Question

What happens when hydrated copper sulphate is heated?
(A) It becomes colourless
(B)It turns into anhydrous salt
(C) It loses water of crystallization
(D) All the above

Answer 1

Hint: The chemical formula of hydrated copper sulphate is $CuS{O}_4.5H_{2}O$. It contains five molecules of water of crystallization. It appears as blue coloured crystals.

Complete answer:
Hydrated copper sulphate, i.e. $CuS{O}_4.5H_{2}O$ is copper sulphate pentahydrate. It has five water molecules of crystallization (or water of hydration) which means that five water molecules are present in crystals of copper sulphate.
- When copper sulphate pentahydrate is heated, it loses water of crystallization as a result of evaporation.
- Losing water of crystallization turns hydrated copper sulphate into anhydrous copper sulphate salt.
- Hydrated copper sulphate is blue coloured crystals. On heating, blue coloured copper sulphate crystals become white. Heating turns crystals of $CuS{O}_4.5H_{2}O$ into anhydrous $CuS{O}_4$ crystals which are colourless.
 The chemical reaction occurring on heating $CuS{O}_4.5H_{2}O$ is given below:

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

All the given statements are correct.

So, the correct answer is “Option D”.

Additional Information:
 If heating continues, anhydrous copper sulphate undergoes decomposition to form copper (II) oxide (cupric oxide) which is black in colour. Oxygen and sulphur dioxide are also released. The chemical reaction for the decomposition of copper sulphate on heating is given below:

     $$2CuS{O}_4\to 2CuO+O_2+2S{O}_2$$

Note: Salts containing no water or crystallization are called anhydrous salts. Copper sulphate pentahydrate is a hydrated salt which on heating loses its five molecules of water of crystallization and converts into anhydrous copper sulphate.