Separation of mixtures
2 marks · HardA student is given a mixture containing sand, sodium chloride, and water. For each of the following, state the most suitable separation technique and explain your choice briefly.
To obtain dry sand from the mixture.
To obtain pure sodium chloride crystals from the mixture.
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(a) Filtration, because sand is insoluble and can be separated from the liquid by filtration.
(b) Crystallisation, because sodium chloride is dissolved in water and can be recovered as crystals by evaporating the water and cooling the solution.
Common mistakes: confusing filtration with crystallisation; selecting distillation for either part
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Marking points
B1 (a) · 1 mark · For correctly naming and briefly explaining filtration as the method to separate dry sand from the mixture. Explanation must reference insolubility or sand remaining as residue.
B2 (b) · 1 mark · For correctly naming and briefly explaining crystallisation as the method to obtain pure sodium chloride crystals from the solution. Explanation must mention recovery of solid from a solution by evaporation and cooling.
Worked solution
a) The suitable separation technique is filtration. Sand is insoluble in water and can be separated from the mixture by pouring it through filter paper. The sand remains on the filter paper (residue), and the solution passes through (filtrate).
b) The suitable separation technique is crystallisation. The sodium chloride is dissolved in water. By heating the solution to evaporate most of the water and then cooling the remaining solution, pure sodium chloride crystals can be formed and collected.