Question

What happens when the plaster of Paris is heated?

Answer 1

Hint: The chemical name of plaster of paris is calcium sulphate hemihydrate and its molecular formula is $ CaS{O_4}.\dfrac{1}{2}{H_2}O $ . It is a white powdery chemical compound which is obtained by heating gypsum at 393 K. Therefore, it is also referred to as gypsum plaster.

Complete answer:
The chemical formula of gypsum is $ CaS{O_4}.2{H_2}O $ and it is chemically known as calcium sulphate dihydrate because of the presence of two moles of water of crystallization. When gypsum is heated at 393 K, then a dehydration reaction takes place and plaster of paris is formed along with removal of water molecules. The reaction is as follows:
 $ 2CaS{O_4}.2{H_2}O\xrightarrow{{393{\text{K}}}}CaS{O_4}.\dfrac{1}{2}{H_2}O + 3{H_2}O $
When plaster of paris, i.e., calcium sulphate hemihydrate is further heated over 393 K, then its water of crystallization is lost and formation of anhydrous calcium sulphate takes place which is also known as dead burnt plaster. The reaction takes place as follows:
 $ CaS{O_4}.\dfrac{1}{2}{H_2}O\xrightarrow{{473{\text{K}}}}CaS{O_4} $
Anhydrous calcium sulphate on further heating to 1373 K, converts into calcium oxide and removal of sulphur trioxide takes place. The reaction is as follows:
 $ CaS{O_4}\xrightarrow{{1373{\text{K}}}}CaO + S{O_3} $
Hence, when plaster of paris is heated the formation of anhydrous calcium sulphate takes place.

Note:
It is important to note that the name dead burnt plaster is given to anhydrous calcium sulphate because unlike plaster of paris, it does not set back to its hydrate form when moistened with water. Therefore, dead burnt plaster is considered as a more superior form of plaster than normal gypsum plaster. It is widely used in places of high traffic and friction.