Question

The sodium compound which is used for softening hard water:
A. Washing soda
B. Bleaching powder
C. Caustic soda
D. Lime soda

Answer 1

Compounds used for removal of ions should not dissolve themselves into the water. Instead, they should precipitate components present as salts mixed in water. For these kinds of purposes, chemical methods like Substitution reactions have wide application. Chemical compounds used for treatment substitutes one of their components which further does not dissolve and settles at the base of the container.

Complete step by step answer:
Generally, hard water contains magnesium and calcium salts in it, due to which they do not get rinsed by detergents. Presence of these compounds also prevents lather formation with soaps and are very harmful for skin, drinking, clothes and in short for daily use.
To remove these salts, a widely known process called ion exchange has been used in which ionic compounds are used which can react with these chemical compounds to form new compounds and precipitates.
Washing Soda or $N{a_2}{\left( {CO} \right)_3}$ is a chemical compound which is commonly used to soften hard water. This compound does not dissolve in water and releases $N{a^ + }$ and $CO_3^ -$ ions in water. These carbonate ions react with calcium and form $CaC{O_3}$, which is insoluble in water and precipitates down.
Bleaching powder or $Ca{\left( {ClO} \right)_2}$, also known as calcium hypochlorite is a chemical compound soluble in water. On dissolution, it releases chlorine which dissolves in water. Due to its property to dissolve it cannot be further used to precipitate salts from hard water.
Caustic Soda or $NaOH$ (Sodium Hydroxide) is a compound soluble in water. It is widely used to form washing soda but due to its water solubility property, it cannot be used to precipitate salts from hard water.
Lime Soda or $CaHN{O_2}$ is used in face masks to absorb moisture and $C{O_2}$from air. It cannot be used for softening of hard water.

Hence, the correct answer is A.

Note: Washing soda and baking soda are somewhat similar. The only difference is the water and $C{O_2}$. When baking soda is heated at high temperature the water is removed and washing soda is formed. So $NaHC{O_3}$ is heated and after releasing $C{O_2}$and ${H_2}O$ are released, $N{a_2}{\left( {CO} \right)_3}$is left. Washing soda is a useful household tool and used widely to remove hard stains.