Water — Question 27

Temporary hardness in water is due to the presence of calcium or magnesium bicarbonates.

On boiling temporary hard water, calcium and magnesium bicarbonates in water decompose to give insoluble carbonates, carbon dioxide and water.

The insoluble carbonates are filtered out, CO₂ gas escapes and water is rendered soft.

$\underset{\text{Calcium bicarbonate}}{\text{Ca(HCO}_3)_2}\xrightarrow{\text{Boil}} \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + \text{H}_2\text{O} + \text{CO}_2 [\text{g}]$

$\underset{\text{Magnesium bicarbonate}}{\text{Mg(HCO}_3)_2}\xrightarrow{\text{Boil}} \underset{\text{Magnesium carbonate}}{\text{MgCO}_3↓} + \text{H}_2\text{O} + \text{CO}_2 [\text{g}]$