The Concept of Hardness of Water

The importance of water and its hardness

Water is an essential substance for all living things. Over 70% of the earth's surface is covered by water, and there are various sources of water used in daily life. These sources include:

1. Rivers
2. Lakes
3. Wells
4. Ponds
5. Springs
6. Streams

Hardness of water

Hardness of water is caused by dissolved calcium and magnesium compounds. These compounds include:

1. Calcium sulphate
2. Calcium hydrogen carbonate
3. Magnesium sulphate
4. Magnesium hydrogen carbonate

Reaction causing hardness

When soap is added to hard water, a reaction occurs, producing scum. For example:

Calcium sulphate + Sodium stearate → Calcium stearate (scum) + Sodium sulphate

The calcium stearate forms an insoluble scum, while the sodium sulphate dissolves in water.

Formation of calcium hydrogen carbonate

Calcium hydrogen carbonate is the most common cause of water hardness. It forms when rainwater, containing dissolved carbon dioxide, reacts with rocks such as limestone and chalk. These rocks are primarily composed of insoluble calcium carbonate.

The process is as follows:

Water + Carbon dioxide + Calcium carbonate → Calcium hydrogen carbonate

$$\text{H}_2\text{O}(l) + \text{CO}_2(g) + \text{CaCO}_3(s) \rightarrow \text{Ca(HCO}_3)_2(aq)$$

Sources of hard water

Other rocks that contribute to water hardness include:

• Dolomite: Composed of calcium magnesium carbonate.
• Gypsum: Composed of calcium sulphate dihydrate.