Question

A student requires hard water for an experiment in his laboratory which is not available in the neighbouring area. In the laboratory there is hard water. Select from the following groups of salts, a group, and each salt of which when dissolved in distilled water will make it hard.
A. sodium chloride, potassium chloride
B. sodium sulphate, potassium sulphate
C. sodium sulphate, calcium sulphate
D. calcium sulphate, calcium chloride

Answer 1

Hint: On the basis of the concentration of ions, particularly calcium and magnesium, the water is divided into two parts: Hard water and soft water.
Hard water: the water contains a high amount of dissolved salts.
Soft water: the water contains a low amount of dissolved salts.

Complete step by step answer:
Hard water contains a high amount of minerals. The main salts present in hard water are carbonates and bicarbonates of alkaline earth metals.
Hard water has a multivalent cation. The commonly present cations are calcium ${\text{C}}{{\text{a}}^{{\text{2 + }}}}$ and magnesium ${\text{M}}{{\text{g}}^{{\text{2 + }}}}$. The ions enter into the water for leaching. The ions are mixed into the water in the form of their minerals.
The common minerals of calcium are gypsum and calcite and the mineral of magnesium is dolomite.
The sodium chloride and potassium chloride do not contain multivalent cation, so they do not cause hardness, so option (A) is incorrect.
The sodium sulphate and potassium sulphate do not contain multivalent cation, so they do not cause hardness, so option (B) is incorrect.
The sodium sulphate does not contain multivalent cation however calcium sulphate contains so option (C) is incorrect.
The calcium sulphate and calcium chloride both contain multivalent cation, so they cause hardness in water, so option (D) is correct.

Therefore, option (D) calcium sulphate, calcium chloride, is correct.

Note: Soft water contains monovalent cations such as sodium chloride and potassium chloride whereas hard water contains multivalent cations. The hard water can be converted into soft water by the ion exchange method.