Question

By boiling, the _______ hardness of water can be removed.

Answer 1

Hard water consists of additional substances in the form of salts, these salts can be temporarily removed by the application of thermal energy. As the process of boiling leads to evaporation of the pure water and the salts are being deposited.

Complete answer:
Water hardness is the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, largely calcium and magnesium. By application of heat, the divalent salts present in the hard water such as calcium and magnesium salts of bicarbonates get precipitated out and they get deposited. These salts are thereby precipitated by liberating carbon dioxide, hence, boiling of water removes bicarbonate form of the salts, whereas, magnesium and calcium salts of chlorides are not being precipitated and hence the boiling of water removes only bicarbonate form of the salts and therefore, temporary hardness of water is being removed.
Reaction: $Ca{(HC{O_3})_2}\xrightarrow{\Delta }CaC{O_3} + {H_2}O + C{O_2}$
Here, the calcium bicarbonate on heating form calcium carbonate precipitate, water and liberation of carbon dioxide.

By boiling, the temporary hardness of water can be removed.

Note:
Hardness of water can be known when the water does not lather with soap. The hardness of water can also be removed by Clark’s method by using calcium hydroxides. The presence of hard water can cause harmful effects such as clogging of pipes, residues on the dishes, strains in the shower, clothes looking dull and rough. Hardness of water can be calculated in units such as part per million ($ppm$), or milligrams per liter $mg/l$.