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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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

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.

Memory hook

Crystals need concentration plus cooling, not careless drying.

Examples and method

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 to apply

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

Diagram support

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

How CBSE asks it

Exam questions ask students to name the method for obtaining pure solid crystals and arrange the steps correctly.

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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.

Study the separation by crystallization diagram carefully

Use the labelled diagram to keep separation by crystallization clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of separation by crystallization in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on separation by crystallization.

Revision cue

Revise separation by crystallization through the labels before writing the answer.

Answer writing and exam use

1-mark answer

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

2-mark answer

Crystallization is a separation method in which pure solid crystals are obtained from a solution by concentrating it and then cooling it. Process rule: dissolve impure solid, filter insoluble impurities, concentrate solution, cool slowly, collect crystals. Impure copper sulphate can be purified by dissolving it in water, filtering insoluble impurities, concentrating the solution, and cooling it to form crystals.

3-mark answer

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. Process rule: dissolve impure solid, filter insoluble impurities, concentrate solution, cool slowly, collect crystals. 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. Exam questions ask students to name the method for obtaining pure solid crystals and arrange the steps correctly. Heating till all water is gone is not the best answer for crystallization because soluble impurities may remain with the residue.
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