Question

(a) What is meant by "water of crystallisation" in a substance? Explain with an example.

(b) How would you show that blue copper sulphate crystals contain water of crystallisation?

(c) Explain how anhydrous copper sulphate can be used to detect the presence of moisture (water) in a liquid.

Answer 1

(a) The water molecules which form part of the structure of a crystal are called water of crystallization.

Example: $CuS{O}_4.5H_{2}O$

(b) When copper sulphate crystals (chemical formula $CuS{O}_4.5H_{2}O$) are heated strongly, they lose all the water of crystallisation and form anhydrous copper sulphate ($CuS{O}_4$), which is white. As the blue colour of the crystals disappeared on heating, we can conclude that the colour was due to the water of crystallisation. This proves that blue copper sulphate crystals contain water of crystallisation.

$CuS{O}_4.5H_{2}O\to CuS{O}_4+5H_{2}O$

(c) Anhydrous copper sulphate turns blue on adding water. This property of anhydrous copper sulphate is used to detect the presence of moisture in a liquid.