Making soap

Fats and oils are known as triglycerides. Each molecule of fat or oil contains three COO links.

Esters react with sodium hydroxide solution to form an alcohol and the sodium salt of the acid: CH 3 COOCH 3 + NaOH → CH 3 COO – Na + + CH 3 OH methyl ethanoate sodium ethanoate methanol In the same way, triglycerides react with sodium hydroxide to form the sodium salt(s) of their component acids, and glycerol.

The traditional source of fat for soap was tallow (from sheep), which is mainly composed of the saturated fatty acid C 17 H 35 COOH (stearic acid). Modern soaps are usually made with a mixture of fats and oils from different source, including vegetable oils such as palm or olive oil (to make Palmolive). To prove that any fat will do, in this demonstration we use butter.

Put about 5 g of butter in one test tube, and an equal volume of alcoholic sodium hydroxide solution in another test tube. Using alcohol as well as water to dissolve the NaOH stops the butter floating on top instead of reacting.

Heat the two tubes together in a water bath until the butter is melted. Then pour the warm sodium hydroxide solution into the butter and stir with a stirring rod while still heating.

The mixture will thicken and become quite viscous. Use the stirring rod to withdraw a little of the jelly-like substance and mix it with warm water in a separate test tube.

Bubbles will form: we have made soap. This soap works best in warm water.

We can separate out the soap from the glycerol by adding saturated sodium chloride solution. The solid soap rises to the top and can be skimmed off. Increasing the concentration of sodium ions in the solution makes the soap less soluble due to the common ion effect.

Soap.

A soap molecule has a long hydrocarbon ‘tail’ which is non- polar, and an ionic ‘head’ which attracts water molecules. The non-polar end of the molecule is attracted to droplets of grease or oil, while the polar ends point outwards, making the grease/soap unit miscible in water. How soap works