Acid Base & Salt - Class 10th Science

Water of Crystallisation

Are the crystals of salts really dry?

Crystals of salts appear dry. But many salts contain some water molecule. This water molecules contain by crystals of slats is known as water of crystallization. Salts that contain water molecules are known as hydrated salts. For example: copper sulphate, gyspsum, sodium carbonate, etc.

Example: (1)

Sodium Carbonate (Washing soda):

Sodium carbonate which is also known as washing soda, contains 10 (ten) water molecules, because of this sodium carbonate is called sodium carbonate decahydrate. Sodium carbonate which does not contains any water molecule is called anhydrous sodium carbonate. Anhydrous sodium carbonate is also known as soda ash.

The chemical formula of sodium carbonate is Na₂CO₃ . 10 H₂O

Sometimes, crystals of sodium carbonate contain seven water molecules. In such case chemical formula of sodium carbonate is written as Na₂CO₃ . 7 H₂O and it is called sodium carbonate heptahydrate.

Example: (2)

Copper Sulphate

Crystals of copper sulphate appears blue because it contains 5(five) water molecules. This is the cause that chemical formula of copper sulphate is CuSO₄ . 5 H₂O.

When crystals of copper sulphate is heated, it loses its water molecule and its colour and becomes grey white.

But when water is added to anhydrous copper sulphate, or it exposes to moisture present in air, it turns blue again.

Example: (3)

Ferrous Sulphate:

Crystals of ferrous sulphate appears bluish green in colour. This colour in ferrous sulphate is due to the presence of water molecule in it. Crystals of ferrous sulphate contains 7(seven) water molecules. Because of it the chemical formula of ferrous sulphate is FeSO₄ . 7 H₂. Since ferrous sulphate contains seven (7) water molecules, thus it is also called ferrous sulphate heptahydrate.

Example: (4)

Gypsum:

The chemical formula of Gypsum CaSO₄ . 2 H₂. The chemical name of gypsum is calcium sulphate dehydrate as it contains two molecules of water. Gypsum is also a hydrated salt. It contains two molecules of water.

Plaster of Paris

Plaster of paris is also known as gypsum plaster. The chemical formula of plaster of paris is CaSO₄ . ½ H₂O. This got its name ‘Plaster of Paris’ because a large gypsum deposit at Montmartre in Paris.

Conversion of Plaster of Paris from gypsum:

Plaster of Paris is obtained by heating of gypsum at about 150^{0C. At this temperature 1.5 molecules of water is evaporated and gypsum turns into plaster of Paris.}

Rehydration of Plaster of Paris:

When plaster of Paris is rehydrated by mixing of water, it turns into gypsum and when this gypsum left open, it starts drying after about 10 minutes and in about 45 minutes it sets as solid. In about 72 hours it dried completely.

The unique property of quick drying and setting into solid after some hours makes plaster of Paris very useful.

Properties of Plaster of Paris:

Plaster of Paris is a white powder.

Mixing of water into the powder of Plaster of Paris is exothermic.

Some Uses of Plaster of Paris:

• Plaster of Paris is used in medical treatment. It is used to cover as plaster in the case of fractured bones. Mounding of plaster of Paris over the part of body after setting of bones ensures to keep bones at right place.
• Plaster of Paris is used for making beautiful and designer false ceiling and other room interiors.
• Plaster of Paris is used in casting into clay molds.
• Plaster of Paris is used in making toys, and other decorative materials.

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