Fat, turned inside out
How Is Soap Made?
Soap is made by cooking fat with something strongly alkaline until it becomes a completely different substance.
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Step 1 of 5
Water alone cannot shift grease.
Water molecules cling to each other and to other watery things. Grease is not watery, so water runs straight past it — you can rinse a greasy plate all day and it stays greasy.
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Step 2 of 5
Soap has one end of each kind.
Nothing else about soap matters as much as those two ends being on one molecule. A soap molecule is a long tail that likes grease with a head on the end that likes water. That is the entire invention — one molecule that can hold onto both things at once.
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Step 3 of 5
It surrounds the grease and lifts it.
The tails bury themselves in a spot of grease while all the heads point outward into the water. The grease ends up in a little ball wrapped in soap, and the water can now carry it away.
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Step 4 of 5
Making it changes the fat completely.
Lye burns skin on the way in. A finished bar has none of it left. Cook fat with a strong alkali and the fat molecules are cut apart and rebuilt with a water-loving end. Neither ingredient cleans anything; the thing you make out of them does.
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Step 5 of 5
It is how you stop a virus, too.
Twenty seconds is roughly how long the tails need to work their way in Many viruses are wrapped in a greasy coat. Soap tears that coat apart in exactly the same way it lifts grease off a plate — which is why washing hands works so well and so cheaply.
Hard water contains dissolved minerals that grab soap and make grey scum. Detergents were invented partly to get around exactly that.
The short version
Soap is made by cooking fat with an alkali, giving molecules with a grease-gripping tail and a water-gripping head.
Try it yourself
Put pepper on a bowl of water, then touch the middle with a soapy finger. The pepper flees — soap has changed the surface.