Discussion
Temporary hardness of water is caused due to :
Answer: C
Temporary hardness is caused by magnesium carbonate among the options.
Temporary hardness comes from the carbonates and bicarbonates of calcium and magnesium. It is called temporary because boiling removes it: the bicarbonates decompose and the carbonate precipitates, as in
\[ \begin{aligned} &\mathrm{Mg(HCO_{3})_{2}} \\ &\rightarrow \mathrm{MgCO_{3}} + \mathrm{H_{2}O} + \mathrm{CO_{2}} \end{aligned} \]
Strictly the dissolved species is the bicarbonate, but textbooks list the carbonate under temporary hardness, which is the reading the key follows. Chlorides and sulphates of calcium and magnesium, such as magnesium chloride, magnesium sulphate and calcium sulphate, cause permanent hardness, which boiling does not remove.
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