Filtration and crystallisation

Question 1

Why is filtration used?

Answer 1

You use filtration to separate an insoluble substance from a liquid or solution. This could be to purify a liquid or solution by removing solid impurities from it, e.g. sand from sea water. It could also be to separate the solid you want from the liquid it is mixed with, e.g. to separate crystals from a solution after crystallisation.

Question 2

How does filtration work?

Answer 2

Filtration works because the filter paper has tiny pores. These are large enough to let water molecules and dissolved substance through but small enough to stop insoluble solid particles going through.

Question 3

What is crystallisation used for?

Answer 3

Producing solid crystals from a solution.

Question 4

What happens in crystallisation?

Answer 4

The solution is heated to remove enough solvent to produce a saturated solution (one that cannot hold any more solute). The saturated solution is allowed to cool and crystals form in the solution. The crystals are separated from the liquid and dried. A hot water bath gives you more control over heating than using a Bunsen burner flame directly on the evaporating basin.

Question 5

How does crystallisation work?

Answer 5

Solubility is the mass of solute that dissolves in a given volume of solvent at a given temperature. Crystallisation works because the Solubility of the solute decreases as the saturated solution cools. Crystals form from the excess solute.