In some areas, extra soap is necessary for washing because the water is too “hard” to produce good suds. Hard water contains dissolved mineral salts, such as magnesium and calcium. Soap combines with the salts in the water and forms a gummy scum that is difficult to rinse off. The salts also form a crust-like deposit that clogs pipes and boilers.
Sea water, which contains large amounts of salts, is very hard, while rainwater is quite soft. Water softeners work by chemically exchanging harmless elements for the salts in the water.
Hard water is formed when water percolates through deposits of limestone and chalk which are largely made up of calcium and magnesium carbonates.
The more calcium and magnesium dissolved in the water, the harder the water becomes. This is why certain cities and counties within the same state can have varying degrees of water hardness – from slightly hard to very hard.
Temporary hard water can be softened by boiling it. Permanent hard water stays hard, even when it is boiled. Temporary hard water contains dissolved hydrogen carbonate ions, HCO3–.
When heated, these ions decompose (break down) to form carbonate ions, CO32–. The carbonate ions in the boiled water react with dissolved calcium and magnesium ions to form insoluble precipitates (calcium carbonate and magnesium carbonate).
Permanent hard water contains dissolved sulfate ions, SO42–. These do not decompose when heated. They remain dissolved and do not react with calcium and magnesium ions – so the water stays hard even when boiled.
Hard water has some benefits compared to soft water. For example, the dissolved calcium compounds in hard water:
- can improve the taste of the water
- are good for the development and maintenance of bones and teeth
- can help to reduce heart disease
But hard water also has some drawbacks compared to soft water. For example:
- More soap is needed to produce lather, which increases costs. This happens with temporary or permanent hardness.
- The scum produced is unsightly – spoiling the appearance of baths and shower screens, for example.
- Temporary hardness can reduce the efficiency of kettles and heating systems. This is because limescale (a solid containing calcium carbonate) is produced when the water is heated. It coats the heating element in kettles, and the inside of boilers and hot water pipes. This means more energy is needed to heat the water, again increasing costs. Pipes may become blocked by limescale – causing the heating system to break down.