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.

Soln:

The sulphate salt should be calcium sulphate which is white and soft substance. Calcium sulphate is popularly known as plaster of paris.

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.