Hardness in Water

Hard water is water that contains high amounts of minerals, mainly calcium and magnesium ions.
The hardness of water can be categorised into two types: temporary hardness and permanent hardness.
Temporary hardness is caused by the presence of calcium hydrogencarbonate and magnesium hydrogencarbonate. This type of hardness can be removed by boiling the water which decomposes the hydrogencarbonates to give carbon dioxide, water and the corresponding carbonate.
Permanent hardness is caused by sulphates and chlorides of calcium and magnesium. Boiling does not remove this type of hardness.
Hard water can be a problem because it causes limescale to form in kettles and pipes. This is due to the thermal decomposition of calcium hydrogencarbonate.
Limescale can cause heating elements to become less efficient and it can block pipes, reducing water flow.
While hard water is not harmful to drink, it can cause other issues such as making it harder to create a lather with soap. This is because the calcium and magnesium ions react with the soap to form scum.
Hard water can be softened by adding chemicals such as sodium carbonate. The sodium carbonate reacts with the calcium and magnesium ions to form insoluble carbonates which can be removed from the water.
Another way to soften hard water is to use an ion-exchange resin which swaps the calcium and magnesium ions with sodium ions as the water passes through it.
It is important to note that softened water will often still contain other minerals and impurities, and so may not be suitable for use in certain laboratory experiments where pure water is required.
While hard water may have some downsides, it is also thought to have health benefits due to its higher mineral content. Calcium, for example, is important for bone health.