Question

Which one of the following processes will produce hard water ?
A Saturation of water with $MgC{{O}_{3}}$
B Saturation of water with $CaS{{O}_{4}}$
C Addition of $N{{a}_{2}}S{{O}_{4}}$ to water
D Saturation of water with $CaC{{O}_{3}}$

Answer 1

Water with a high mineral content is referred to as hard water (in contrast with "soft water"). Hard water is created when water percolates through deposits of calcium and magnesium carbonates, bicarbonates, and sulphates found in limestone, chalk, or gypsum.

Complete answer:
The concentration of multivalent cations in water determines the permanent hardness of the water. Positively charged metal complexes having a charge larger than ${{1}^{+}}$ are known as multivalent cations. The cations usually have a charge of ${{2}^{+}}$. $C{{a}^{2+}}$ and $M{{g}^{2+}}$ are two common cations found in hard water. These ions leak from an aquifer's minerals into the water supply. Calcite and gypsum are two common calcium-containing minerals. Dolomite is a common magnesium mineral (which also contains calcium). Because they contain few ions, rainwater and distilled water are soft.
The dissolving and production of calcium carbonate and calcium bicarbonate (on the right) are described by the following equilibrium reaction:
$N{{a}_{2}}C{{O}_{3}}+CaS{{O}_{4}}\to CaC{{O}_{3}}\downarrow +~N{{a}_{2}}S{{O}_{4}}$
$CaC{{O}_{3}}~\left( s \right)\text{ }+\text{ }C{{O}_{2}}~\left( aq \right)\text{ }+\text{ }{{H}_{2}}O\text{ }\left( l \right)\text{ }\rightleftharpoons \text{ }C{{a}^{2+}}~\left( aq \right)\text{ }+\text{ }2~HC{{O}^{-}}_{3}~\left( aq \right)$
Calcium ions can be carried away by rain that contains dissolved carbon dioxide. As carbon dioxide is released to the atmosphere, calcium carbonate may re-deposit as calcite, generating stalactites and stalagmites. Water softeners can sometimes remove calcium and magnesium ions. Boiling is often ineffective at removing permanent hardness. The presence of calcium sulfate/calcium chloride and magnesium sulfate/magnesium chloride in the water, which do not precipitate out as the temperature rises, is frequently the cause. A water softener or an ion-exchange column can remove ions that cause permanent hardness in water.
The presence of dissolved bicarbonate minerals causes temporary hardness (calcium bicarbonate and magnesium bicarbonate). When these minerals are dissolved, they produce calcium and magnesium cations ($C{{a}^{2+}},\text{ }M{{g}^{2+}}$) as well as carbonate and bicarbonate anions ($C{{O}_{3}}^{2-}\text{ },\text{ }HC{{O}_{3}}^{2-}$). Water becomes hard due to the presence of metal cations. Unlike the permanent hardness induced by sulphate and chloride compounds, however, this "temporary" hardness can be decreased by boiling the water or adding lime to the water through the lime softening process. Boiling encourages the synthesis of carbonate from bicarbonate and causes calcium carbonate to precipitate out of solution, resulting in water that is softer when cooled.

Hence option c is correct.

Note:
Drinking hard water may provide some health benefits. In industrial settings, where water hardness is monitored to minimise costly breakdowns in boilers, cooling towers, and other water-handling equipment, it can cause serious difficulties. Hard water is commonly indicated in home settings by a lack of froth when soap is agitated in water, as well as the production of limescale in kettles and water heaters. Water softening is often utilised to alleviate the negative impacts of hard water wherever it is a problem.