Experiment 3:

How does soap work?

Soap has a split personality. Its tail loves fat, but its head is wild for water. This experiment will demonstrate soap's split personality.

Dishwashing liquid
Vegetable oil
Two small transparent glasses
Water

Half-fill each glass with water. Add a tablespoon of vegetable oil to each glass.

Add a teaspoonful of dishwashing liquid to one of the glasses.

Shake the contents of both glasses.

The oil will also be at the surface of the second glass, but it will be in the form of droplets, instead of a smooth layer. This is due to the action of the soap.

Soap molecules are like pins: they consist of a head and a tail. The head attracts water molecules. The tail is attracted to fat molecules, like those in vegetable oil.
When you shake up the mixture, the oil becomes dispersed and takes the form of little droplets. The tails of the soap molecules then attach themselves to these droplets.
The soap molecules surround the droplets, forming a thin film around them. This film prevents the droplets from joining together and forming a uniform layer again - like they did in the first glass.

Since the oil stays dispersed, it is easily carried away by running water (when you wash your hands, for instance).
This is how soap deals with dirt, which contains fat. The tail of the soap molecule becomes attached to the fat in the dirt, and the head of the molecule binds with the water, which is running.

Leave the glasses for a few minutes.

Now observe the oil in both glasses. In the one without soap, there will be a smooth layer of oil at the surface.