Question:medium
Which of the following does NOT cause hardness of water?
Which of the following does NOT cause hardness of water?
Show Hint
Hardness needs a soluble salt; carbonate of calcium is insoluble.
Updated On: Jun 24, 2026
- Calcium carbonate
- Calcium sulphate
- Calcium bicarbonate
- Magnesium bicarbonate
Show Solution
The Correct Option is A
Solution and Explanation
Step 1: Water hardness reflects how much dissolved $Ca^{2+}$ and $Mg^{2+}$ salt content the water carries. The question asks which listed salt fails to add to that load.
Step 2: Soluble bicarbonates give temporary hardness. Both calcium bicarbonate and magnesium bicarbonate dissolve readily and are knocked out by heating, so they qualify as hardness-producers.
Step 3: Sulphate and chloride salts such as calcium sulphate give permanent hardness that boiling cannot clear. So calcium sulphate also counts as a cause.
Step 4: Calcium carbonate, by contrast, has very low solubility in ordinary water and stays as a precipitate rather than dissolving. Because it is not present in dissolved form, it cannot raise hardness. Therefore the odd one out is calcium carbonate.
\[\boxed{\text{Calcium carbonate}}\]
Step 2: Soluble bicarbonates give temporary hardness. Both calcium bicarbonate and magnesium bicarbonate dissolve readily and are knocked out by heating, so they qualify as hardness-producers.
Step 3: Sulphate and chloride salts such as calcium sulphate give permanent hardness that boiling cannot clear. So calcium sulphate also counts as a cause.
Step 4: Calcium carbonate, by contrast, has very low solubility in ordinary water and stays as a precipitate rather than dissolving. Because it is not present in dissolved form, it cannot raise hardness. Therefore the odd one out is calcium carbonate.
\[\boxed{\text{Calcium carbonate}}\]
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