Hard Water

There are 2 types of water – Soft and Hard.  The type depends on the level of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions are dissolved in the water.

Water is considered to be soft if it contains less than 100mg of Ca²⁺ ions per litre.  Concentrations greater than this are consider to be hard.

How hard water forms

Rain water is naturally acidic as it contains dissolved carbon dioxide forming a weak carbonic acid.  When rainwater passes along certain rocks e.g. limestone (calcium carbonate, CaCO₃) or gypsum (calcium sulfate, CaSO₄) on its way to a reservoir, the acidic water will react with the limestone producing dissolved calcium ions – Ca²⁺:

CaCO3(s)	+	H2O(l)	+	CO2(aq)	→	Ca2+(aq)	+	2HCO3-(aq)
		Water and carbon dioxide combine to form
		carbonic acid				‘hardness’

Magnesium ions, Mg²⁺ can also collect in water in a similar way.

The following clip explains hard water:

Types of hard water – Temporary and permanent

Temporary - This water contains dissolved calcium and magnesium hydrogen carbonates – CaHCO₃ and MgHCO₃.  When boiled:

Ca²⁺(aq)  +  2HCO₃^-  →  CaCO₃(s)  +  H₂O(l)  +  CO₂(g)

The calcium ions become trapped in calcium carbonate which is insoluble, this is seen as scale on a kettle and thus removing the hardness. 

Permanent - This water contains dissolved calcium and magnesium sulfates – CaSO₄ and MgSO₄.  When boiled the calcium and magnesium ions remain in solution so the water is still hard.

The following clip explains how hard water can be turned into soft water: