Question

Show the decomposition of gypsum to P.O.P and back interconversion of P.O.P to gypsum.

Answer 1

We should know the chemical formula of gypsum and P.O.P to answer this question. The gypsum is converted into P.O.P by heating it, so we should also know the heating temperature. P.O.P is back converted into gypsum on reacting with water.

Complete step by step answer:
Calcium sulphate dihydrate is known as gypsum. The chemical formula of gypsum is ${\text{CaS}}{{\text{O}}_{\text{4}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}\,$. When gypsum is heated at low temperature, some amount of water escapes from the calcium sulphate to give P.O.P. the chemical formula of P.O.P is ${\text{CaS}}{{\text{O}}_{\text{4}}}\,.\frac{1}{2}{{\text{H}}_{\text{2}}}{\text{O}}$.
When calcium sulphate dihydrate or gypsum is heated at ${\text{120}}{\,^{\text{o}}}{\text{C}}$ , some amount of water escapes from the calcium sulphate. The calcium sulphate having half water molecule $\left( {{\text{CaS}}{{\text{O}}_{\text{4}}}\,.\frac{1}{2}{{\text{H}}_{\text{2}}}{\text{O}}} \right)$is known as Plaster of Paris or P.O.P.
The reaction for the decomposition of gypsum to P.O.P is as follows:
${\text{CaS}}{{\text{O}}_{\text{4}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}\,\mathop \to \limits^{{\text{120}}{\,^{\text{o}}}{\text{C}}} \,{\text{CaS}}{{\text{O}}_{\text{4}}}\,.\frac{1}{2}{{\text{H}}_{\text{2}}}{\text{O}}\,\, + \,\,\frac{3}{2}{{\text{H}}_{\text{2}}}{\text{O}}$
When the plaster of Paris is reacted with water it forms gypsum again. The reaction for the back interconversion of P.O.P to gypsum is as follows:
${\text{CaS}}{{\text{O}}_{\text{4}}}\,.\frac{1}{2}{{\text{H}}_{\text{2}}}{\text{O}} + \frac{3}{2}{{\text{H}}_{\text{2}}}{\text{O}}\, \to {\text{CaS}}{{\text{O}}_{\text{4}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}}\,$

So, the decomposition of gypsum occur at ${\text{120}}{\,^{\text{o}}}{\text{C}}$ to give P.O.P and back interconversion of P.O.P to gypsum is take place by treating the P.O.P with water.

Note: The temperature of the heating is important. At low temperature, some amount of water remains in compound calcium sulphate which is known as Plaster of Paris whereas at high temperature (at ${\text{200}}{\,^{\text{o}}}{\text{C}}$) complete water escapes from calcium sulphate dihydrate to give dead burnt plaster$\left( {{\text{CaS}}{{\text{O}}_{\text{4}}}} \right)$. When anhydrous calcium sulphate is strongly heated it converts into oxide of calcium known as lime$\left( {{\text{CaO}}} \right)$. The temperature of heating is important because heating gypsum at different temperatures leads to the formation of different products.