When water is heated, certain metal ions in the water, due to evaporation and concentration, will form scale and adhere to the heated surface, the concentration of these ions is called hardness. For natural water, these ions are mainly calcium ions magnesium ions, other ions such as iron, manganese, zinc ions, etc. also form hardness, but as these ions are very small in natural water, they are negligible. So the hardness of water is the total concentration of magnesium ions in the water. How is the hardness of water classified?
The hardness of water can be divided into two categories: carbonate hardness (also known as 'temporary hardness') and non-carbonate hardness (also known as 'permanent hardness').
(1) Carbonate hardness: This type of hardness is mainly the bicarbonate of calcium and magnesium ions. This type of hardness is called "temporary hardness" as it forms precipitates after heating and precipitates out of the water.
(2) Non-carbonate hardnesses: These are mainly sulphates and chlorides of calcium and magnesium ions. As they cannot be precipitated out of the water by heating, they are called "permanent hardness".
