How soap cleans

Soap gets round the problem by being two things at once. Each soap molecule is a long tail of carbon and hydrogen, much like a strand of oil, with a small charged head at one end. In soap made from olive oil a typical tail holds seventeen carbon atoms, and the head just one, joined to two oxygens.

The head is hydrophilic, Greek for "water-loving": its negative charge draws water molecules to it. The tail is hydrophobic, "water-fearing", and mixes easily with grease. Molecules built this way, with one end for water and one for oil, are called surfactants, short for surface-active agents. They gather wherever water meets oil or air, with their heads in the water and their tails pointing out of it.

Now add soap to the greasy plate. The tails push into the grease, the one place they mix, while the heads stay out in the water. Rubbing breaks the grease into smaller and smaller drops, and soap molecules crowd round each drop and wrap it, until it sits inside a tiny ball: grease at the core, tails packed around it, and the charged heads facing out into the water. A cluster like this is a micelle, far too small to see.

Two things now keep the grease in the water. The outside of each ball is all water-loving heads, so water holds on to it. And every head carries the same negative charge, so the balls repel one another instead of merging back into a greasy film. The rinse water carries them off the plate.

Soap is made from fat or oil. A fat molecule is shaped like a comb with three long teeth: a small backbone called glycerol, with three fatty chains hanging from it. Olive oil, butter and animal tallow are all built this way.

Heat the fat with lye, a strong alkali such as sodium hydroxide, and the alkali snaps each fatty chain off the backbone. Every freed chain now ends in a charged head: it has become a soap molecule. So one fat molecule gives three molecules of soap and one of glycerol, a sweet, syrupy liquid. The reaction is called saponification, from the Latin sapo, soap.

Before chemical works, the alkali came from fire. Burn certain plants, such as saltwort from salty ground, and their ash is rich in soda, sodium carbonate. The Arabic name for such ash, al-qaly, is where the word alkali comes from. But soda is only a mild alkali, so soapmakers made it stronger with lime, which pulls the carbonate out of the soda and leaves caustic soda, sodium hydroxide, in the water.

A treatise on the crafts attributed to al-Muẓaffar Yūsuf, sultan of Yemen in the thirteenth century, sets out the method: two parts ash to one part lime, water poured through them again and again, and the sharp liquid that drains out cooked with oil. That caustic liquid is lye. Nablus made its lye the same way until factory-made caustic soda began to replace the ash in the 1860s.

Soap is far older than Islam, but in the olive country of Palestine and Syria it grew into a great export trade. Writing in 985, the Jerusalem geographer al-Muqaddasī listed soap among the goods of Palestine, beside olives, figs and raisins. About three centuries later al-Dimashqī wrote that Nablus made fine soap from its olive oil and sent it to every land and to the islands of the Mediterranean.

Levantine soap was hard, which let it be cut up and shipped. The metal in a soap sets its texture: soap made with sodium, from the soda in saltwort ash, sets firm enough to cut into bars and cubes, while soap made with potassium, from the potash in wood ash, stays soft. Nablus still makes its olive-oil soap in cubes, and Aleppo's adds oil from laurel berries. UNESCO added both crafts to its heritage lists in 2024.

In some towns soap lathers at once; in others it leaves a scum and a ring round the bath. The difference is in the water. Rain soaking down through soil and rock dissolves minerals on the way, and water that carries a lot of dissolved calcium and magnesium is called hard water.

Calcium and magnesium float in water as ions, each with a double positive charge, and each one grabs the negative heads of two soap molecules. The trio no longer dissolves: it drops out as an insoluble white solid, soap scum. Every soap molecule caught this way is lost to cleaning. So in hard water the calcium uses up the soap first, and only the soap added after that can make a lather and lift grease.

The way round soap scum came out of a war, though not by design. During the First World War, Germany ran short of the fats needed for soap, and its chemists made the first synthetic detergents from coal tar.

A detergent is built on soap's plan, a water-loving head on a grease-loving tail, but it is made in a chemical works rather than from fat, and its head is different: a group built around sulfur, such as a sulfonate, in place of soap's head of carbon and oxygen. Calcium holds on to that head far more weakly, so a detergent stays dissolved and keeps cleaning in hard water. After the Second World War detergents took over the washing machine, and by the end of the 1950s they had largely replaced soap for laundry in the richer countries.

Soap does more than lift grease. Some viruses, including flu and the coronavirus behind COVID-19, wrap their genes in an envelope: a thin, fatty membrane taken from the cell they were made in. Studded through it are the proteins the virus uses to latch on to its next cell.

A fatty layer is exactly what soap's tails push into. They wedge into the envelope and break it up, carrying its fatty molecules off into micelles, and the virus falls apart. With its envelope gone it can no longer latch on to a cell and infect it. That is one reason soap works so well against these viruses: their weak point is fatty, like grease.

Not every virus has an envelope. Norovirus, the stomach bug that sweeps through cruise ships and hospital wards, keeps its genes inside a tough shell of protein, with no fatty layer for soap to break. Alcohol hand gel, which works well against enveloped viruses such as flu, does poorly against it too.

Soap and water still get rid of norovirus, by a different route. The same two-ended molecules that lift grease also loosen germs stuck to the skin, rubbing works them free, and running water carries them away. Against a virus it cannot break, soap works by removal: it need not destroy the virus, because it washes it off your hands and down the drain. That is why health agencies tell people facing norovirus to wash with soap and water rather than rely on gel.

Does soap really beat a rinse? A study published in 2011 put it to the test. Twenty volunteers touched door handles and railings in public places, then left their hands unwashed, rinsed them in water alone, or washed them with plain soap and water. Their hands were then sampled for bacteria of the kind found in faeces.

Such bacteria turned up on 44 per cent of the samples from unwashed hands, 23 per cent after water alone, and 8 per cent after soap and water. Water on its own helps, but soap's surfactants lift off grime and microbes that cling to the skin, cutting the rate to about a third of what water alone left. Plain soap was all it took: the soap had no antibacterial ingredient. And in 2016 America's drug regulator ruled that makers of antibacterial washes had not shown them to beat plain soap and water.

How long should a wash last? Health agencies advise lathering and scrubbing for at least 20 seconds, about as long as humming "Happy Birthday" twice. Scrubbing matters because friction helps lift dirt, grease and germs from skin, and the soap needs time to reach the backs of the hands, between the fingers and under the nails.

The evidence behind the number is thinner than the slogan. Studies suggest that a 15 to 30 second wash removes more germs than a shorter one, but few have checked whether it prevents illness. Hot water is no shortcut either: the temperature of the water makes no clear difference to how many germs come off, and warmer water is harder on the skin.