Separation of Substances

Many substances we use are mixtures, and we often need to separate their components, either to remove harmful or useless parts or to obtain useful ones. In this chapter you will learn methods based on differences in size, weight and other properties. Handpicking removes stones from rice and pulses, threshing separates grain from stalks, and winnowing uses wind to blow away light husk from heavier seeds. Sieving separates fine flour from bran and larger particles, as builders do with sand and pebbles. You will also study how insoluble solids are separated from liquids: in sedimentation the heavier particles settle at the bottom, decantation pours off the clear liquid above, and filtration passes the mixture through filter paper. Evaporation recovers salt from sea water, and condensation turns water vapour back into liquid. Churning separates butter from curd. Finally, you will learn that water dissolves only a limited amount of salt to form a saturated solution, and more dissolves when the water is heated.

Detailed Worksheet: Separation of Substances Section A - Definitions (10 marks) 1. Why do we need to separate substances? Give two reasons. (2 marks) 2. Define threshing and winnowing. (2 marks) 3. What are sedimentation and decantation? (2 marks) 4. What is filtration? Name the material through which liquid is filtered in the laboratory. (2 marks) 5. What is a saturated solution? How can more salt be dissolved in it? (2 marks) Section B - Calculations and Applications (15 marks) 6. Name the method of separation for each: (i) stones from rice (ii) husk from wheat grains (iii) bran from flour (iv) tea leaves from tea (v) butter from curd (vi) salt from sea water. (3 marks) 7. A student dissolves salt in 50 mL of water at room temperature. She can dissolve 18 g before the solution becomes saturated. (i) How much salt can she dissolve in 100 mL of water at the same temperature? (ii) How much in 25 mL? (iii) What happens if she heats the saturated solution and adds more salt? (3 marks) 8. Sea water contains about 35 g of salt in every litre. (i) How much salt can be obtained by evaporating 10 litres of sea water? (ii) How much water would be needed to obtain 1.4 kg of salt? (iii) Name the method used in salt pans. (3 marks) 9. Suggest the sequence of methods to separate (i) a mixture of sand, salt and water (ii) a mixture of chalk powder and water (iii) a mixture of iron filings, sand and sawdust. (3 marks) 10. Explain why each method works: (i) winnowing (ii) sieving (iii) sedimentation. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of filtration in the laboratory. Label the filter paper folded into a cone, funnel, beaker, residue and filtrate. (4 marks) 12. Draw a labelled diagram of winnowing showing a farmer, the falling grain, the husk being blown away and the wind direction. (3 marks) 13. Draw a labelled diagram showing evaporation of salt water in a china dish heated on a burner and condensation of vapour on a cold metal plate held above it. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Paheli has a mixture of salt and sand. (i) Describe step by step how she can get back both salt and sand. (ii) Name each method used. (iii) Why does salt not stay on the filter paper? (iv) How can she get back the water too? (5 marks) 15. Muddy water from a river is to be made clean enough to use. (i) What will happen if it is left in a bucket for some time? (ii) How can the clear water be removed? (iii) Why is this water still not safe to drink? (iv) Suggest one way to make it safe. (5 marks) 16. A student mixes 20 g of sugar into 100 mL of water and keeps adding sugar until no more dissolves. (i) What kind of solution is formed? (ii) Why does extra sugar settle at the bottom? (iii) What will happen if the solution is heated? (iv) What will happen when the hot solution cools? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part.