Question: How does soap work? (Like shampoo) And why does it sting when you get it in your eyes?

Susan Cartwright answered on 18 Jun 2015:

Soap and shampoo are both examples of detergents, and they both work the same way. The key point here is that oil and water do not mix: if you take some water, and add some cooking oil to it, they may mix at first (especially if you shake the mixture!) but eventually the oil will settle out on top of the water.

Some chemicals will dissolve in water, and some will dissolve in oil, but usually not in both. Detergent molecules have one end which prefers water (hydrophilic, from the Greek for “likes water”) and one end that prefers oil (hydrophobic, from the Greek for “fears water”). Most dirt is fat-based, and when you use soapy water, the soap tries to stick all its fat-loving ends into the fat, and keep all its water-loving ends in the water. The net result is that it lifts little balls of fat off the dirty object (dish, clothes, skin) and carries them off into the washing water, making you cleaner.

I don’t exactly know why they sting your eyes. It may just be that they are molecules which are most definitely not the type of molecule that normally occurs in your eyes, and so your body wants you to get rid of them in case they’re damaging. If they sting, two things happen: it stimulates more tears, which help to wash the stuff out, and you rub your eyes, which helps to clear away the stuff that the tears have washed out. So I’d guess it’s a protective reaction.