A water containing certain mineral constituents in solution, chiefly
calcium and magnesium, which form insoluble compounds with soap, is said
to be hard. Carbon dioxide in water increases the solubility of calcium
and magnesium carbonates, forming bicarbonate. If carbon dioxide is
removed from the water by boiling the bicarbonate is decomposed and
calcium and magnesium are partly precipitated. The proportion of calcium
or magnesium carbonate that a water can hold in solution depends on the
concentration of carbon dioxide, which in turn depends on the
temperature of the water and the proportion of carbon dioxide in the
atmosphere with which the water has been in contact. Consequently, when
the carbon dioxide is removed from the water by boiling or otherwise the
carbonates of calcium and magnesium are partly, but not completely,
precipitated, and the hardness of the water is thus diminished and the
water is softened to the extent to which these substances are
precipitated. The hardness thus removed is called temporary hardness.
The hardness which still remains after boiling is due mainly to calcium
and magnesium in equilibrium with sulfate, chloride, and nitrate, and
residual carbonate, and it is called permanent hardness. Non-carbonate
hardness is the hardness caused by sulfates, chlorides, and nitrates of
calcium, magnesium, iron, and other metals that form insoluble soaps.