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Distillation can be used to separate and collect a liquid from a solution. Distillation can also be used to soften water as the water evaporates as it is heated and the ions are left behind.

Hard water can be softened by adding sodium carbonate (washing soda) or by passing the water through an ion-exchange column.

Washing soda

Sodium carbonate, Na₂CO₃, is also known as washing soda. It can soften water that has temporary hardness and it can soften water that has permanent hardness.

It is an inexpensive method of removing hardness but leads to the formation of limescale, which can block narrow water pipes.

[Higher tier only]

Sodium carbonate is soluble in water and adds a large amount of carbonate ions to the water. These react with dissolved calcium ions, forming a precipitate of calcium carbonate:

Ca²⁺(aq) + CO₃^2–(aq) → CaCO₃(s)

The calcium ions come from the hard water and the carbonate ions from the washing soda.

Ion-exchange

Ion-exchange columns can also soften water that has temporary hardness as well as water that has permanent hardness. The hard water passes through the column continuously, where the calcium and magnesium ions are removed. This technique is more convenient than using washing soda because you don’t need to change the column very often. However, the ion-exchange columns are expensive.

[Higher tier only]

The resin within the column is made into small balls around 1-2 mm in diameter, which are packed into a tube or ‘column’.

The ion-exchange resin starts with sodium ions stuck to it. As the hard water passes through the column, sodium ions come off the resin and go into the water, while calcium ions come out of the water and stick to the resin. In effect, calcium ions that cause hardness are swapped for sodium ions that do not cause hardness.

Dishwashers contain ion-exchange resin to soften the water. The resin needs recharging with dishwasher salt (sodium chloride) once it becomes full of calcium ions.