Unit 6M.3: Adding materials to water. Chemical reactions Skill you will use: Classifying Observing Predicting By the end of this unit you should: Know that when substances are added to water, some will react while others either dissolve or remain suspended. Distinguish between reversible and irreversible changes and know that reversible changes are physical and irreversible ones involve chemical changes in which new substances are formed. 67
6.12.1, 6.12.2 Materials Mixing materials to water Many types of materials dissolve when mixed into water, this forms a solution. Salt is a soluble material, it dissolves in water. A material that doesn t dissolve in water is called insoluble, for example sand. A material that is insoluble creates a suspension when mixed in water. A suspension is not clear, you cannot easily see through it. As the large, insoluble particles settle you can see the separate layers. Muddy water particles are suspended in water making it cloudy The large mud particles have settled to bottom Physical changes in water are temporary and reversible Some materials can react with water, like sodium. Signs of a chemical change in water are the presence of bubbles, change in colour, light or heat, even a flame. 68
Activity 1: Investigate what happens when a variety of substances are added to water. You will need: Five 500 ml plastic bottles with caps Teaspoon Plastic funnel Samples: sugar, dirt, tea leaves (cut open a tea bag), flour, food colouring Water and measuring beaker Steps: 1. Use the funnel to add 1 teaspoon of each of the sample to the empty plastic drink bottle. 2. Predict (P) what will happen to each sample after the water is added: Sugar..... Dirt Tea leaves Flour.. Food colouring 69
2. Use the measuring beaker and funnel to add 250ml of water to each plastic bottle and screw on the lid. 3. Observe (O) what happens to contents initially. 4. Now, shake each bottle until the contents are mixed. 5. Choose some key changes you observed to add to the top line of the Attribute List Organiser (ie dissolved, suspended, cloudy etc). 6. Record the changes that you observed for each sample by placing ticks in the corresponding box. You can also draw examples. Attribute List Organiser Sample Change 1 Change 2 Change 3 Change 4 Sugar Dirt Tea leaves Flour Food Colouring 70
Observe (O): Other observations you want to comment on?.. Explain (E): Did your observations match your predictions? Did anything happen that you did not predict? Activity 2: Sort into a T-chart the following common kitchen materials according to how they change when added to water Milo, detergent, pepper, milk, salt, juice, rice, coffee granules, cooking oil. Dissolves Suspended 71
Changing Materials with Water: 1-Weathering is the process of breaking down a material over time. There are two types of weathering; physical weathering and chemical weathering. Physical weathering erodes a material over time, but the properties of the material stay the same. Chemical weathering also breaks down a material over time but it changes the properties that the material is made of. For instance, a hard material may change to a soft material after chemical weathering. The time taken for the weathering process depends on the properties of the material, the amount of water, the movement/ action and the type of water. 72
2- Extracting colour: The colour found in our soils and rocks, plant leaves and flowers can be extracted using water. If you collect a sample of soft rock and scratch the surface to make a powder. Mix the powder with water it can make a paint known as ochre. Traditionally tribes used the ochre pigment for art. 3- Extracting scent: The natural oils in plants, fruits and flowers have a scent that can be extracted using water. The water is used to heat and reduce down the liquid from the plant. The liquid left produced in this process has a fragrance that is used for perfumes and fragrances. 73
4- Putting out fires: Water has two main properties that help put out certain types of fires. Water when applied to a very hot fire changes to steam, this steam prevents the air getting to the fuel thus putting out the fire. The water also cools the fuel to below the ignition point helping to put out the fire. Water only helps for fires of paper, wood or cloth/ material. Aerial Water Bombing Other types of fires may need chemicals to extinguish the flame for eg. petrol. Foam Extinguisher 74
Chemical Changes of Materials A chemical change is when the atoms in a substance or material link together to make a new substance. This is known as a chemical reaction. The properties of the original material have changed. Chemical changes can be permanent and irreversible but like physical changes they can also be reversible. You can usually identify when a chemical change of a material has taken place if: - a gas or smell is given off - there is permanent color change - a new substance (e.g. rust) is formed and is visible - the liquid changes temperature - the solute is not recoverable when the water is left to evaporate. These children are studying chemical reactions that make materials change permanently. The substances they start with are called reactants. After the reactants change, they form a new substance called a product. 75
Activity 3: Observing a Chemical Reaction You will need: Plastic bottle, Baking soda, teaspoon, balloon, funnel,vinegar. Steps: 1. Use the funnel to add 2 teaspoons of baking soda to the balloon. 2. Tip a small amount of vinegar into the bottom of the plastic bottle. 3. Attach the balloon to the top of the plastic bottle, CAREFUL not to tip the contents in yet! 4. Draw a picture of what the experiment looks like BEFORE in the box below. 5. Now tip the contents of the balloon, the baking soda into the bottle. 6. Draw another picture of the experiment AFTER adding the baking soda. BEFORE AFTER 76
What evidence do you have that the Baking soda and vinegar experienced a chemical change?... Design your own experiment using baking soda and vinegar to demonstrate a chemical change. Remember the scientific process Show your experiment design and planning to the teacher before you test it out. Write some initial ideas down here: 77
Using electricity to change things: Water and solutions, particularly strong salt solutions or acid solutions, like lemons are known as conductive substances. This means that electricity flows through them easily. Research the Lemon Juice Electricity Experiment What are the procedures? How does it work? Test it out yourself! Why cells and batteries eventually run down when used? A chemical cell converts chemical energy into electrical energy. Most batteries are chemical cells. A chemical reaction takes place inside the battery and causes electric current to flow. This process is known as electrolysis. 78
Key Ideas: Some materials when added to water dissolve and others form suspensions. Materials can have physical or chemical change. Sometimes change in some materials is temporary and other times permanent. Chemical changes can create new substances. Key Terms: Soluble/ insoluble Dissolve Suspension Physical and chemical change Weathering Permanent Temporary Key Questions: Q1 Complete this concept map for the word Dissolve DISSOLVE 79
Q2. Amal added some substances to water and stirred them. She noted her observations in the table below. Substance Observation 1 - Sugar Dissolved 2 - Salt Dissolved 3 - Sand Remained at the bottom 4 - Stomach powder Bubbles of gas given off 5 - Flour Liquid turned white and eventually the white substance collected at the bottom What substances experienced chemical changes?. b. How could Amal tell if it was a chemical change?.. c. Which two samples could the change have been physical or chemical?... 80
Q2: The two lists below contain raw materials and substances made from them. Draw lines to connect each raw material to the substance produced from it. On each line write whether the change is a physical or a chemical one. Raw material Clay Ore Sand Seawater Wood Substance produced Glass Metal Paper Pottery Salt Q3: Classify the list of changes as physical or chemical change A. Making yoghurt from milk. ---------------. B. Making drink cans from aluminium sheet. -----------. C. Boiling water in a kettle. -------------. D. Boiling an egg. -----------. E. Making a cup of tea. ---------------. F. Using a torch cell. ---------------. G. Burning a candle. ---------------. 81