Salts are ionic compounds formed when acids and bases react with each other. These salts are found in crystal form.
Normally crystals of salts look dry but they contain
water in them. When crystals of certain salts are formed, they combine with a certain
number of water molecules which chemically combined in a definite molecular proportion with the
salt in its crystalline state. This water is called crystalline water.
A substance containing water of crystallisation is called
a hydrous substance (hydrate)
What is a water of crystallisation?
A fixed or definite number of Water molecules present in one formula unit of salt is called water of
crystallisation.
Examples of salts with water of crystallisation
| Salt | Formula of salt | Chemical name of salt |
|---|---|---|
| Gypsum | `CaSO_4. 2H_2O` | Calcium sulphate |
| Saltpetre | `CaCl_2. 6H_2O` | Calcium chloride |
| Dow flake | `BaCl_2. 2H_2O` | Barium chloride |
| Glauber’s salt | `Na_2SO_4. 10H_2O` | Sodium sulphate |
| Washing soda | `Na_2CO_3. 10H_2O` | Sodium carbonate |
| Potash alum | `K_2SO_4. Al_2(SO_4)_3. 24H_2O` | Potassium aluminium sulphate |
| Epsom salt | `MgSO_4. 7H_2O` | Magnesium sulphate |
| White vitriol | `ZnSO_4. 7H_2O` | Zinc sulphate |
| Green vitriol | `FeSO_4. 7H_2O` | Ferrous sulphate |
| Blue vitriol | `CuSO_4. 5H_2O` | Copper sulphate |
| Plaster of Paris | `CaSO_4. frac{1}{2}H_2O` | Calcium sulphate hemi hydrate |
How do we know that a salt crystal contains water of crystallisation?
Ans. – Water of crystallisation is responsible for the shape and colour of the salt crystal.
When the salt is heated, the water of crystallisation
gets evaporate and the colour of the salt gets changed so we can say salt contains
water of crystallisation
How would you show that blue copper sulphate crystals contain water of crystallisation?
Ans. – When copper sulphate crystals are heated, the
blue colour of crystals disappears.
But when water is added again to copper sulphate, the
blue colour appears again. So, we can say that copper sulphate crystals contain water of crystallisation.
FAQ
1. When salt water evaporates what happens to the salt
Ans. – When salt water evaporates, the water changes
from liquid to a gaseous state and salt remain behind in a solid state.
So the process of evaporation separates the water from
the salt.
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