Plaster of paris, CaSO4.1/2 H2O or (CaSO4)2.H2O

It occurs in nature as gypsum and the anhydrous salt as anhydride. It is prepared by precipitating a solution of calcium chloride or nitrate with dilute sulphuric acid.

The effect of heat on gypsum or the dihydrate presents a review of interesting changes. On heating the monoclinic gypsum is first converted into orthorhombic form without loss of water. When the temperature reaches 120°C, the hemihydrate or plaster of paris is the product. The latter losses water, becomes anhydrous above 200°C and finally above 400°C, it decomposes into calcium oxide.

2CaSO4 ——→ 2CaO + 2SO2­↑ + O2­↑


The following conditions are necessary 

(i) The temperature should not be allowed to rise above 393 K because above this temperature the whole of water of crystallization is lost. The resulting anhydrous CaSO4 is called dead burnt plaster because it does not have the properties of setting with water.

(ii) The gypsum should not be allowed to come in contact with carbon containing fuel otherwise some of it will be reduced to calcium sulphite.


It is a white powder. On mixing with 1/3rd its weight of water, it forms a plastic mass which sets into a hard mass of interlocking crystals of gypsum within 5 to 15 minutes. It is due to this reason that it is called plaster. The addition of common salt accelerates the rate of setting, while a little borax or alum reduces it. The setting of plaster of paris is believed to be due to rehydration and its reconversion into gypsum.

2CaSO4. 1/2 H2O + 3H2O ——→ 2CaSO4. 2H2O
plater of paris                         gypsum

(i)  Plaster of pairs is used for producing moulds for pottery and ceramics & casts of statues & busts.

(ii) It is used in surgical bandages used for plastering broken or fractured bones.

(iii) It is also used in dentistry.

Industrial uses of lime and Limestone

Uses of lime

Calcium oxide is called lime or quick lime. It main industrial uses are

(i)   It is used in steel industry to remove phosphates and silicates as slag.

(ii)  It is used to make cement by mixing it with silica, alumina or clay.

(iii) It is used in making glass.

(iv) It is used in lime soda process for the conversion of Na2CO3 to NaOH & vice versa.

(v)  It is used for softening water, for making slaked lime Ca(OH)­2 by treatment with water and calcium carbide CaC2.

Uses of Slaked lime [Ca(OH)2]

(i) Slaked lime is used as a building material in form of mortar. It is prepared by mixing 3 – 4 times its weight of sand and by gradual addition of water. Its sets into a hard mass by loss of H2O and gradual absorption of CO2 from air.

(ii) In manufacture of bleaching powder by passing Cl2 gas.

(iii) In making glass and in the purification of sugar and coal gas.
(iv) It is used in softening of hard water.

Uses of lime stone (CaCO3)

(i)  It is used as building material in form of marble.

(ii) In manufacture of quick lime.

(iii) It is used as a raw material for the manufacture of Na2Co3 in solvay – ammonia process.

(iv) Commercial limestone contains iron oxide, alumina, magnesia, silica & sulphur with a CaO content of 22 – 56% MgO content upto 21%. It is used as such as a fertilizer.


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