Water that does not form an immediate lather with soap is called hard water. Hardness of water is due to the presence of soluble salts of bicarbonates (hydrogen carbonates), sulphates and chlorides of calcium and magnesium.
Water that forms an immediate lather with soap is called soft water. Soft water is free from bicarbonates, sulphates and chlorides of calcium and magnesium. Distilled water, rain water are the examples of soft water.

Types of hardness:

Temporary hardness:

Temporary hardness of water is due to the presence of calcium and magnesium bicarbonates (hydrogen carbonates).
Temporary hardness is also called as carbonate hardness.
Softening of water: the process of removal of Ca²⁺ and Mg²⁺ ions from water is called softening of water.

Removal of temporary hardness of water:
Temporary hardness can be removed by boiling the water. When water is boiled, the soluble bicarbonate is decomposed into insoluble carbonate or hydroxide.

Mg(HCO₃)₂ Mg(OH)₂ + 2CO₂

Ca(HCO₃)₂ CaCO₃ + H₂O + CO₂

Clark’s method or calcium hydroxide method:
In Clark’s method, calculated amount of lime is added to hard water. If excess of lime is used, permanent hardness is produced in place of temporary hardness.

Ca(HCO₃)₂ + Ca(OH)₂ 2CaCO₃ + 2H₂O

Mg(HCO₃)₂ + 2Ca(OH)₂ 2CaCO₃ + Mg(OH)₂ + 2H₂

Permanent hardness:

Permanent hardness of water is due to the presence of chlorides and sulphates of calcium and magnesium.
Permanent hardness is also called as non-carbonate hardness.
Treatment of water is required to remove this type hardness.

Removal of permanent hardness of water:
By washing soda (Na₂CO₃.10H₂O) treatment:
Temporary hardness can be removed by boiling the water. When water is boiled, the soluble bicarbonate is decomposed into insoluble carbonate or hydroxide.

CaCl₂ + Na₂CO₃ CaCO₃ + 2NaCl

By Ion exchange method or Permutit method or Zeolite method:
Ion exchange method is also called as Permutit method or Zeolite method. Zeolites or permutits are naturally occurring hydrated sodium alumino silicate minerals. When hard water is introduced into the tank with loosely packed permutit, the cations present in hard water are exchanged for sodium ions. Therefore this method is also called ion exchange method.

Na₂Al₂Si₂O₈ + CaCl₂ Ca(Al₂Si₂O₈)₂ + 2NaCl

The zeolite can be regenerated by treatment with sodium chloride solution.

Ca(Al₂Si₂O₈)₂ + 2NaCl Na₂Al₂Si₂O₈ + CaCl₂

By Ion exchange resins method:
This method is more efficient than Zeolite method. Giant organic molecules having acidic or basic groups are known as ion exchange resins.
Cation exchange resins:
Cation exchange resins contain acidic groups like sulfonic acid groups, e.g. sodium polystyrene sulfonate or polyAMPS. Acid resins exchange their H+ ions with other cations such as Ca₂₊, Mg₂₊, etc., present in hard water. Therefore acid resins are known as base exchange resins. This process results in proton release and it makes the water acidic.

Mg²⁺ + 2H-resin Mg resin₂ + 2H⁺

Anion exchange resins:
Anion exchange resins contain basic groups like quaternary amino groups, for example, trimethylammonium groups, e.g. polyAPTAC. Basic resins exchange their OH-ions with the other anions such as HCO₃^-, Cl^-, SO₄^2-, present in hard water. Therefore basic resins are known as acid exchange resins.

Cl^- + HO-resin Cl-resin + OH^-

The exhausted cation and anion exchange resins are regenerated with dilute acid and alkali (NaOH, Na₂CO₃ solutions respectively.

Ca resin₂ + 2HCl 2H-resin + CaCl₂

Cl-resin + NaOH HO-resin + NaCl