Question

Which of the following substances can remove the temporary as well as the permanent hardness of water?

• A. Baking soda
• B. Common salt
• C. Washing soda
• D. Rock salt

Answer 1

The correct option is C

Washing soda

Explanation for the correct option:

(C) Washing soda: Washing soda or Sodium carbonate $({\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O})$ is used to remove permanent hardness of water.

• Washing soda when mixed with the calculated quantities of Calcium hydroxide $\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$ is used to remove temporary as well as permanent hardness of water.
• Calcium hydroxide removes the temporary hardness.

Chemical Reactions

$\underset{(\mathrm{Sodium}\mathrm{carbonate})}{{\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)}+\underset{(\mathrm{Calcium}\mathrm{chloride})}{{\mathrm{CaCl}}_2\left(\mathrm{aq}\right)}\to \underset{(\mathrm{Calcium}\mathrm{carbonate})}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}+\underset{(\mathrm{Sodium}\mathrm{chloride})}{2\mathrm{NaCl}\left(\mathrm{aq}\right)}$

• Calcium carbonate in the form of solid precipitates is filtered and permanent hardness is removed.

$\underset{(\mathrm{Calcium}\mathrm{hydroxide})}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}+\underset{(\mathrm{Calcium}\mathrm{bicarbonate})}{\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{aq}\right)}\to \underset{(\mathrm{Calcium}\mathrm{carbonate})}{2{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

• Calcium carbonate in the form of solid precipitates is filtered and temporary hardness is removed.

Explanation for the incorrect options:

(A) Baking soda: Baking soda or Sodium bicarbonate $\left({\mathrm{NaHCO}}_3\right)$ mixed with a mild acid like tartaric acid is used to make the cakes and breads spongy.

• It is also used to prepare antacids to give relief from the problem of acidity.

(B) Common salt: Common salt is Sodium chloride $\left(\mathrm{NaCl}\right)$ and is used as a raw material to prepare various useful chemicals products like baking soda etc.

(D) Rock salt: Rock salt also contains sodium chloride as a major component but it also contains minerals like Iron $\left(\mathrm{Fe}\right)$, Manganese $\left(\mathrm{Mn}\right)$, Magnesium $\left(\mathrm{Mg}\right)$ etc.

Hence, option (C) is correct .