Acids, Bases and Salts

Calcium sulphate with half a molecule of water per molecule of the salt (hemi-hydrate) is called plaster of paris (plaster of paris).

Preparation

plaster of paris is prepared by heating gypsum (CaSO₄.2H₂O) at 120°C in rotary kilns, where it gets partially dehydrated.

The temperature should be kept below 140°C otherwise further dehydration will take place and the setting property of the plaster will be partially reduced.

Properties

It is a white powder. When mixed with water (1/3 of its mass), it evolves heat and quickly sets to a hard porous mass within 5 to 15 minutes. During setting, a slight expansion (about 1%) in volume occurs so that it fills the mould completely and takes a sharp impression. The process of setting occurs as follows:

The first step is called the setting stage, and the second, the hardening stage. The setting of plaster of Paris is catalyzed by sodium chloride, while it is reduced by borax, or alum.

Uses

• In surgery for setting broken or fractured bones
• For making casts for statues, in dentistry, for surgical instruments, and toys etc
• In making black board chalks, and statues
• In construction industry