Question

Which of the following do not cause hardness of water ?

• A. Calcium carbonate
• B. Calcium sulphate
• C. Calcium bicarbonate
• D. Magnesium bicarbonate

Answer 1

The Correct Option is A

In understanding the hardness of water, it is important to identify which compounds contribute to this property and which do not. Hardness in water is typically caused by the presence of dissolved minerals, primarily calcium and magnesium ions. The compounds that commonly contribute to hardness in water include:

• Calcium sulfate (CaSO₄): A soluble calcium compound that contributes to permanent hardness, as it doesn't precipitate out of the water.
• Calcium bicarbonate (Ca(HCO₃)₂): Responsible for temporary hardness, as it can be removed by boiling the water, which causes the bicarbonate to decompose into insoluble carbonates.
• Magnesium bicarbonate (Mg(HCO₃)₂): Like calcium bicarbonate, it causes temporary hardness and can also be removed by boiling.

Comparatively:

• Calcium carbonate (CaCO₃): Although it is a calcium compound, it is largely insoluble in water at neutral pH. When present, it precipitates out of the water rather than dissolving, thereby not contributing to water hardness in the typical sense.

Thus, calcium carbonate is the compound from the options provided that does not cause hardness of water, because it does not remain dissolved in significant amounts when in water, thus not contributing to the dissolved mineral content responsible for hardness.