Soft water and Hard water: Definition, example, How hard water is formed

Soft Water: Water that gives lather with soap easily and does not form scum is called soft water.

Hard Water: Water that gives little lather with soap easily and forms scum is called soft water.

How hard water is formed?

Rainwater dissolves CO2 as it passes through atmosphere. Carbon dioxide reacts with water to form carbonic acid (weak acid).

When carbonated water passes over or through the rocks containing carbonates of calcium and magnesium, acid present in it reacts with these carbonates. As a result calcium hydrogen carbonate and magnesium hydrogen carbonate are formed which slowly dissolve rain water.

Some rocks may contain gypsum (CaSO4.2H2O) or anhydride (CaSO4) or Kieserite (MgSO4.2H2O) which is sparingly soluble in water. The presence of these dissolved salts causes the water to become hard.

Self-Assessment

1. List substances that cause hardness in water.

2. Differentiate between soft and hard water.

Answer.

i. Calcium hydrogen carbonate (Ca(HCO3)2)

ii. Magnesium hydrogen carbonate (Mg(HCO3)2)

iii. Gypsum (CaSO4.2H2O)

iv. Calcium sulphate anhydride (CaSO4)

v. Magnesium sulphate (MgSO4.H2O)

Hard water contains large amount of calcium and magnesium minerals while soft water is free from calcium and magnesium minerals. Hard water prevent the cleansing action while soft water while soft water increases the cleansing action of soaps and detergents.

Types of Hardness of Water

There are two types of hardness in water: temporary hardness and permanent hardness.

Temporary Hardness:

The harness of water that can be removed by boiling is called temporary hardness. Temporary hardness is caused by the presence of dissolved calcium and magnesium hydrogen carbonates.

Permanent Hardness:

The harness of water that cannot be removed by boiling is called permanent hardness. Permanent hardness is caused by the presence of dissolved sulphates and chlorides of calcium and magnesium.

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