Question

Treatment with lime remove hardness of water caused by:
A.$\text{CaC}{{\text{l}}_{\text{2}}}$
B.$\text{CaS}{{\text{O}}_{4}}$
C.$\text{Ca}{{\left( \text{HC}{{\text{O}}_{\text{3}}} \right)}_{\text{2}}}$
D.$\text{CaC}{{\text{O}}_{\text{3}}}$

Answer 1

Hint: The amount of calcium and magnesium present in water is measured by the hardness of water. This type of water is rich in minerals mainly calcium and magnesium.

Complete step by step answer:
This hardness of water makes it very difficult to use detergents and soap in it. In hard water the bases and EDTA present in the soap react with the calcium, which is relatively high in amount, to form the soap scum. Hence when working with hard water, more soap or detergent is needed than is generally required. To treat the hardness of water lime is used and this process is known as the Clark’s process. As lime in the form of lime water is introduced in the calcium and magnesium rich water, the pH of the water is raised. Hence the dissolved carbon dioxide is changed into bicarbonate and then into carbonate. This causes the precipitation of calcium carbonate due to exceeding the solubility product. Apparently magnesium is also precipitated as magnesium hydroxide in a double displacement reaction.

Hence, option C is the correct answer.

Note:
In contrast to lime softening, the calcium and magnesium removal by the ion exchange method involves the exchange of the cations for sodium cations. So lime softening causes a reduction in the Total Dissolved Solids (TDS) of the water but the use of ion-exchange does not lower the TDS of the water.