Are the crystals of salts really dry explain with example?

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.