Is Matter Around Us Pure?

This chapter explains what scientists mean by a pure substance and how mixtures can be separated. A pure substance consists of a single type of particles, while a mixture contains two or more substances that keep their own properties. You will distinguish homogeneous mixtures, or solutions, such as salt in water, from heterogeneous mixtures such as sand in water. You will learn the parts of a solution, solute and solvent, saturated and unsaturated solutions, solubility, and how to express concentration as mass by mass or mass by volume percentage. You will compare true solutions, suspensions such as chalk in water, and colloids such as milk, and see how colloids scatter a beam of light, showing the Tyndall effect. You will study methods of separating mixtures, including evaporation, centrifugation, separating funnels, sublimation, chromatography, simple and fractional distillation and crystallisation, and see how air is separated into gases. Finally, you will distinguish physical and chemical changes and classify pure substances as elements, either metals, non-metals or metalloids, and compounds.

Detailed Worksheet: Is Matter Around Us Pure? Section A - Definitions (10 marks) 1. What is a pure substance? Distinguish between a homogeneous and a heterogeneous mixture with one example each. (2 marks) 2. Define solute, solvent and solution. Identify them in a solution of sugar in water. (2 marks) 3. What is a saturated solution? Define solubility. (2 marks) 4. What is the Tyndall effect? Which type of mixture shows it? (2 marks) 5. Distinguish between a physical change and a chemical change with one example each. (2 marks) Section B - Calculations and Applications (15 marks) 6. A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution. (3 marks) 7. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration as a mass by mass percentage. How much salt will dissolve in 250 g of water at the same temperature? (3 marks) 8. A student dissolves 15 g of sugar in water to make 250 mL of solution. Find the concentration in mass by volume percentage. How much sugar is needed to make 400 mL of a solution of the same concentration? (3 marks) 9. Suggest the method of separation for each mixture: (i) ammonium chloride from salt (ii) butter from curd (iii) kerosene from water (iv) dyes in black ink (v) acetone from water (vi) salt from sea water. (3 marks) 10. Classify the following as elements, compounds or mixtures: sodium, soil, sugar solution, silver, calcium carbonate, tin, silicon, coal, air, methane, carbon dioxide, blood. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the apparatus for simple distillation to separate acetone and water. Explain the principle. (4 marks) 12. Draw a labelled diagram of separation of two immiscible liquids using a separating funnel. (3 marks) 13. Draw a labelled diagram showing the separation of camphor or ammonium chloride from salt by sublimation. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Compare true solutions, suspensions and colloids on five properties: particle size, visibility, stability, filtration and the Tyndall effect. Give one example of each. (5 marks) 15. Draw a flowchart and explain how the gases of air are separated by fractional distillation of liquid air. Why is crystallisation better than evaporation to purify solids? (5 marks) 16. Explain how iron filings and sulphur powder show different properties when mixed and when heated together to form iron sulphide. What does this show about mixtures and compounds? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show the formula and all steps in concentration calculations.