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Salts > Water in Salt Crystals

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Definition: Plaster of Paris
Activity
Chemical Equation: Plaster of Paris
Key Points: Water in Salt Crystals

CBSE: Class 10

Definition: Plaster of Paris

On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO₄ . \[\frac {1}{2}\] . H₂O). This is called Plaster of Paris.

CBSE: Class 10

Chemical Equation: Plaster of Paris

\[\mathrm{CaSO_{4}\cdot\frac{1}{2}H_{2}O+1\frac{1}{2}H_{2}O\rightarrow CaSO_{4}\cdot2H_{2}O}\]
(Plaster of Paris) (Gypsum)

CBSE: Class 10

Key Points: Water in Salt Crystals

Many salts like copper sulphate (CuSO₄·5H₂O) appear dry but actually contain water of crystallisation, which is released on heating and restores the salt’s original colour upon rehydration.
Water of crystallisation refers to the fixed number of water molecules present in a formula unit of a salt (e.g., 5 in copper sulphate, 10 in washing soda, 2 in gypsum).
Plaster of Paris (CaSO₄·½H₂O) is formed by heating gypsum and, when mixed with water, reforms into gypsum (CaSO₄·2H₂O); it is used in medical casts, toys, and decorative items.