Temporary hardness is caused by presence of dissolved bicarbonates of calcium, magnesium and other heavy metals and the carbonate of iron.

Temporary hardness is mostly destroyed by mere boiling of water, when bicarbonates are decomposed, yeilding insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel. Thus:

$\begin{matrix} & \text{Ca(HCO}_3)_2 & \xrightarrow {heat} & \text{CaCO}_3\downarrow \\ &\text{Calcium Bicarbonate} & & \text{Calcium Carbonate (insoluble)}\\& & &+ \text{H}_2\text{O} + \text{CO}_2\uparrow \end{matrix}$

$\begin{matrix} & \text{Mg(HCO}_3)_2 & \xrightarrow {heat} & \text{Mg(OH)}_2\downarrow\\ &\text{Magnesium Bicarbonate} & & \text{Magnesium Hydroxide}\\ & & & + \text{2CO}_2\uparrow \end{matrix}$

The insoluble calcium carbonate forms a layer of lime-scale.

This may coat the heating element in kettles and irons. For example, making it less efficient.

Limescale is unsightly and it clogs up hot water pipes and boilers.