Page 4 - 3.LESSON NOTES-(CH-2 WASHING SODA& PLASTER OF PARIS)

5. Plaster of Paris (Common Name)

OR
• Chemical Name – Calcium Sulphate Hemi Hydrate
• Chemical Formula – CaSO4.½ H2O

Please Note that it is not possible to have half a molecule of water.

The Formula actually means that two molecules ( or two formula units of CaSO4)
share one molecule of water so that the effective water of crystallization for
one CaSO4 unit comes to half molecule of water

2 CaSO4 sharing one molecule of H2O in POP

Plaster of Paris stands for calcium sulphate hemihydrate which is obtained by
heating the mineral gyspum

Paris then had rich deposits of gyspum which was heated in large quantities to
manufacture plaster and make it strong enough to withstand weight of massive
buildings and hence the name Plaster of Paris .

 Preparation of Plaster of Paris-
• Plaster of Paris is prepared from Gypsum by heating it in a controlled
o
manner at a temperature is 373K/100 C,
• Chemical Equation for the preparation of POP:

CaSO4.2H2O ---heat→ CaSO4.½ H2O + 1½ H2O
Gypsum 373K/100oC Plaster of Paris

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