Micelles

A soap molecule has one end for water and one end for grease, so it can hold on to both at once. That is how soap lifts grease off a plate.

In soapy water on a greasy plate, the tails of the soap molecules push into the grease, and the heads stay out in the water. Thousands of them coat the grease all over. As you scrub, the grease breaks up into tiny droplets, and each droplet ends up wrapped in soap molecules: tails in, heads out. This little ball is called a micelle (say my-SELL).

The outside of every micelle is covered in charged heads, and water molecules crowd round them, so the water holds each micelle and carries it away when you rinse. All the heads carry the same negative charge, and like charges push apart, so the droplets stay separate in the water. The tiny droplets scatter light, which is why washing-up water turns cloudy.

Warm water helps, because it melts solid fats so they break up easily. Soap on its own in water gathers into micelles too, each only a few millionths of a millimetre across, with the tails tucked away inside.

Washing-up liquid

Washing-up liquid works the same way. Its cleaning molecules are detergents, made in factories from crude oil or plant oils, most with a long hydrocarbon tail and a charged head. Many have a sulfate head, -⁠OSO₃⁻. Sodium laureth sulfate, in many washing-up liquids, is a close relative of the sodium lauryl sulfate that makes toothpaste and shampoo foam.