioned in the unit. I created this presentation after doing extensive reading and research on the history of the mole (Dierks 1981, Furio et al. 2000, Gorrin 1994, Lumpe 1995). This process significantly clarified my own thinking and understanding of the concepts involved. The arguments between chemists at the end of the 19 th century were illustrated to show that the concept of amount of substance was developed over time, and why the need for the unit mole was required. Gorrin (1994, p.115) sums up the points I tried to make very well when he states that the mole forges the crucial link between Avagadro s Hypothesis and Newton s mechanical philosophy, and that any element can be used as reference and its atomic weight can be assigned any numerical value. But to do so in the absence of a consensus would result in utter confusion. By allowing students to build connections between experiment and theory themselves, and to see that concepts are constructed in a collaborative social endeavour by the science community, I hoped to avoid some of the confusion traditionally associated with the teaching of the mole. I will be very interested to see if my new teaching approach for introducing the mole has been helpful to the students when they apply these concepts to solving stoichiometric problems. The formal assessment for this topic is set by an external qualifications authority and I will use this to judge student understanding. However, throughout the remainder of the topic I will continue to probe for understanding and construction of meaning using my improved questioning techniques. Duit & Treagust (1998, p.14) argue that for students to accept a new idea like the mole, the new conceptions must be intelligible, initially plausible and fruitful. Having made the mole intelligible and plausible through the practical and historical activities outlined, I have confidence that the students will find it fruitful when applied to solving problems. References Bodner, G. M. (1986). Constructivism: A theory of knowledge. Journal of Chemical Education, 63(10), Conférence Générale des Poids et Mesures (CGPM), (1971). Resolution 3 of the 14th meeting of the CGPM: SI unit of amount of substance (mole). Retrieved May 8, 2010, from CGPM/db/14/3/ 19

5 Davis, J. (Producer), & Whittle, G. (Writer). (2002). The History of the Atom [Video]. Australia: Classroom Video. Dierks, W., (1981). Teaching the mole. International Journal of Science Education, 3(2), Driver, R., Asoko, H., Leach, J., Mortimer, E. & Scott, P. (1994). Constructing scientific knowledge in the classroom. Educational Researcher, 23(7), Duit, R. & Treagust, D.F (1998). Learning in science from behaviourism towards social constructivism and beyond. In B. J. Fraser & K. G. Tobin (Eds), International handbook of science education (pp.3-25). Dordrecht, Netherlands: Kluwer. Furio, C., Azconza, R., Guisasola, G. and Ratcliffe, M. (2000). Difficulties in teaching the concepts of amount of substance and mole. International Journal of Science Education, 22(12), Gorrin, G., (1994). Mole and Chemical Amount; A discussion of the fundamental measurements of chemistry. Journal of Chemical Education, 71(2) Gunstone, R. F. (1995). Constructivist learning and the teaching of science. In B. Hand & V. Prain (Eds), Teaching and learning in science: The constructivist classroom (pp 3-20). Sydney: Harcourt Brace. Lumpe, A. T. (1995). Two investigations of students understanding of the mole concept and its use in problem solving. Journal of Research in Science Teaching, 32(2), Novak, J. D. (1978). An alternative to Piagetian psychology for science and mathematics education. Studies in Science Education, 5, Sfard, A. (1998). On two metaphors for learning and the dangers of just choosing one. Educational Researcher, 27(2), Venville, G. (2003). Fostering thinking in science through the early years of schooling. International Journal of Science Education, 25(11), Von Glasersfeld, E. (1995). A constructivist approach to teaching. In L. P. Steffe & J. Gale (Eds), Constructivism in education, (pp. 3-15). Hillsdale, NJ: Laurence Erlbaum. Some schools in the United States, South Africa, Canada and Australia celebrate Mole Day on 23 October. Why this date? If activities are scheduled between 6.02 a.m. and 6.02 p.m. on October 23, this makes the date 6.02: 10/23 in the American format of writing dates. If Mole Day was to be celebrated in New Zealand perhaps it should be from a.m. to p.m. on 6 February, i.e., 6/