Question

Write the chemical names and formulae of the plaster of paris and gypsum?

Answer 1

Hint: In this question first you can try to recall the chemical formulae and then assign their name, it would be easier. Their formulae don’t have much difference, they just differ by one and a half water molecules from each other.

Complete step by step answer:

Plaster of Paris -
The chemical name of plaster of paris is Calcium sulphate hemihydrate.
The chemical formula of Plaster of paris = $CaSO_{ 4 }.1/2H_{ 2 }O$
The chemical name of the plaster of Paris is calcium sulfate hemihydrate because half-molecule of water is attached with calcium sulphate.

Advantages:
1. It is light in weight and more durable.
2. It has low thermal conductivity.
3. It is very good fire resistant and hence a very good heat-insulating material.
4. It is not likely to cause cracking of surfaces.
5. It has no appreciable chemical action on paint and does not cause an alkali attack.
The plaster of Paris gives a decorative interior finish.

Gypsum - The chemical name of plaster of paris is calcium sulphate dihydrate. Gypsum is a crystalline mineral of hydrated calcium sulphate (chemical formula $CaSO_{ 4 }.2H_{ 2 }O$).

Gypsum is colourless or white, is not highly water-soluble and is not at all hard.

A mixture of gypsum and water can be poured; the gypsum hardens as the water evaporates.
In art, gypsum is mainly used in the partly dehydrated form of plaster of Paris ($2CaSO_{ 4 }.H_{ 2 }O$) to make casts of objects or works of art in sculpture (moulds).

Note: When plaster of paris is mixed with water, crystals of gypsum are produced and set into a hard mass, which has a vast number of applications.
$CaSO_{ 4 }.1/2H_{ 2 }O\quad +\quad 3/2H_{ 2 }O\quad \rightarrow \quad CaSO_{ 2 }.2H_{ 2 }O$