Water can be softened by adding sodium carbonate (washing soda) or by passing the water through an ion-exchange column.
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.
Ca2+(aq) + CO32–(aq) → CaCO3(s)
The calcium ions come from the hard water and the carbonate ions from the washing soda.
Ion-exchange resins can also soften water that has temporary hardness as well as water that has permanent hardness. The resin 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.