Water of crystallisation is a fixed number of water molecular present in one formulas unit of a salt. One formula unit of copper sulphate contains five water molecules (5H2O). The water molecules which form part of the structure of a crystal are called water of crystallisation. When hydrated salts are heated strongly, they lose their water of crystallisation.
On strong heating, blue copper sulphate turn white (due to the loss of water of crystallisation).
. 𝐶𝑢𝑆𝑂4. 5𝐻2𝑂 {\overset{heat}{\longrightarrow}} 𝐶𝑢𝑆𝑂4 + 5𝐻2𝑂
𝐻𝑦𝑑𝑟𝑎𝑡𝑒𝑑 𝑐𝑜𝑝𝑝𝑒𝑟 𝑠𝑢𝑙𝑝ℎ𝑎𝑡𝑒 𝐴𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 𝑐𝑜𝑝𝑝𝑒𝑟 𝑠𝑢𝑙𝑝ℎ𝑎𝑡𝑒 𝑊𝑎𝑡𝑒𝑟
. (𝐵𝑙𝑢𝑒) (𝑊ℎ𝑖𝑡𝑒) (𝐺𝑜𝑒𝑠 𝑎𝑤𝑎𝑦)
Anhydrous copper sulphate turns blue on adding water.
. 𝐶𝑢𝑆𝑂4 + 5𝐻2𝑂 → 𝐶𝑢𝑆𝑂4. 5𝐻2𝑂
𝐴𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 𝑐𝑜𝑝𝑝𝑒𝑟 𝑠𝑢𝑙𝑝ℎ𝑎𝑡𝑒 𝑊𝑎𝑡𝑒𝑟 𝐻𝑦𝑑𝑟𝑎𝑡𝑒𝑑 𝑐𝑜𝑝𝑝𝑒𝑟 𝑠𝑢𝑙𝑝ℎ𝑎𝑡𝑒
. (𝑊ℎ𝑖𝑡𝑒) (𝐵𝑙𝑢𝑒)
