Question

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 1

• The substance which is used for making different shapes is Plaster of Paris. 
• Its chemical name is calcium sulphate hemihydrate (CaSO₄ . H₂O). 
• The two formula unit of CaSO₄ share one molecule of water. 
• As a result, it is soft. When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum, which is a hard solid mass.

\(CaSO_4.\frac{1}{2}H_2O+1\frac{1}{2}H_2O\rightarrow CaSO_4.2H_2O\)

• Plaster of paris has half molecule of water of crystallization.
•  When we leave plaster of paris open for some time, it absorbs moisture to gain number of molecules of crystallization. 
• This newly formed compound is called Gypsum which is hard to make moulds.