2.1 Hard water
Hard water contains dissolved compounds such as calcium and magnesium salts.
The calcium and/or magnesium ions in hard water react with soap producing a precipitate called scum.
One type of hard water can produce a solid scale when it is heated, reducing the efficiency of heating systems and kettles because scale is a poor conductor of energy.
Hard water is better than soft water for developing and maintaining teeth and bones. It may also help to prevent heart disease.
2.2 Removing hardness
Soft water does not contain ions that produce scum or scale.
Hard water can be softened by removing the ions that produce scum and scale.
Temporary hardness is removed from water by heating it. Permanent hardness is not changed by heating.
2HCO3- (aq) --> CO32- (aq) + H2O (l) + CO2 (g)
The hydrocarbonate ions in temporary hard water decompose on heating. The carbonate ions formed react with Ca2+ (aq) and Mg2+ (aq) ion, making precipitates.
Either type of hard water can be softened by adding washing soda or by using an ion-exchange resin to remove calcium and magnesium ions.
2.3 Water treatment
Water for drinking should only contain low levels of dissolved substances and microbes.
Water is made fit to drink by filtering it to remove solids and adding chlorine to reduce the number of microbes.
We can make pure water by distillation but this requires large amounts of energy which makes it expensive.
To produce water that is fit to drink:
- use a suitable source
- remove solids by filtration
- kill microbes by adding chlorine