What is Hard water ?

What is hard water ? (or) hardness of water ?

Water which does not give good lather with soap immediately is called hard water.

Types of hard water

Hard water is two types.

  1. Temporary hardness
  2. Permanent hardness

Temporary hardness is due to the presence of bicarbonate of Ca and Mg.

Permanent hardness is due to the presence of chlorides and sulphates of Ca and Mg.

Temporary hardness can be removed by (a) Boiling (b) Clark’s process.

Boiling : When hard water is boiled the bicarbonates will be converted into insoluble carbonates.

\text {Mg (HCO}_{3})_{2}         \xrightarrow{\text {boiling}}       \text {MgCO}_{3} \downarrow + \text {H}_{2} \text {O + CO}_{2} \uparrow

Clark’s process :

When a required quantity of milk of lime is added to hard water to remove temporary hardness. It forms insoluble carbonates and hydroxides.

\text {Ca(HCO}_{3})_{2} + \text {Ca(OH)}_{2} \longrightarrow \text {2CaCO}_{3} \downarrow + \text {2H}_{2} \text {O}

Permanent hardness can be removed by (a) permutit process (b) Calgon method and (c) Ion-exchange method.

Harness of water diagram
Permutit process : Permutit is an artificial zeolite, chemically it is sodium aluminum orthosilicate. Its formula is

\text {Na}_{2} \;\text {A}l_{2} \; \text {Si}_{2} \; \text {O}_{8}. \; \times \; \text {H}_{2} \text {O}.

Permutit is packed in a suitable container and hard water is percolated through it. Calcium and magnesium ions which cause hardness in water are replaced by sodium ions which do not cause hardness. Thus water is softened and is taken out through the outlet of the container.

After some use, all the sodium ions in the permutit are replaced by \text {Ca}^{2+} or \text {Mg}^{2+} ions and then it is said that the permutit is exhausted.

The exhausted permutit can be regenerated for subsequent use by shaking it with 10% brine solution. (NaCl solution).