1.4 Filtration and crystallisation
5 exam questions worth 11 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Which method would you use to separate sand from water? Explain your choice.
Mark scheme
- filtration (1)
- sand is insoluble (so stays as residue) (1)
Model answer
Filtration, because sand is insoluble and stays in the filter paper while the water passes through.
In filtration, what is meant by the residue and the filtrate?
Mark scheme
- residue: solid left in the filter paper (1)
- filtrate: liquid that passes through (1)
Model answer
The residue is the solid left in the filter paper, and the filtrate is the liquid that passes through.
Describe how to obtain pure, dry salt crystals from a mixture of salt and sand.
Mark scheme
- add water and stir to dissolve the salt (1)
- filter to remove sand (1)
- heat the filtrate to evaporate some water / until crystals start to form (1)
- leave to cool and crystallise; filter and dry crystals (1)
Model answer
Add water and stir so the salt dissolves. Filter the mixture to remove the sand. Heat the filtrate gently to evaporate some of the water until crystals start to form, then leave it to cool. Filter off the crystals and dry them between filter papers.
Why is the solution heated gently and then left to cool, rather than boiled until dry?
Mark scheme
Any two from:
- larger / purer crystals form (1)
- avoids decomposing the solid / losing water of crystallisation (1)
- avoids spitting (1)
Model answer
Heating gently and cooling slowly gives larger, purer crystals, and stops hydrated crystals losing their water or decomposing.
Which method is used to get a soluble solid from its solution?
Choose one answer.
Correct Not quite: the answer is Crystallisation
Mark scheme
B: crystallisation
Model answer
B: crystallisation.
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