Question

(a) The blue colour of crystals of a substance changed on heating in a closed test tube but the colour was regained after sometime on cooling. Name the substance and write its chemical formula. Explain the phenomenon involved.

(b) Write name and chemical formula of two such compounds whose one formula unit is associated with 10 and 2 water molecules respectively.

Answer 1

The substance is Copper sulphate crystals $\left(CuS{O}_4.5H_{2}O\right)$, which are blue in colour. When copper sulphate crystals are heated strongly, they lose all the water of crystallisation and forms anhydrous copper sulphate (which is white) :
$\begin{array}{cccc}& & & \\ CuS{O}_4.5H_{2}O& \stackrel{Heat}{\to }& CuS{O}_4& +5H_{2}O\\ \text{Hydrated}& & \text{Anhydrous}& \text{Water}\\ \text{Copper}& & \text{Copper}& \text{(goes}\\ \text{Sulphate (Blue)}& & \text{Sulphate (White)}& \text{away)}\end{array}$

Thus, on strong heating, blue copper sulphate crystals turn white due to the loss of water of crystallisation.

The dehydration of copper sulphate crystals is a reversible process. So when water is added to anhydrous copper sulphate, it gets hydrated and turns blue due to the formation of hydrated copper sulphate.

$\begin{array}{cccc}& & & \\ CuS{O}_4+& 5H_{2}O& ⟶& CuS{O}_4.5H_{2}O\\ \text{Anhydrous}& \text{Water}& & \text{Hydrated}\\ \text{Copper}& & & \text{Copper}\\ \text{Sulphate}& & & \text{Sulphate}\\ \text{(White)}& & & \text{(Blue)}\end{array}$

(b) Chemical Formula of two such compounds whose one formula unit is associated with 10 and 2 water molecules respectively-

• Washing soda- $N{a}_{2}C{O}_3.10H_{2}O$ (Sodium Carbonate Decahydrate)

• Gypsum - $CaS{O}_4.2H_{2}O$ (Calcium Sulphate Dihydrate)