MYP 8 Science - Chemistry

MYP 8.6.2: Hardness in water


  • To define hardness as a reaction that prevents soaping action
  • To know the difference between temporary and permanent harness
  • To recognise the appearance of 'scum' as a sign of hardness
  • To know the consequences of hardness on domestic and industrial equipment
  • To know some of the treatments for hardness in water
  • To know that calcium and magnesium (positive) ions are responsible for hardness in water by reacting with the stearate (negative) ions of soap.


The soaping action involves removal of hydrophobic particles by soap micelles. The effectiveness of soap can be measured by its lathering action on shaking. Hardness can then be measured by looking at how much water is needed to prevent a lathering action.

The students have prepared soaps in the previous section of this unit. This could be used against commercial soaps and detergents.

Student activity

Four samples of water are tested on a standard volume of soap solution, prepared by dissolving soap in distilled or deionised water.

The water samples are added 1 ml at a time and the test tube shaken. When there is no lather the soap is assumed to have all reacted with the harness in the water.

It is possible to rate the samples in order of hardness.

Fresh samples of the four solutions are then boiled for 1 minute and then re-tested after cooling.

The students should then be able to identify which sample contains temporarily hard water.





Technical notes

The four samples are:

1. Distilled water

2. Tap water

3. A solution containing magnesium sulphate (2g / 100ml)

4. A solution containing calcium hydrogencarbonate (prepared by bubbling carbon dioxide through limewater until colourless, then filtering)

These should be coded A, B, C, D (not necessarily in this order)

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