Hard Water and Soft Water

Water forms lather with soap which is used for cleaning purposes. It is called soft water. Sometimes water from some sources like rivers or hand pumps does not produce any lather with soap. It is called hard water.

Water, which we get from taps, contain lesser amounts of dissolved salts in it than water that we get from hand pumps. The dissolved salts are usually bicarbonates, sulphates and chlorides of calcium and magnesium. Their presence prevents formation of soap lather.

Soap is a sodium salt called sodium stearate. It is soluble in water. When soap is added to hard water, which contains calcium and magnesium ions, a precipitate of Ca or Mg stearate is formed. These calcium and magnesium steartes are insoluble in water and appears as a greasy scum. The formation of scum in place of lather makes it more difficult to clean things.

Sodium stearate + Calcium sulphate → Calcium stearate + Sodium sulphate

Hard water can be converted into soft water, by removal of Ca and Mg ions which are responsible for hardness. This is called softening of water. 

Hardness of water is of two types:

  1. Temporary hardness
  2. Permanent hardness

Temporary Hardness

Temporary hardness of water is due to the presence of soluble bicarbonates of calcium and magnesium. It is also called carbonate hardness. It can be removed by boiling and by soda lime process.

1. By boiling: Upon boiling hard water, calcium or magnesium bicarbonate present in it are decomposed to give magnesium or calcium carbonate. These carbonate salts are insoluble in water. They settle down easily and water can be decanted.

Ca(HCO3)2 + Heat → CaCO3 + H2O + CO2

Mg(HCO3)2 + Heat → MgCO3 + H2O + CO2

 

2. By soda lime process (Clark’s method): When a calculated amount of lime is added to hard water, then the soluble bicarbonates are converted to insoluble carbonates.

Ca(HCO3)2 + Ca(OH)2 + Heat → CaCO3 + 2H2O

Mg(HCO3)2 + Ca(OH)2 + Heat → 2MgCO3 + CaCO3 + 2H2O

Permanent Hardness

Permanent hardness of water is due to the presence of soluble chlorides and sulphates of calcium and magnesium. It is also known as non-carbonate hardness. It can be removed by addition of washing soda or by the ion exchange method.

1. By addition of washing soda: The hard water is treated with the calculated quantity of washing soda (sodium carbonate). Washing soda reacts with chloride and sulphate of calcium and magnesium to form precipitate of calcium and magnesium carbonate. 

CaCl2 + Na2CO3 + Heat → CaCO3 + 2NaCl

MgSO4 + Na2CO3 + Heat → MgCO3 + Na2SO4

The precipitate settles down and can be removed by decantation.

2. By ion exchange method: Two types of ion exchangers can be used - inorganic ion exchanger and organic ion exchanger. In inorganic ion exchange process, complex compounds known as Zeolite are used to soften the hard water. 

The salts causing hardness of water are precipitated as insoluble zeolite of calcium and magnesium and are replaced by soluble sodium salts. After using it for sometime the zeolite is regenerated by soaking it in 10% solution of NaCl (brine) and then washing away chlorides. The washings are removed and are replaced by soluble sodium salts.

By using organic ion exchanger, water obtained is free from cations and anions and is known as deionized water or demineralized water.