Water is said to be temporarily hard when it contains

Analysis

The question asks for the specific chemical composition that makes water "temporarily hard." Hard water is classified into two types: temporary and permanent. Understanding the difference between the two is a common requirement for JAMB Chemistry.

Concept Explanation

• Temporary Hardness: This is caused by the presence of dissolved bicarbonates (also known as hydrogen carbonates) of calcium or magnesium. It is called "temporary" because it can be easily removed by boiling the water or by adding slaked lime (calcium hydroxide).
• Permanent Hardness: This is caused by the presence of chlorides or sulfates of calcium or magnesium. This type of hardness cannot be removed by boiling; it requires chemical treatment (like adding washing soda or using ion-exchange resins).

Step-by-Step Solution

1. Identify the cause of temporary hardness: As defined above, temporary hardness is strictly due to the presence of $Ca(HCO_3)_2$ (Calcium bicarbonate) and $Mg(HCO_3)_2$ (Magnesium bicarbonate).
2. Evaluate the options: • Option A: Contains both bicarbonates. This matches our definition. • Option B: Contains $CaCO_3$ (Calcium carbonate). Calcium carbonate is insoluble in water and does not contribute to hardness. • Option C: Contains $CaSO_4$ (Calcium sulfate). Sulfates cause permanent hardness, not temporary. • Option D: Contains $CaSO_4$. Again, sulfates cause permanent hardness.

Conclusion

Since temporary hardness is solely attributed to the bicarbonates of calcium and magnesium, Option A is the correct description.

Exam Tip

To remember this easily, think of the "B" in Bicarbonate as standing for Boiling. If you can remove the hardness by Boiling, it must be caused by Bicarbonates!