Question

Heated calcium sulphate with half a water of crystallization is commonly called as:
(A) Washing soda
(B) Plaster of Paris
(C) Nausadar salt
(D) Bleaching powder

Answer 1

Hint: Heated calcium sulphate forms two types of compounds on hydration. The compound with half a water of crystallization is also known as bassanite. The compound which has water of crystalline two is known as gypsum.

Complete step by step answer:
> Calcium sulphate is an inorganic molecule, with chemical formula \[CaS{{O}_{4}}\]. It is anhydrous in nature and therefore, has a tendency to form hydrates, which is known as ‘water of crystallization’. It refers to the water molecules that are incorporated in the formation of crystals from aqueous solutions. These molecules do not react with the main component, it is just present inside crystals physically. Hence, it is vulnerable to heat.
> Calcium sulphate commonly forms the following hydrates:
\[\text{(i) }CaS{{O}_{4}}.2{{H}_{2}}O\]
This compound is known as Gypsum. It has two water molecules per calcium sulphate and hence is known as a dihydrate.
\[\text{(ii) }CaS{{O}_{4}}.\dfrac{1}{2}{{H}_{2}}O\]
This compound is known as bassanite, or more commonly – Plaster of Paris. It has half water molecule per calcium sulphate and hence is known as a hemihydrate.
>Let us discuss the other options given to us in the question.
- Washing soda – It is the common name for sodium bicarbonate
- Nausadar salt – It is the common name for ammonium chloride
- Bleaching powder – It is the common name for calcium hypochlorite
Therefore, the answer is – option (b) – Plaster of Paris

Additional Information:
A crystal salt containing water of crystallization is known as a hydrate. Hence, water of crystallization is also known as water of hydration.

Note: Water of crystallization is defined as, “water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation”.