Question

What happens when plaster of paris is heated ?

Answer 1

1. Plaster of Paris is Calcium sulphate hemihydrate and the molecular formula is ${\mathrm{CaSO}}_{4.}\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$.
2. It is also referred to as gypsum plaster because it is obtained from gypsum and is a white colored compound.
3. When we heat gypsum, a dehydration reaction takes place and plaster of Paris is obtained as the product.
4. Dehydration reaction is a reaction in which loss of water occurs from a molecule.
   $$\begin{gathered} 2{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{}{\stackrel{∆}{\to }\mathrm{CaSO}4}.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Calcium}\mathrm{sulphate}\mathrm{dihydrate} \\ \left(\mathrm{Gypsum}\right)(\mathrm{Plaster}\mathrm{of}\mathrm{paris}) \end{gathered}$$
5. When this plaster of Paris is heated, we get anhydrous Calcium sulphate also known as dead burnt plaster.
   $$\begin{gathered} {\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{}{\stackrel{∆}{\to }{\mathrm{CaSO}}_4}\left(\mathrm{s}\right) \\ \mathrm{Calcium}\mathrm{sulphate} \end{gathered}$$