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SAI INTERNATIONAL SCHOOL
CLASS-VII
SUBJECT- CHEMISTRY
TOPIC-PHYSICAL AND CHEMICAL CHANGES
SUB TOPIC- CRYSTALLISATION

Lesson Note
CRYSTALLISATION

What is a crystal?

The purest form of a solid having definite geometrical shape is called crystal.

Sugar, salt, alum etc are all made of crystals.

Crystallisation:

The process in which crystal of pure substances are obtained from their solution is
called crystallisation.

It is a physical change as no new substance is formed in this process.

To understand crystallisation we must know what is saturation and supersaturation.

SOLUTION:

We know that when a solid substance is dissolved in a liquid, it forms a solution.

The substance which dissolves in a solvent is called solute and the substance in
which the solute is dissolved is called solution.

Water can dissolve a number of substances. That is why it is called universal
solvent.

Solution are of three types.

Unsaturated solution- The solution formed, where more solute can be dissolved in
the solvent is called an unsaturated solution.

Saturated solution: The solution where no more solute can be added at a particular
temperature is called a saturated solution.

Supersaturated solution: In a saturated solution, if we add more solute the solution
becomes to supersaturated solution.

For crystallisation we need to prepare supersaturated solution.

Let us see the following activity.

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