Soap

Soap has been made since at least the Roman times. A type of toilet soap used by the Roman nobility was made using animal fat together with wood ashes mixed with lime.

Wood ashes contain potash, and potash soaps, which are still used today, are the kind known as soft soaps. These soft soaps always make good lather when mixed with water.

Hard soaps do not make such a good lather, but they last longer than soft soap. For this reason they were suitable for such purposes as scrubbing doorsteps and floors. Hard soaps are made in a similar way to soft soaps, but using soda instead of potash.

From the 18th century onwards, soap has been made on a larger scale in factories. In a soap factory, soap is made enormous metal pans known as soap kettles.

In a soap kettle, animal or vegetable fat is mixed with an alkali and water. For soft soap, the alkali is caustic potash and for hard soap the alkali is caustic soda.

The mixture is then boiled, and brine, a solution of common salt is added. This causes the soap to separate from the mixture as a floating curd. After more boiling all the fats remaining in the curd are made into soap, or saponified. The purified curd can then be pressed into moulds to make bars of soap or flaked to make soap flakes.

The cleansing action of soap depends on the way its molecules behave. Soap molecules have the long, rather tadpole like shape. The tails of these molecular tadpoles are repelled by water, but are attracted to oily or greasy substances. This causes them to attach to particles of greasy dirt, for example clothes being washed. In contrast to the tails, the head of the molecules are attracted by water and are repelled by oil or grease. They surround each dirt particle, so separating it from the article being washed. The dirt particles, with their coating of soap molecules, float off and are seen as a soapy scum on the surface of the water.