Question

Give reasons for the following:

On strong heating, blue-coloured copper sulphate crystals turn white.

Answer 1

• Hydrated Copper Sulphate (${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$) has blue coloured crystals. It has five water molecules of crystallization.
• Heating turns crystals into anhydrous crystals, which are colourless.
• On strong heating, blue Copper Sulphate crystals turn white due to the formation of anhydrous copper sulphate.
  $\underset{\mathrm{Copper}\mathrm{sulphate}\mathrm{crystals}\left(\mathrm{blue}\right)}{{\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\to \underset{\mathrm{Anhydrous}\mathrm{Copper}\mathrm{sulphate}\left(\mathrm{white}\right)}{{\mathrm{CuSO}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Steam}}{5{\mathrm{H}}_2\mathrm{O}\uparrow \left(\mathrm{g}\right)}$
• Also, when water is added to anhydrous copper sulphate, it gets hydrated and turns blue.
  $\underset{\mathrm{Anhydrous}\mathrm{copper}\mathrm{sulphate}\left(\mathrm{white}\right)}{{\mathrm{CuSO}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Hydrated}\mathrm{Copper}\mathrm{sulphate}\left(\mathrm{Blue}\right)}{{\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

Thus, on strong heating, blue-coloured copper sulphate crystals turn white.