Hard water is water that contains high amounts of mineral ions, especially calcium and magnesium, which can precipitate out and cause problems in water conducting or storing vessels like pipes/Descriptive_Chemistry/Main_Group_Reactions/Hard_Water). Hard water can be distinguished from other types of water by its metallic, dry taste and the dry feeling it leaves on skin. It is responsible for the scum rings seen in bathtubs, as well as the inability of soap to lather/Descriptive_Chemistry/Main_Group_Reactions/Hard_Water). Hard water can be classified into two types:

• Temporary hard water: This type of hard water consists primarily of calcium and bicarbonate ions. Heating causes the bicarbonate ion in temporary hard water to decompose into carbonate ion, carbon dioxide, and water. The resultant carbonate ion can then react with other ions in the solution to form insoluble compounds, such as CaCO3 and MgCO3/Descriptive_Chemistry/Main_Group_Reactions/Hard_Water).

• Permanent hard water: This type of hard water is caused by the presence of calcium and magnesium ions in water, which react with the higher fatty acids of soap to form an insoluble gelatinous curd, thereby causing a waste of the soap. In boilers, the calcium and magnesium in hard waters form a hard, adherent scale on the plates. As a result of the poor heat conductivity of the scale, fuel consumption is increased, and the boiler deteriorates rapidly through the external overheating of the plates.

Hard water can be softened through various processes, including ion exchange and the use of chelating agents/Descriptive_Chemistry/Main_Group_Reactions/Hard_Water).