A student requires hard water for an experiment in his laboratory which is not available in the neighbouring area. In the laboratory there are some salts, which when dissolved in distilled water can convert it into hard water. Select from the following groups of salts, a group, each salt of which when dissolved in distilled water will make it hard.

Hardness of water is caused by the presence of soluble salts of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions, such as their chlorides, sulphates, and hydrogen carbonates. Distilled water is pure water and contains no dissolved minerals. To prepare hard water synthetically in the laboratory, one must dissolve salts that release $Ca^{2+}$ or $Mg^{2+}$ ions. Calcium sulphate ($CaSO_4$) and calcium chloride ($CaCl_2$) both contain calcium ions ($Ca^{2+}$). Therefore, dissolving them in distilled water will make it hard (Option D).

Why other options are incorrect:

• Option A: Sodium chloride ($NaCl$) and potassium chloride ($KCl$) contain sodium ($Na^+$) and potassium ($K^+$) ions, which do not cause hardness in water.
• Option B: Sodium sulphate ($Na_2SO_4$) and potassium sulphate ($K_2SO_4$) contain $Na^+$ and $K^+$ ions, which do not cause hardness.
• Option C: Although calcium sulphate ($CaSO_4$) causes hardness, sodium sulphate ($Na_2SO_4$) does not. Both salts in the group must make the water hard.