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Plaster of Paris

Chemistry Formulas

About Plaster of Paris, (Calcium sulphate hemihydrate) (2CaSO4).H2O

It is obtained when gypsum, is heated at 120 degree.

2[CaSO4.2H2O] →  (2CaSO4).H2O + 3H2O

 Gypsum Plaster of Paris.    (Calcium sulphate hemihdrate)

Properties of Plaster of Paris

(i) Plaster of Paris is a white powder.

(ii) Plaster of Paris has the property of setting to a hard mass when a paste with water is allowed to stand aside for some time. Slight expansion occurs during the setting as water is absorbed to reform CaSO4.2H2O (gypsum). The setting process is exothermic. The process of setting involves two stages. In the first stage, there is conversion of plaster of Paris into orthorhombic form of gypsum (setting step) and in the second stage orthorhombic form changes into monoclinic form (hardening step).

(2CaSO4).H2O  → CaSO4.2H2O→   CaSO4.2H2O

Plaster of Paris      Orthorhombic Monoclinic 

The setting of plaster of Paris is catalysed by sodium chloride and is retarded by borax or alum. for Chemistry Formulas check out the formulas page.

Find the Pdf of prepartion and properties of Plaster of Paris 

Plaster of Paris

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