Making soap

Soap is made by reacting a fat with sodium hydroxide. Traditionally sheep fat (tallow) was used, but modern toilet soaps are often made using 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.

When sodium hydroxide reacts with fat, the ester links break to form the sodium salt of the fatty acids contained in the fat (which is soap), and glycerol (propan-1, 2, 3, triol). 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.

Soap.