CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 6(b)

Acids, Bases and Salts 3 marks Medium

A sulphate salt of group two element of the periodic table is white, soft substance, which can be moulded into different shapes by making it dough. When this compound is left in open for sometime, it becomes a solid object and cannot be used for moulding purposes. Identify this sulphate salt and explain why does it show such a behaviour. Also give the equation for the reaction involved.

Answer

The salt is Plaster of Paris, $CaSO_{4} \cdot \frac{1}{2} H_{2} O$ , which takes up water and sets as gypsum.

The salt is Plaster of Paris, calcium sulphate hemihydrate,

CaSO_{4} \cdot \frac{1}{2} H_{2} O

(calcium is a group 2 element). Reason: Plaster of Paris has only half a molecule of water of crystallisation per formula unit. Mixed with water it forms a dough that can be moulded, but on standing, or when left in the open where it takes up moisture from the air, it combines with water and changes into gypsum, a hard solid mass. So it sets and can no longer be used for moulding. Equation:

CaSO_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O \to CaSO_{4} \cdot 2 H_{2} O

(gypsum)

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CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 6(b)

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