when we talk about temporary hardness of water we talk about boling soluble magnesium hydrogen carbonate which gives insoluble magnesium hydroxide and not magnesium carbonate. we say that this is due to higher solubility product of magnesium hydroxide that it precipitates first. But why is it so? Please explain.
Boiling the water causes the precipitation of solid calcium carbonate or solid magnesium carbonate. This removes the calcium ions or magnesium ions from the water, and so removes the hardness. Therefore, hardness due to hydrogencarbonates is said to be temporary.
Generally an increase in water temperature causes an increase in the solubility of most salts, there are exceptions like CaCO3, CaSO4, MgCO3, Mg(OH)2 all of which become less soluble as the temperature increases.