Treating Water Hardness

The process of removing Ca²⁺ and Mg²⁺ from the water is known as water softening. Two minerals, lime (Ca(OH)₂) and soda ash (Na₂CO₃), are typically used to soften public water supplies. (Incidentally, one important source of lime is near St. Genevieve, Missouri.) When lime is added to water, it dissolves to give three aqueous (solvated) ions: one Ca²⁺ ion and two OH^- ions for each unit of Ca(OH)₂. Likewise, soda ash dissolves to give two Na⁺ ions and one CO₃^2- ion for each unit of Na₂CO₃ that dissolves.

A number of reactions occur to generate the insoluble precipitates CaCO_3(s) and Mg(OH)_2(s) from the Ca²⁺ and Mg²⁺ ions. The most important reaction for the removal of Mg²⁺ is shown in Equation 4. 

Notice that Ca²⁺ appears on both sides of Equation 4. The calcium ion from lime does not actually participate in the reaction to generate insoluble Mg(OH)₂. Hence, this ion is called a spectator ion and can be omitted from the equation. We can write the reaction more correctly with the net ionic equation, given by Equation 5.

The important reaction for the removal of Ca²⁺ ions is given in Equation 6.  

The solids generated by the water-softening precipitation reaction are then removed by sedimentation or filtration.  If an excess of lime was used to precipitate magnesium ions in the water (Equation 4), some unused hydroxide (OH^-) ions will remain in the water after the calcium is precipitated, resulting in a high (or basic) pH.  If necessary, the pH can be lowered by bubbling carbon dioxide gas through the water.  The net ionic equations for this recarbonation are given in Equations 7 and 8.  

Bicarbonate (HCO₃^-) remaining in the water is nontoxic and does not negatively affect the flavor of the water.