1. Hard and Soft Water

2. Learning intentions
Describe the difference between hard and soft water
Explain the difference between permanent hard water and temporary hard water
Explain the precipitation reaction that takes place when temporary hard water is boiled.

3. Different types of water
In Wales we have soft water. It readily forms a lather with soap and does not cause problems with limescale in kettles, washing machines and dishwashers.
However in other parts of the country this is not the case and hard water can cause problems in appliances.

4. Key facts about hard and soft water
Soft water readily forms lather with soap
Hard water reacts with soap to form scum and so more soap is needed to form lather
Soapless detergents do not form scum

5. Hard water
What causes hard water? As water passes through rocks containing calcium and magnesium compounds these dissolve in the water causing hardness. Fact: ions in hard water are Mg2+ and Ca2+

6. Equations for forming hard water
Rain water is slightly acidic. This means when it falls on the rocks the following reaction happens. CaCO3 + H+  Ca2+ + HCO3-

7. How scum wastes soap
It is the dissolved Ca2+ and Mg2+ that react with soap to form scum.
Sodium stearate + calcium ions  calcium stearate (scum) + sodium ions
This means more soap is needed because of the soap wasted as scum. A lather is only formed when the calcium/magnesium ions have been used up.

8. How scale damages appliances
Temporary hard water contains hydrogencarbonate ions (HCO3-) that decompose on heating to produce carbonate ions
These react with calcium and magnesium ions to form precipitates of calcium carbonate (limestone) and magnesium carbonate.
These are solid scale

9. Damaging appliances
When temporary hard water is heated in a kettle, washing machine, dishwasher or central heating this leads to deposits of these metal carbonates which we call limescale.
It can significantly reduce the efficiency of these systems and in some cases prevents them from working at all.

10. You turn – exam style question

11. Scale vs. scum
Scale is caused by heating temporary hardwater to make precipitates of calcium and magnesium carbonate
Scum is caused by the reaction of magnesium and calcium ions with sodium stearate (soap)

12. Permanent hard water This is not softened by boiling
Because it does not contain HCO3- so the Mg2+ and Ca2+ ions remain after boiling rather than being removed
Seawater is permanent hard water
Permanent hard water can be softened by adding washing soda.