Physical and Chemical Changes

Changes around us are of two kinds. A physical change alters properties such as shape, size, colour or state, but no new substance is formed, and it is often reversible; melting ice, tearing paper and dissolving salt are examples. Crystallisation, in which pure crystals of a substance such as copper sulphate form from its solution, is also a physical change. A chemical change forms one or more new substances and may be accompanied by heat, light, sound, a change of colour or smell, or the release of a gas. When a magnesium ribbon burns with a dazzling white light, it forms magnesium oxide, which dissolves in water to give magnesium hydroxide, a base. An iron nail in copper sulphate solution turns brown as copper is deposited, and the blue solution turns green due to iron sulphate. Vinegar and baking soda give carbon dioxide, which turns lime water milky. Rusting needs both oxygen and water: iron + oxygen + water -> rust. It is prevented by painting, greasing or galvanisation, that is, coating iron with zinc.

Detailed Worksheet: Physical and Chemical Changes Section A - Definitions (10 marks) 1. Define physical change and chemical change with one example each. (2 marks) 2. What is rusting? Write its word equation. (2 marks) 3. What is galvanisation? Why is it done? (2 marks) 4. What is crystallisation? Is it a physical or chemical change? (2 marks) 5. List four signs that indicate a chemical change has taken place. (2 marks) Section B - Calculations and Applications (15 marks) 6. Classify as physical or chemical change: (i) melting of wax (ii) burning of paper (iii) cooking food (iv) dissolving sugar in water (v) rusting of iron (vi) cutting a log of wood. (3 marks) 7. Write word equations for: (i) burning of magnesium (ii) magnesium oxide dissolving in water (iii) reaction of carbon dioxide with lime water. (3 marks) 8. Write the word equation for the reaction between copper sulphate solution and iron. State two observations. (3 marks) 9. Name the gas formed when vinegar reacts with baking soda. How will you test it? Write the word equation for the test. (3 marks) 10. Explain why (i) rusting is faster in coastal areas (ii) ships suffer a lot of damage from rusting (iii) stainless steel does not rust easily. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the set-up to pass the gas formed from vinegar and baking soda through lime water. (4 marks) 12. Draw a labelled diagram showing an iron nail in a beaker of copper sulphate solution before and after the reaction. (3 marks) 13. Draw a flowchart showing the steps of obtaining crystals of copper sulphate from its solution. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Boojho placed three iron nails in three test tubes: one in dry air with calcium chloride, one in boiled water with an oil layer, and one in ordinary water open to air. (i) In which tube will the nail rust? (ii) Why not in the other two? (iii) What does this prove? (iv) Suggest two ways to prevent rusting of a bicycle. (5 marks) 15. Explain whether each is a physical or chemical change, giving reasons: (i) burning of a candle (both changes) (ii) ripening of fruit (iii) formation of curd from milk (iv) explosion of a firecracker (v) folding a paper. (5 marks) 16. A cut apple turns brown when left in air. (i) Is this a physical or chemical change? (ii) What causes it? (iii) Name one way to slow it down. (iv) Give one more example of a chemical change in daily life that is useful and one that is harmful. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write word equations with arrows and do experiments only under teacher supervision.