How does boiling remove any temporary hardness, caused by the presence of calcium hydrogen carbonate, in a water sample? - Leaving Cert Chemistry - Question i - 2017

Boiling water containing temporary hardness due to calcium hydrogen carbonate leads to a chemical reaction where calcium hydrogen carbonate decomposes.

This decomposition can be represented by the following chemical equation:

$\text{Ca(HCO}_3\text{)}_2 (aq) \rightarrow \text{CaCO}_3 (s) + \text{CO}_2 (g) + \text{H}_2\text{O} (l)$

As the water is heated, calcium hydrogen carbonate breaks down to form calcium carbonate, which is an insoluble precipitate referred to as 'limescale' or 'limestone'.

The formation of this insoluble product effectively removes the hardness from the water sample, as it precipitates out and can be filtered away.