What if soap did not exist?

You eat a buttery snack. Your fingers feel slippery.

You rinse them with water. They still feel greasy! Then you use soap, and the grease washes away.

What if there were no soap anywhere?

After a buttery snack, try rinsing your fingers with water alone. They stay slippery, because oil and water don't mix: the water just runs over the grease. Add soap, rub, rinse, and the greasy feeling is gone.

So what is soap doing that water can't? And what if soap did not exist? We can't make soap vanish, but we can follow the clues, from the molecules in a soap bubble to the germs on your hands.

Plain water is a poor cleaner of oily soil. Water molecules are polar and attract one another strongly, while fats and oils are nonpolar, so the two separate instead of mixing. Soap bridges that gap.

This thought experiment asks what would change if soap disappeared. It follows three threads: why oil and water stay apart, how a soap molecule grips grease, and what health researchers have measured about washing hands with soap. Some consequences are well supported; others are reasoned ideas, and each is labeled.

We use soap every day. We wash our hands, our bodies and our dishes.

Soap makes bubbles when we rub it. The rubbing helps lift dirt off our skin.

Soap is so ordinary that it's easy to forget how clever it is. Scientists at the CDC, the U.S. public health agency, say that washing hands with soap removes germs from hands, and that soap works better than water alone.

Here is how they say to do it: wet your hands, lather them with soap, scrub for at least 20 seconds, rinse well under running water, and dry them. The rubbing helps too: it makes friction, which helps lift dirt, grease and germs off the skin.

Soap is a mixture of the sodium or potassium salts of long-chain fatty acids. It is made by saponification: treating animal fat with an alkali such as sodium hydroxide, which gives soap and glycerol.

The U.S. Centers for Disease Control and Prevention (CDC) describes how handwashing works in practice: the surfactants in soap lift soil and microbes from skin, and lathering and scrubbing add friction that helps. Scrubbing for at least 20 seconds removes more germs than shorter washes, and the CDC notes that water temperature does not appear to affect germ removal.

Now imagine there is no soap. You rinse with water only.

Some dirt comes off. But the grease stays, and more germs stay on your hands.

Without soap, water would still rinse off some loose dirt, and scrubbing would still help. But greasy dirt would cling on, because water can't mix with grease. The CDC says using soap removes germs more effectively than water alone, so more germs might stay on people's hands.

Dishes, clothes and floors would be harder to clean too. People would probably look for other ways to lift grease, like scraping, scrubbing harder, or using other cleaners. These last ideas are our guesses, not facts.

Remove soap and the cleaning job falls to water and friction. Water rinses away water-soluble dirt, and scrubbing dislodges some particles, but oily films and the microbes held in them are poorly removed: the CDC states that using soap is more effective than using water alone.

From there the consequences become ideas. Laundering and dishwashing would lose an important cleaning agent; people might lean on synthetic detergents, or on scrubbing and scraping. Whether illness would rise, and by how much, is reasoned from handwashing studies rather than observed in a world without soap.

A soap molecule is tiny. It has a head and a long tail.

The tail holds on to grease. The head holds on to water. So the water can carry the grease away!

Each soap molecule is shaped a bit like a tadpole. Its head is attracted to water, so scientists call it water-loving (hydrophilic). Its long tail avoids water but mixes happily with oil, so it is water-fearing (hydrophobic).

When you wash, the tails push into the grease and the heads stay in the water. Many soap molecules surround a tiny blob of grease, tails inside and heads outside, making a little ball called a micelle. The water grabs the heads, and the whole ball, grease and all, rinses down the drain.

A soap molecule's carboxylate end is ionic and therefore hydrophilic; its long hydrocarbon chain is nonpolar and hydrophobic. In water, the tails cluster together inside a tangled ball while the ionic heads face outward into the water: a micelle.

Grease and oil droplets are solubilized when they are coated by the hydrophobic tails at the center of micelles, and they can then be rinsed away. Hard water weakens soap: its calcium and magnesium ions convert soluble sodium carboxylates into insoluble salts, the scum that rings a bathtub. Synthetic detergents based on sulfonate heads avoid this, because their calcium and magnesium salts stay dissolved.

Germs are too small to see. They can get on our hands when we touch things.

Washing with soap helps keep germs off our food and out of our mouths. That helps us stay healthy.

Germs can get onto hands when people use the toilet or touch things that have germs on them. People touch their eyes, nose and mouth without even noticing, and that is one way germs get into the body.

The CDC reports that teaching people in a community to wash their hands cuts the number of people who get sick with diarrhea by 23 to 40 percent, and cuts colds and other breathing illnesses by 16 to 21 percent. Need a timer for your 20 seconds? The CDC suggests humming "Happy Birthday" from beginning to end twice.

Soap has a long history. As early as 2800 BC, people in Babylon boiled fats with ashes to make a soap-like substance, and Egyptian medical papyri from 1550 BC show that people there bathed regularly with soap. Wood ash supplied the alkali until the early 1800s, when the Leblanc process became available.

Today the evidence for soap is measured. The CDC reports that handwashing education in communities reduces the number of people who get sick with diarrhea by 23 to 40 percent and respiratory illnesses such as colds by 16 to 21 percent. Germs on unwashed hands can also get into food and drinks while people prepare or eat them.