Making soap is a simple piece of applied chemistry. All that is needed is an oil or fat and an alkali; heating the materials together produces soap.
Originally, animal fats were used, including lard and tallow; now we generally use vegetable oils, such as palm and olive oils.
The alkali needed used to be extracted from plants by burning seaweed and stirring the ash with water to dissolve the alkali.
You will need some wood ash from the remains of a fire. Alternatively, burn some weeds in an old tin can and use the ash that remains.
Stir the cold ash with a small amount of water and filter the mixture. Use an indicator to test the solution - litmus (goes blue) or red cabbage indicator (goes green) are both suitable. The filtered solution will be strongly alkaline.
Making soap in school is rather hazardous since the alkali required is very corrosive. However, samples of commercial soaps can be tested safely, although some people have very sensitive skins and find that certain soaps cause irritation.
Ask pupils to bring in a small sample of the soap they use at home - a 1cm cube is sufficient.
Shake a sample of 5mm with half a tube of warm distilled water. Test the resulting solutions with indicator to find the most and least alkaline soaps. If you use pH indicator paper, the results can be ranked by pH value. Pupils who have sensitive skin may find that the more alkaline soaps cause skin problems.
Washing with soap in sea water doesn't work very well. The presence of salt reduces the solubility of the soap in water.
Test this by shaking a 5mm sample with salty water and comparing the appearance with the distilled water sample.
Both tap water and bottled waters contain dissolved minerals derived from rocks. Children can use a soap solution to compare how much material is dissolved.
Dissolve a small amount of soap in methylated spirits or ethanol, well away from any flames. Place equal volumes of bottled water samples in test tubes and add drops of soap solution.
After shaking, those samples with the most minerals dissolved will give the cloudiest mixtures.