NCERT Exemplar Solutions
Class 10 - Science - CHAPTER 2: Acids, Bases and Salts - Long Answer Questions
Question 47

Question. 47

47. A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why does it show such a behaviour? Give the reaction involved.

Answer:

The salt is Plaster of Paris (calcium sulphate hemihydrate, \(\mathrm{CaSO_4\cdot\tfrac{1}{2}H_2O}\)). On absorbing moisture it converts to gypsum (calcium sulphate dihydrate, \(\mathrm{CaSO_4\cdot2H_2O}\)) which is hard. Reaction: \(\mathrm{CaSO_4\cdot\tfrac{1}{2}H_2O + \tfrac{3}{2}H_2O \to CaSO_4\cdot2H_2O}\) or written as \(\mathrm{CaSO_4\cdot\tfrac{1}{2}H_2O + H_2O \to CaSO_4\cdot2H_2O}\) in stoichiometric form.

Detailed Answer with Explanation:

Steps: Plaster of Paris contains less water and is soft when mixed to a paste; exposure to moisture converts it to the dihydrate (gypsum) forming a hard mass (setting reaction used in casts).

Open NCERT PDF