Hard and Soft Water

What makes water "hard". Hard water carries high concentrations of dissolved mineral ions -- most commonly the soluble cations of magnesium and calcium, though iron, aluminium and manganese can also contribute in some areas. It is the presence of these metal ions as bicarbonates, chlorides and sulphates that gives water its "hardness"; boiling hard water precipitates out the magnesium and calcium carbonates it carries. Water free of dissolved calcium and magnesium salts is, correspondingly, called soft water. Hardness comes in two kinds -- temporary and permanent -- distinguished by which salts cause it and, in turn, by how each is removed.

Temporary hardness and its removal. Temporary hardness comes mainly from soluble bicarbonates of magnesium and calcium. Simply boiling the hard water, and filtering the result, removes it: on heating, the bicarbonates decompose into insoluble carbonate, which precipitates out, while the magnesium carbonate so formed hydrolyses further to insoluble magnesium hydroxide:

Filtration then removes the resulting precipitates. Alternatively, Clark's method adds a calculated amount of lime (Ca(OH)₂) to the hard water, converting the dissolved magnesium and calcium bicarbonates into insoluble carbonates and hydroxides that can then be filtered off:

Permanent hardness and its removal. Permanent hardness, unlike temporary hardness, comes from soluble chlorides and sulphates of magnesium and calcium -- salts that do not decompose on boiling. It is instead removed by adding washing soda (Na₂CO₃), which converts these metal (M = Ca or Mg) chlorides and sulphates into insoluble carbonates:

$MSO_4 + Na_2CO_3 \rightarrow MCO_3 + Na_2SO_4$ …