Hardness of Water

Types of Water:

There are two types of water. They are:

1.     Hard water

2.     Soft water

1. Hard Water:

Presence of calcium and magnesium salts in the form of hydrogen carbonate, chloride and sulphate in water makes water ‘hard’.

·        Hard water does not give lather with soap.

·        Hard water forms precipitate with soap.

2. Soft Water:

Water free from soluble salts of calcium and magnesium is called Soft water.

·        It gives lather with soap easily.

·        Because soft water has few calcium ions, there is no inhibition of the lathering action of soaps.

Hardness of Water:

The hardness of water is of two types:

a)    Temporary hardness and

b)    Permanent hardness.

a) Temporary Hardness:

Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates. It can be removed by:

·       Boiling:

During boiling, the soluble Mg(HCO3)2 is converted into insoluble Mg(OH)2 and Ca(HCO3)2 is changed to insoluble CaCO3. Thus Mg(OH)2 is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

Mg(HCO3)2

Mg(OH)2   + 2CO2

Ca(HCO3)2

CaCO3  + H2O CO2

·       Clark’s Method :

In this method calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.

Ca(HCO3)2 + Ca(OH)2

2CaCO3  + 2H2O

Mg(HCO3)2 + 2CaCO3

2CaCO3+Mg(OH)2  + 2H2O

b) Permanent Hardness:

It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness is not removed by boiling. It can be removed by the following methods:

·       Treatment with Washing Soda (sodium carbonate):

Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.

MCl2 + Na2CO3

MCO3  + 2NaCl          (M = Mg,Ca)

MSO4 + Na2CO3

MCO3  + Na2SO4

·       Calgon’s Method:

            Sodium hexametaphosphate (Na6P6O18) commercially called ‘calgon’. When it is added to hard water, the following reactions take place:

Na6P6O16

2Na + (Na4P6O18)2-             (M = Mg, Ca)

M2+ + (Na4P6O18)2-

(Na2MP6O18)2- + 2Na+

The complex anion keeps the Mg2+ and Ca2+ ions in solution.

·       Ion-exchange Method:

This method is also called zeolite or permutit process. Hydrated sodium aluminium silicate is known as zeolite or permutit. For the sake of simplicity, sodium aluminium silicate (NaAlSiO4) can be written as NaZ. When this is added in hard water, exchange reactions take place.

2NaZ (s) + M2+ (aq)  MZ2 (s) + 2Na+ (aq)                  (M = Mg, Ca)

·       Synthetic Resins Method:

This method is more efficient than zeolite process. Cation exchange resins contain large organic molecule with - SO3H group and are water insoluble. Ion exchange resin (RSO3H) is changed to RNa by treating it with NaCl. The resin exchanges Na+ ions with Ca2+ and Mg2+ ions present in hard water to make the water soft. Here R is resin anion.

2RNa (s) + M2+ (aq)  R2M (s) + 2Na+ (aq)

Heavy Water (D2O):

It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries. It is used for the preparation of other deuterium compounds.

For example,

CaC2 + 2D2O

C2D2 + Ca(OD)2

SO3 + D2O

D2SO4

Al2C3 + 12D2O

3CD4 + 4Al(OD)3