Soaps and detergents

Oil doesn't dissolve in water. So how does soap get an oily stain out of your shirt?

A soap molecule has two ends that want different things:

• an ionic head that loves water (hydrophilic);
• a long hydrocarbon tail that loves oil and avoids water (hydrophobic).

Soaps are sodium or potassium salts of long-chain carboxylic acids.

In water, soap molecules gather into balls called micelles. Picture people on a ship: those who love the sea stand at the railing facing the water, and the seasick ones hide in the middle. The heads face the water on the outside; the tails point inwards and grab the oily dirt in the middle.

The micelles stay floating in the water, carrying the dirt with them. Rinse with water and they are washed away. Rubbing or beating the cloth helps knock the dirt loose. The water gets dirty, and the cloth gets clean.

Soap solution looks cloudy because the micelles are big enough to scatter light. Soap is slightly basic: it turns red litmus blue.

In hard water, soap doesn't lather well. It reacts with calcium and magnesium salts in the water and forms a sticky white scum, so much more soap is needed. Detergents (ammonium or sulphonate salts of long-chain acids) do not form scum, so they work even in hard water. That is why shampoos and washing powders are detergents.