1. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press Washing soda and ion-exchange columns

2. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  Hard water can be softened by adding soda crystals, for example when washing clothes.  Both temporary and permanent hardness are removed by washing soda. Without it, extra detergent would be needed in hard water areas.  It can also be used to remove grease from cookers and pans, and unblock sinks and drains.

3. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  Washing soda is sodium carbonate.  The white crystals dissolve in water to make an alkaline solution.  It is unlike most other carbonates, including calcium carbonate and magnesium carbonate, which are insoluble.

4. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press When washing soda is added to hard water, a precipitation reaction happens. For example: Ca(HCO 3 ) 2 (aq) + Na 2 CO 3 (aq)  CaCO 3 (s) + 2NaHCO 3 (aq) The hard water is softened because the calcium ions and magnesium ions are removed as a precipitate. calcium hydrogencarbonate sodium carbonate calcium carbonate sodium hydrogencarbonate +  +

5. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  There is another way to remove both temporary and permanent hardness from water.  Ion-exchange columns contain ion-exchange resins, tiny beads packed into a plastic or metal tube.  Hard water is softened as it passes through an ion- exchange column.

6. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  This is ideal as a permanent solution to hard water in the home.  The ion-exchange column is usually plumbed into the home’s water supply so that all the water used in the home is softened.

7. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  Ion-exchange resins work by swapping ions in the water for ions from the resin.  As the hard water passes through, calcium ions and magnesium ions from the water attach to the resin and sodium ions leave it.  The water is softened because the calcium ions and magnesium ions are removed.

8. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  Eventually, all the sodium ions in the resin are replaced by calcium ions and magnesium ions.  The resin is regenerated by flushing sodium chloride solution through it.  Sodium ions are swapped back into the resin. Calcium ions and magnesium ions are washed away in waste water.

9. IC6.12.3 Washing soda and ion-exchange columns © Oxford University Press  Dishwashers contain ion- exchange columns to soften water.  This stops white streaks of precipitates being left on plates and glasses as they dry.  Salt has to be added regularly to regenerate the ion-exchange resin.