Question

What are the methods used for softening hard water?

Answer 1

The process of removing dissolved salts of magnesium and calcium from hard water to get soft water is called the softening of water.

It can be done by the following methods.

By boiling:

• The temporary hardness of water can be easily removed by boiling.
• On boiling, the magnesium and calcium bicarbonates are converted into their respective carbonates. These carbonates are insoluble and can be removed by filtration to obtain soft water.

Reaction:-

$$\begin{gathered} \mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\stackrel{\mathrm{Heat}}{\to }{\mathrm{MgCO}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2 \\ \mathrm{Magnesium}\mathrm{Magnesium}\mathrm{Water}\mathrm{Carbon}\mathrm{dioxide} \\ \mathrm{bicarbonate}\mathrm{carbonate} \end{gathered}$$

The permanent hardness of water cannot be removed by boiling because the salts (chlorides and sulfates of calcium and magnesium) are stable to heat.

By treating with chemicals like washing soda (Na₂CO₃):

• The permanent hardness of water can be removed by adding washing soda or sodium carbonate.
• During this process, the sulfates and chlorides of magnesium and calcium are converted into their respective carbonates.
• These carbonates are insoluble and can be removed by the filtration process to obtain soft water.

Reaction:-

$$\begin{gathered} {\mathrm{MgSO}}_4+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{MgCO}}_3+{\mathrm{Na}}_2{\mathrm{SO}}_4 \\ \mathrm{Magnesium}\mathrm{Sodium}\mathrm{Magnesium}\mathrm{Sodium} \\ \mathrm{Sulphate}\mathrm{Carbonate}\mathrm{Carbonate}\mathrm{Sulphate} \end{gathered}$$