Question:

In the neighbourhood of your school, hard water required for an experiment is not available. Select from the following groups of salts available in your school, a group each member of which, if dissolved in distilled water, will make it hard: sodium chloride, calcium chloride; potassium chloride, sodium chloride; sodium chloride, magnesium chloride; calcium chloride, magnesium chloride.

sodium chloride, magnesium chloride

sodium chloride, calcium chloride

potassium chloride, sodium chloride

calcium chloride, magnesium chloride

Solution:

Water described as "hard" is high in dissolved minerals, specifically calcium and magnesium. Calcium and magnesium dissolved in water are the two most common minerals that make water "hard." Calcium chloride and magnesium chloride can be dissolved in distilled water to make it hard. Hence, calcium chloride and magnesium chloride when dissolved in water make it hard. Therefore, option D is correct.