Question

A white chemical compound becomes hard on mixing with the proper quantity of water. It is also used to maintain joints in a fixed position. Name the chemical compound and write its chemical formula and also write a chemical equation to show what happens when water is added to this compound in proper quantity.

Answer 1

1. The water of crystallization is the number of water molecules that are found in the crystalline framework of a metal complex or salts, which is not directly bonded with the metal cation.
2. Plaster of Paris is the compound that becomes hard on mixing with water and is also used to maintain joints in a fixed position.
3. The chemical name of Plaster of Paris is calcium sulphate hemihydrate and the chemical formula is ${\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$

Chemical reaction when it reacts with water:

$\underset{\left(\mathrm{Plaster}\mathrm{of}\mathrm{Paris}\left(\mathrm{POP}\right)\right)}{{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}(\mathrm{s}})+\underset{\left(\mathrm{Water}\right)}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\left(\mathrm{Gypsum}\right)}{{\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$

When the Plaster of Paris reacts with water, water of crystallization happens and it formed Gypsum (CaSO₄.2H₂O) which is quite hard.

Therefore, the compound is plaster of Paris which becomes hard and is also used to fix joints.