1. Water hardness and soaps

2. Soaps are made of molecules that are both fat and water soluble
Soaps are made of molecules that are both fat and water soluble. The molecule has a long hydrocarbon tail that allows it to dissolve grease, and an ionic head that is water soluble.
Ionic head
Hydrocarbon tail

3. Fats and oils do not dissolve in water, so to remove them soap is needed.
The hydrocarbon tail attaches to the grease, while the ionic head dissolves in water. The grease particle can therefore be removed from the stain.

4. Soap and hard water 2RCOO-Na+ + Ca2+ (RCOO)2Ca + 2Na+
Sodium stearate calcium ions calcium stearate + sodium ions
(from soap) (in hard water) (scum)
Once all the calcium (and magnesium ions) have been precipitated out as scum, the soap can then lather up and remove grease.
Hardwater uses a lot more soap than soft water.

5. Temporary hardness
This is formed when limestone (calcium carbonate) reacts with acidic rainwater. It forms a solution of calcium hydrogencarbonate.
What is the formula for calcium hydrogencarbonate?

6. When temporary hard water is boiled, the calcium hydrogencarbonate breaks down.
It turns back into insoluble calcium carbonate thus removing the calcium ions (and therefore hardness) from solution.
calcium calcium + carbon + water
hydrogencarbonate carbonate dioxide
Ca(HCO3)2(aq) CaCO3(s) CO2(g) + H2O(l)
heat
heat

7. The calcium carbonate formed on heating temporary soft water is the limescale you get inside kettles and hot-water pipes.
Limescale build up can block water pipes and can damage washing machines, dishwashers etc.

8. Permanent hardness
This is caused by other calcium and magnesium compounds such as calcium sulfate (from gypsum).
Permanent hardness cannot be removed by boiling as the compounds do not decompose on heating.

9. Other methods of dealing with hardness
Washing soda (sodium carbonate). Sodium carbonate is soluble in water. When it is added to hard water, the carbonate ions remove the calcium ions in a precipitation reaction:
Ca2+(aq) + CO32-(aq) CaCO3(s)

10. Using a soapless detergent does not produce scum.
A water softener uses an ion exchange column. An ion exchange column is a glass tube filled with a resin. Calcium (or magnesium) ions become exchanged for sodium ions as water flows through the column.