medium difficulty5 min

Separation by Crystallization

Crystallization is a separation method in which pure solid crystals are obtained from a solution by concentrating it and then cooling it.

In one line: Crystals need concentration plus cooling, not careless drying.

Practice This Concept

Understand it

Crystallization is useful when a soluble solid contains impurities or when the solid has to be recovered in a purer form. The solution is heated to become concentrated, filtered if needed, and then cooled so crystals separate out.

Example

Impure copper sulphate can be purified by dissolving it in water, filtering insoluble impurities, concentrating the solution, and cooling it to form crystals.

Rule to remember

Process rule: dissolve impure solid, filter insoluble impurities, concentrate solution, cool slowly, collect crystals.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

To purify impure salt containing insoluble dust, dissolve it in water, filter out dust, heat the filtrate until concentrated, cool it, and collect the formed crystals.

Method you can reuse

1Make a solution of the impure solid.
2Filter if insoluble impurities are present.
3Heat gently to concentrate.
4Stop heating before complete dryness.
5Cool slowly.
6Separate and dry crystals.

Sketch it in your answer

Draw an evaporating dish over gentle heat followed by a labelled cooled solution containing crystals at the bottom.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is crystallization preferred over simple evaporation for obtaining pure crystals?

Think first, then show answer

Crystallization is preferred because crystals separate in a more ordered and purer form, while simple evaporation may leave soluble impurities mixed with the solid residue.

Common wrong answer

Heating till all water is gone is not the best answer for crystallization because soluble impurities may remain with the residue.

Don't mix this up

Students sometimes choose evaporation instead of crystallization even when the solid may decompose or impurities may remain mixed.

Exam tip

Choose crystallization when the question asks for a purer soluble solid from its solution.