The calcium ions in the water are usually originated from limestone and chalk, which is made up of calcium carbonate CaCO3, or from other mineral deposits in the form of calcium sulphate, CaSO4. Dolomite deposits (CaMg(CO3)2) contribute to the magnesium ions in the hard water.
Compared to soft water, hard water does not lather easily with soap or toothpaste. The presence of multivalent cations prevents lathering with soap solutions; instead, scum, a white precipitate is formed. The hardness in water causes water to resist soap by forming scales. For example, sodium stearate reacts with calcium:
2C17H35COONa + Ca2+ --> (C17H35COO)2Ca + 2Na+
Hard water can be defined as temporary hard water and permanent hard water. The temporary hardness in water is caused by a combination of Ca2+ ions and bicarbonate ions. Simple steps such as boiling or adding lime (calcium hydroxide, CaOH) can be used to convert to soft water.
Boiling helps to promote the formation of carbonate from bicarbonate and precipitate calcium carbonate. As a result, the cooled water is softened. The reaction of calcium carbonate CaCO3 being dissolved in water can be seen as below:
CaCO3 + CO2 + H2O < -- > Ca2+ +2HCO3-
Permanent water on the other hand cannot be softened by boiling. The calcium and magnesium sulphates or chlorides present in the water become more soluble as the temperature is raised. Although it is termed “permanent”, this hardness can be removed using water softeners, or ion exchange columns, where the calcium and magnesium ions are exchanged with the sodium ions in the column.
Ion exchange column used for softening hard water.
It is considered preferable to soften hard waters, although they do not bring any adverse health effects. However, hard water often cause calcification- the buildup of calcium in pipes and taps. This clogs pipes and causes leakages and bursts.
Calcification on a water tap.