In this method, the soluble calcium and magnesium salts in waer are chemically converted into insoluble compounds, by adding calculated amounts of lime Ca(OH)2 and soda Mg2CO3.

Calcium carbonate CaCO3 and magnesium hydroxide Mg(OH)2, so precipitated, are filtered off.

Following are the reactions that takes place in this process:

• As slaked lime is added to a water, it will react with any carbon dioxide present as follows:
  $\text{Ca(OH)}_2+\text{CO}_2\rightarrow\text{CaCO}_3\downarrow+\text{H}_2\text{O}\\$.....(i) * The lime will react with carbonate hardness asa follows: $\text{Ca(OH)}_2+\text{Ca(HCO}_3)_2\rightarrow2\text{CaCO}_3\downarrow+2\text{H}_2\text{O}\$.....(ii)
  $\text{Ca(OH)}_2+\text{Mg(HCO}_3)_2\rightarrow\text{MgCO}_3+\text{CaCO}_3\downarrow+2\text{H}_2\text{O}\\$.....(iii) * The product magnesium carbonate in equation (iii) is soluble. To remove it, more lime is added: $\text{Ca(OH)}_2+\text{MgCO}_3\rightarrow\text{CaCO}_3\downarrow+\text{Mg(OH)}_2\downarrow\$.....(iv)
• Also, magnesium non-carbonate hardness, such as magnesium sulfate is removed:
  $\text{Ca(OH)}_2+\text{MgSO}_4\rightarrow\text{CaSO}_4+\text{Mg(OH)}_2\downarrow\\$.....(v) * Lime addition removes only magnesium hardness and calcium carbonate hardness. In equation (v) magnesium is precipitated, however, an equivalent amount of calcium is added. The water now contains the original calcium non-carbonate hardness and the calcium non-carbonate hardness produced in equation (v). Soda ash is added to remove calcium non-carbonate hardness: $\text{Na}_2\text{(CO)}_3+\text{CaSO}_4\rightarrow\text{Na}_2\text{SO}_4+\text{CaCO}_3\downarrow\$.....(vi)
• To precipitate CaCO3 requires a pH of about 9.5; and to precipitate Mg(OH)2 requires a pH of about 10.8, therefore, an excess lime of about 1.25 meq/l is required to raise the pH.
• After softening, the water will have high pH and contain the excess lime and the magnesium hydroxide and the calcium carbonate that did not precipitate. Recarbonation is used to stabilize the water. The excess lime and magnesium hydroxide are stabilized by adding carbon dioxide, which also reduces pH from 10.8 to 9.5 as the following:
  $\text{CO}_2+\text{Ca(OH})_2\rightarrow\text{CaCO}_3\downarrow+\text{H}_2\text{O}\\$ $\text{CO}_2+\text{Mg(OH})_2\rightarrow\text{MgCO}_3+\text{H}_2\text{O}\$
• Further recarbonation, will bring the pH to about 8.5 and stabilize the calcium carbonate as the following:
  $\text{CO}_2+\text{CaCO}_3+\text{H}_2\text{O}\rightarrow\text{Ca(HCO}_3)_2\$
  It is not possible to remove all of the hardness from water. In actual practice, about 50 to 80 mg/l will remain as a residual hardness.