Plaster of Paris, ( CaSO4 . ½ H2O)
Plaster of Paris is calcium sulphate hemihydrate, CaSO4 . ½ H2O .
It is prepared by heating gypsum to 393 K.
2 CaSO4 . 2 H2O ———> 2 CaSO4 . ½ H2O + 3 H2O
The following conditions are necessary:
1) The temperature should not be allowed to rise above 393 K because above this temperature, the whole of water of crystallisation is lost. The resulting anhydrous CaSO4 is called dead burnt plaster because it loses the properties of setting with water.
2 CaSO4 . ½ H2O ——-> 2 CaSO4 + H2O
2) The gypsum should not be allowed to come in contact with carbon containing fuel otherwise some of it will be reduced to calcium sulphite.
1) It is a white powder.
2) It has a remarkable property of setting with water. On mixing with one third its weight of water, it forms a plastic mass which sets into a hard mass of interlocking crystals of gypsum within 5 to 15 min. It is due to this reason that it is called plaster.
The setting of plaster of paris is believed to be due to rehydration and its reconversion into gypsum.
1) It used for producing moulds for pottery and ceramics and casts of statues and busts. It is also used in building industry as well as plasters.
2) It is used for making statues ,models and other decorative materials.
3) It is used in surgical bandages used for plastering broken or fractured bones of the body and for preparing blackboard Chalk.
4) It is used in dentistry.