Soft water does not contain dissolved substances that produce scum and scale.
We can soften hard water by removing the Calcium and Magnesium ions which give it its hardness. Softening water had big benefits for washing ourselves and our clothes, and heating our water too. But people are advised to continue to drink hard water if they can, since scientists think that this is better for them.
Soft water is also important in many industrial processes too, where hardness can produce scale in boilers (making them expensive to run). Hardness may also interfere with chemical processes like dyeing. There are 2 important ways to soften water.
Removing Hardness 2
One way to soften water is to add Sodium Carbonate to it. Sodium Carbonate is also called washing soda, because it has been used when people have washed clothes for many years.
When we add washing soda to hard water, it precipitates out Calcium and Magnesium ions as insoluble carbonates. Once these ions, which cause 'hardness', are no longer in the solution, they cannot react with the soap. This means that the water is soft.
This reaction is similar to formation of limescale when hard water is heated. However, here it happens quickly, where and when we want it to happen.
Removing Hardness 3
Water can also be softened by removing the Calcium and Magnesium ions using an ion-exchange column. These columns contain Sodium ions which are exchanged for the Calcium and Magnesium ions in hard water when it passes through the column. This is how domestic water softening units work. Some people have these units in their homes to soften all of the water used for showering, bathing and washing clothes. A dishwasher contains its own water softening system.
Once all of the Sodium ions in the resin have been exchanged for Calcium and Magnesium ions, the resin is washed with a salt solution to exchange these ions for Sodium ions. This is why water softeners must be kept topped up with salt (Sodium Chloride). The salt keeps the resin supplied with Sodium ions.
For health reasons, houses fitted with water softeners normally have one cold water tap in the kitchen which is supplied with water which has not been softened.