What is Plaster of Paris? Write a short note on it.

Explanation

Gypsum of heating to a temp. of `120^(@) - 130^(@)`C gives a semi hydrate. It is known as Plaster of Paris.
`underset("Gypsum")(CaSO_(4).2H_(2)O) rarr underset("Plaster of Paris")(CaSO_(4).1/2 H_(2)O)`
When a water paste of plaster of paris is allowed to stand for sometime it sets to hard mass. This is called setting of Plaster of Paris. This is an exothermic process and involves tow stages. In the first stage, plaster of paris is converted into orthorhombic dihydrate. In the second stage, the orthorhombic dihydrate is converted into monoclinic dihydrate.
`underset("Plaster of Paris")(CaSO_(4) cdot 1/2 H_(2)O) to underset("Orthorhomibic dihydrate")(CaSO_(4).2H_(2)O) to underset("Monoclinic dihydrate(Gypsum)")(CaSO_(4).2H_(2)O)`
It is used (1) in making casts for statues, toys, etc. (2) in surgical bandages for bone fructure (3) in making white chalks.