Question

Give reasons for the following

When hardness in temporary hard water can be removed by boiling but the hardness of permanent hard water cannot.

Answer 1

• The temporary hardness of water can be removed by boiling. The soluble Calcium bicarbonate [Ca(HCO₃)₂] and Magnesium bicarbonate [Mg(HCO₃)₂] decompose on boiling to form their respective insoluble carbonate. these insoluble salt can be separated from water by filtration and the water then become soft.

$\underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Calcium}\mathrm{Bicarbonate})}}{\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{s}\right)}\stackrel{\mathrm{Heat}}{\to }\underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Calcium}\mathrm{Carbonate})}}{\mathrm{Ca}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbondioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

$\underset{\underset{\left(\mathrm{Soluble}\right)}{(\mathrm{Magnesium}\mathrm{Bicarbonate})}}{\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{s}\right)}\stackrel{\mathrm{Heat}}{\to }\underset{\underset{\left(\mathrm{Insoluble}\right)}{(\mathrm{Magnesium}\mathrm{Carbonate})}}{\mathrm{Mg}{\mathrm{CO}}_3\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Carbondioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

• The permanent hardness of water cannot be removed by boiling as it is caused due to Chloride and Sulfate of Calcium and Magnesium. These salts are stable to heat.