Question

# Temporary hardness of water is due to the presence of:Magnesium sulphateSodium chlorideCalcium sulphateCalcium hydrogen carbonate

- Hint: Temporary hardness of water is caused by the bicarbonates of Magnesium (atomic number 12 ) and Calcium (atomic number 20).

Temporary hardness is because of the presence of bicarbonates of calcium and magnesium. Rainwater dissolves small quantities of $C{{O}_{2}}$ from the atmosphere, forming a very dilute solution of carbonic acid. This water attacks calcium and magnesium carbonates in any rocks over which it flows, and the soluble hydrogen carbonates or bicarbonates are formed.
$CaC{{O}_{3}}+{{H}_{2}}O+C{{O}_{2}}\overset{{}}{ \Leftrightarrow}Ca{{(HC{{O}_{3}})}_{2}}$
Temporary hardness in water is easily removed by boiling, as the bicarbonates decompose readily and the insoluble carbonates are precipitated.
$M{{g}^{2+}}(aq)+2{{C}_{17}}{{H}_{35}}CO{{O}^{-}}(aq)\xrightarrow{{}}{{({{C}_{17}}{{H}_{35}}COO)}_{2}}Mg$
$Ca{{(HC{{O}_{3}})}_{2}}\xrightarrow{boil}CaC{{O}_{3}}+{{H}_{2}}O+C{{O}_{2}}$
(insoluble)
$Mg{{(HC{{O}_{3}})}_{2}}\xrightarrow{boil}MgC{{O}_{3}}+{{H}_{2}}O+C{{O}_{2}}$
(insoluble)
Temporary hardness can also be removed by Clark’s process which involves the addition of slaked lime $[Ca{{(OH)}_{2}}]$.
$Ca{{(HC{{O}_{3}})}_{2}}+Ca{{(OH)}_{2}}\xrightarrow{{}}2CaC{{O}_{3}}+2{{H}_{2}}O$
(insoluble)
$Mg{{(HC{{O}_{3}})}_{2}}+Ca{{(OH)}_{2}}\xrightarrow{{}}\{2CaC{{O}_{3}}+Mg{{(OH)}_{2}}\}+2{{H}_{2}}O$
(insoluble)
It is essential to add only the calculated amount of $Ca{{(OH)}_{2}}$because the excess will cause artificial hardness.
Therefore, we conclude that the temporary hardness of water is caused by Calcium Hydrogen Carbonate. So Option D is the correct answer.

Additional Information: Water is said to be soft if it produces sufficient lather with the soap, and water is described as being hard if it forms an insoluble scum before it forms a lather with the soap. The hardness of water is because of the presence of salts in water. The hardness of natural water is generally caused by the bicarbonates and sulphates of calcium and magnesium, but in fact, all soluble salts that form a scum with the soap cause hardness.
$C{{a}^{2+}}(aq)+2{{C}_{17}}{{H}_{35}}CO{{O}^{-}}(aq)\xrightarrow{{}}{{({{C}_{17}}{{H}_{35}}COO)}_{2}}Ca$
$M{{g}^{2+}}(aq)+2{{C}_{17}}{{H}_{35}}CO{{O}^{-}}(aq)\xrightarrow{{}}{{({{C}_{17}}{{H}_{35}}COO)}_{2}}Mg$
(Anion of Soap) (Insoluble Precipitates)
Soap will not produce a lather with water until all the calcium and magnesium ions have been precipitated. Using hard water thus results in the wastage of soap. Hence, in our household, we use soft water for all purposes. This also saves us from wastage of water.

Note: Permanent hardness of water is caused by the soluble salts of magnesium and calcium when present in the form of chlorides and sulfides in water. The permanent hardness of water cannot be removed by boiling. To remove permanent hardness of water we need to treat the water with washing soda.