Question

Give the reasons for the following statement.

Temporary hardness can be removed by boiling.

Answer 1

• Water that does not form lather with soap solution is known as Hard water.
• Hardness of water due to the presence of dissolved salts of $\mathrm{Calcium}\left(\mathrm{Ca}\right)\mathrm{and}\mathrm{Magnesium}\left(\mathrm{Mg}\right)$.
• There are two types of hardness which are permanent hardness and temporary hardness.
• Hardness of water due to the presence of dissolved $\mathrm{Calcium}\mathrm{and}\mathrm{Magnesium}\mathrm{bicarbonates}$ is called temporary hardness.
• Temporary hardness can be removed by boiling.
• When the temporary hard water is boiled for some time, the soluble $\mathrm{Calcium}\mathrm{and}\mathrm{Magnesium}\mathrm{bicarbonates}$ salts present in it, decompose to form insoluble $\mathrm{Calcium}\mathrm{and}\mathrm{Magnesium}\mathrm{carbonates}$.
• The insoluble $\mathrm{Calcium}\mathrm{and}\mathrm{Magnesium}\mathrm{carbonates}$ formed by the boiled temporary hard water can be removed by filtration and leaving behind the soft water (Water free from dissolved salts).

Chemical equation 1

$\underset{(Solubleinhardwater)}{\underset{\mathrm{Calcium}\mathrm{bicarbonate}}{\mathrm{Ca}{\left(\mathrm{HCO}\right)}_3\left(\mathrm{aq}\right)}}\underset{∆}{\to }\underset{}{\underset{}{\underset{\left(\mathrm{Insoluble}\right)}{\underset{\mathrm{Calcum}\mathrm{carbonate}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}}$

Chemical equation 2

$\underset{(Solubleinhardwater)}{\underset{\mathrm{Magnesium}\mathrm{bicarbonate}}{\mathrm{Mg}{\left(\mathrm{HCO}\right)}_3\left(\mathrm{aq}\right)}}\underset{∆}{\to }\underset{}{\underset{}{\underset{\left(\mathrm{Insoluble}\right)}{\underset{\mathrm{Magnesium}\mathrm{carbonate}}{{\mathrm{MgCO}}_3\left(\mathrm{s}\right)}}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}}}$