Hardness was originally defined as the capacity of water to precipitate soap.
Hardness of water is due to the presence of calcium and magnesium salts: bicarbonates, carbonates, chlorides, sulphates and nitrates. Presence of iron, aluminium and manganese salts also contributes to water hardness, but normally these are not present in appreciable quantities. Normal carbonates are also found, but only occasionally and in highly alkaline water. Nitrates are usually present in minor quantities while it has been found that sulphates on the average exceed chlorides.
Removing hardness from water is called softening and hardness is mainly caused by calcium and magnesium salts. These salts are dissolved from geologic deposits through which water travels. The length of time water is in contact with hardness producing material helps determine how much hardness there is in raw water.