Question

why does it show such a behaviour?

Answer 1

Hey Ayushi ,
Here is your answer..
Calcium belongs to group 2 of periodic table.

The Sulphate salt of Group 2 element of the periodic table which is white , soft and can be moulded into dough of different shapes is Calcium sulphate( Plaster of Paris).

When Plaster of Paris is left for some time in open, it turns into solid mass because of reaction with moisture present in the atmosphere. The solid mass so formed is known as gypsum and cannot be further used for moulding.

CaSO4.0.5H2O + 1.5H2O ⇨ CaSO4.2H2O

Plaster of Paris shows such behavior because of presence half water molecule as water of crystallization.
I hope it helps!!!