Define Crystallisation

Pollution of air and water of Class 8

Define Crystallisation:

When a saturated solid-liquid solution is kept, solid-solute settles down with a highly regular arrangement of its constituent particles (atoms, molecules or ions). This regular arrangement of component particles in a three-dimensional system is uniform throughout the entire solid. This type of solid is called crystalline solid. The smallest unit of this arrangement of particles which gets repeated throughout the crystalline solid is called unit cell. Unit cell of each substance has a definite geometric shape.


Each crystal consists of a set of three axes in a particular geometrical arrangement. There are seven unique crystal systems depending on the orientation of the particles along the three axes.

Types of Crystal Systems Examples
Cubic NaCl, CsCl
Hexagonal Graphite, ZnO
Tetragonal SnO2, TiO2
Rhombohedral CaCo3, HgS
Orthorhombic Rhombic sulphur
Monoclinic Monoclinic sulphur, PbCrO2
Triclinic K2Cr2O7, CuSO4.5H2O


When a solid gets crystallized from its respective hot concentrated aqueous solution, a certain fixed number of water molecules also get attaché to the solid crystals to from unit cells of the crystals. The fixed number of water molecules which combine with a crystal and are necessary for the maintenance of crystalline properties, but capable of being lost either at normal temperature or at a higher temperature is called Water of Crystallization

Example of such solids (salts) are green vitriol (FeSO47H2O), blue vitriol (CuSO4 5H2O), washing soda (Na2CO3.10H2O) etc.


The salts which contain water of crystallization are called hydrated salts. When the hydrated salts completely lose their water molecules, it is called anhydrous salt. When as hydrated salt gets dehydrated and forms anhydrous salt, the colour of the salt changes. For example CuSO4.5H2O is blue in colour. It changes to white colour due to the loss of water molecules on heating.

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