Question

How is Plaster of Paris obtained? What reactions are involved in the setting the paste of Plaster of Paris?

Answer 1

Plaster of Paris is chemically different from gypsum in terms of water of crystallisation. Gypsum has 2 moles of water for every mole of $CaS{O}_4$.Whereas in Plaster of Paris we have one mole of water of crystallisation for two moles of $CaS{O}_4,(2CaS{O}_4.H_{2}O)$. Gypsum when heated at a temperature of 373 K gets converted into Plaster of Paris.

${CaSO}_4$$.2H_{2}O$ $\to$ ${CaSO}_4$$.\frac{1}{2}{H}_{2}O$ + $\frac{3}{2}{H}_{2}O$

Gypsum Plaster or Paris

When Plaster of Paris is mixed with water, it gets converted to gypsum.

${CaSO}_4.\frac{1}{2}$ $H_{2}O$ + $\frac{3}{2}{H}_{2}O$ $\to$ ${CaSO}_4$$.2H_{2}O$

Plaster of Paris Gypsum

Plaster of Paris is used for making statues and for the setting of fractured bones.