Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step. The models are simplified to show the idea, not to measure anything.
Tap the buttons and try things out.
Pick water or soap, then tap the pan. Which one lifts the grease?
Water carries away hydrophilic dust; hydrophobic grease only lifts once soap is added.
Tap the pan with water, then with soap. What happens?
Drag a soap molecule into the water or the oil. Where does it end up, and which way does the tail point?
Drag a surfactant molecule into either phase: it returns to the interface, tail in the oil, head in the water.
Drag a soap bit. Where does its tail go?
Rub back and forth over the grease. How many micelles can you make? Then rinse.
Mechanical scrubbing breaks the coated grease into micelles; rinsing carries them off.
Rub over the grease. Then rinse it away!
Tap each germ to send in the soap. Which ones pop, and which get rinsed off?
Lipid-enveloped microbes are disrupted; non-enveloped ones are removed mechanically.
Tap a germ. Does it pop?
Pour in lye, or tap a fat molecule. How many soap molecules does each fat make?
Each triglyceride + base gives three fatty-acid salts (soap) and one glycerol.
Tap the button and watch fat turn into soap.
Choose water or soap and a scrub time, then wash. What is left behind?
Compare water with soap and change the scrub time. What stays on the skin?
Pick soap, then tap Wash! Is the hand clean?
The odds in this toy are made up to show the idea; they are not measured data.
Water alone can't wash off grease. Oil and water don't mix! Soap helps them get along.
Soap is made of tiny, tiny bits. Each one looks like a tadpole. The round head loves water. The long tail loves grease.
When you scrub, the tails dig into the grease. The heads stay out in the water. The grease gets wrapped up in tiny soap balls. Then the water washes them away.
Some germs have a thin, greasy coat. Soap pops that coat open! Other germs get lifted off your skin and rinsed away.
Wet your hands, add soap, and scrub all over for at least 20 seconds. That's like singing "Happy Birthday" two times. Then rinse and dry.
Soap works because each soap molecule has two very different ends. One end, the head, is attracted to water. The other end, a long tail, avoids water but is attracted to oil and grease.512
When you wash, the tails dig into the grease and dirt on your skin while the heads stay out in the water. The soap molecules then wrap the grease into tiny balls called micelles, with the tails tucked inside and the heads facing out. Because the outside of a micelle is water-loving, running water can carry it away.58
grease + soap → micelles → rinsed away
Soap can also break open some germs. Viruses and bacteria that are wrapped in a fatty coat get pried apart by soap tails, and scrubbing lifts the rest off your skin.89
You already know that oil and water don't mix: shake up a salad dressing and the oil always floats back to the top. Chemists call substances like oil hydrophobic, which means "water-fearing". Things that dissolve in water, like salt or sugar, are hydrophilic, or "water-loving".10
The dirt on your hands is a jumble of both kinds. Dust and dead skin are mostly water-loving, but the oils from your skin and from food are water-fearing. So if you rinse with water alone, you only remove the water-loving bits, and the greasy part stays behind.10 That's why water beads up and runs off a greasy pan.
A soap molecule is shaped a bit like a tadpole or a pin: a small round head and a long tail.810
| Part | What it's made of | What it likes |
|---|---|---|
| Head | An electrically charged group (a carboxylate) | Water: it is hydrophilic |
| Tail | A long chain of carbon and hydrogen atoms | Oil and grease: it is hydrophobic |
Source for the table: 5
Because it likes both, a soap molecule settles right at the border between oil and water, tail in the oil and head in the water. Chemists describe it as a bridge between two things that normally refuse to mix.712 Soap also lowers the surface tension of water, so water spreads out and wets a surface instead of beading up.12
Here is what happens, step by step, when you wash a greasy hand:
Soap doesn't really dissolve grease. It packages it in water-friendly wrappers. The micelles also keep the dirt spread out in the water so it doesn't simply stick back onto the surface you just cleaned.12
Some viruses and bacteria are wrapped in a thin, fatty outer layer called a lipid membrane or envelope. This layer is built from molecules with water-loving heads and water-fearing tails, a lot like a double-layered soap bubble. Coronaviruses, HIV, herpes viruses and the viruses that cause hepatitis B and C all have one.8
When soap meets one of these germs, its tails wedge into the fatty layer and pry it apart. A chemist quoted by Yale Medicine compared soap molecules to crowbars. The contents spill out, the bacteria die and the viruses are made useless.89
Not every germ has a fatty coat. Rhinoviruses and adenoviruses (common causes of colds), the hepatitis A virus and poliovirus don't, and some bacteria protect themselves with tough shields of protein and sugar. Soap breaks these less easily, but vigorous scrubbing with soap and water still removes them from your skin.8
| Kind of germ | What soap does |
|---|---|
| Germs with a fatty coat (for example coronaviruses and herpes viruses) | Pries the coat apart and breaks the germ, then washes it away |
| Germs without a fatty coat (for example rhinoviruses, a common cause of colds) | Lifts them off the skin so water can rinse them down the drain |
Source for the table: 8
The CDC's advice is to wet, lather, scrub, rinse and dry, and to scrub for at least 20 seconds. Need a timer? Hum "Happy Birthday" from beginning to end twice.1 Studies show that scrubbing for 20 seconds removes more germs than washing for a shorter time, and lathering creates the friction that lifts dirt, grease and microbes from the skin, including under your nails and between your fingers.2
Water temperature doesn't seem to affect how many germs are removed, so warm or cold water both work. Hotter water can irritate your skin more.2 (The "Your turn: wash up" toy in the story uses rough, made-up odds to show the idea; it isn't measured data.)
If soap and water aren't available, the CDC suggests a hand sanitizer with at least 60% alcohol, but sanitizer may not work well when hands are visibly dirty or greasy.13
Soap and antibiotics both fight germs, but in very different places. Soap works outside your body: it breaks some germs and washes others off your skin before they can get in. Antibiotics are medicines that work inside your body, and only against bacteria, not viruses.8
Clean hands also help keep antibiotics working. The CDC says handwashing can prevent about 30% of diarrhea-related illnesses and about 20% of respiratory infections like colds. Fewer infections means fewer antibiotics are prescribed when they aren't needed, which slows the rise of antibiotic resistance.2 To see how bacteria multiply so fast in the first place, read how bacteria reproduce, and to see how your body prepares for germs that do get in, read how vaccines work.
Soap starts out as fat. Animal fats and vegetable oils are made of molecules called triglycerides: three long fatty-acid chains joined to a small molecule called glycerol.6
When fat is mixed with a strong base such as sodium hydroxide, known as lye, the fatty chains split off the glycerol. Each chain becomes a salt of a fatty acid, which is a soap molecule: a long grease-loving tail with a charged, water-loving head. This reaction is called saponification.567
fat (or oil) + lye → soap + glycerol
In 1823 the French chemist Michel Eugène Chevreul showed that this is exactly what happens: the fat splits into soap and glycerin (another name for glycerol).12 For most of history, people got their base from wood ashes: pioneers soaked ashes in hot water to make lye and boiled it with fat. Wood ash was the main source until the early 1800s.57
Safety: lye is caustic and can burn skin and eyes, so soap-making with lye needs adult supervision and proper safety gear such as gloves and goggles.14
Soap has been around for nearly 5,000 years.5 As early as about 2800 BC, people in ancient Babylon, in what is now Iraq, boiled fats with ashes to make a soap-like substance. Archaeologists found clay cylinders from that time inscribed with words meaning "fats boiled with ashes".511
An Egyptian medical scroll, the Ebers papyrus from about 1500 BC, describes mixing animal and vegetable oils with alkaline salts to make a soap-like material for washing and for treating skin diseases.11 For a long time, soap was used more for cleaning wool and cloth than bodies.13 In 1791 the French chemist Nicolas Leblanc patented a way to make soda ash from salt, which helped turn soap-making into a big industry.11
Nobody knows who invented soap: it was discovered long before anyone wrote down names. Handwashing as medicine came much later. In the 1840s the doctor Ignaz Semmelweis found that far fewer mothers died after childbirth when doctors washed their hands.8
During World Wars I and II there wasn't enough fat to make soap, so chemists made similar molecules from other raw materials. These are synthetic detergents. Today, most of the things we call "soap", from many liquid hand soaps to dish soaps, are actually detergents.11
Detergents clean the same way: one end is attracted to grease and the other to water.5 Their big advantage shows up in hard water, which contains calcium and magnesium. Real soap reacts with those minerals and forms a scum that can't dissolve, the ring you see around a bathtub. Detergents don't form that scum.57
Each soap molecule has two ends: a head that loves water and a long tail that avoids water but loves oil and grease. When you wash, the tails dig into grease and dirt while the heads stay in the water. The molecules wrap the grease into tiny balls called micelles, and running water carries them away.
The cleaning is done by surfactants: the soap (or detergent) molecules themselves, which have a water-loving end and a grease-loving end. Scents, colours and moisturisers don't do the cleaning. The CDC notes that soap works better than water alone because its surfactants lift dirt and microbes off the skin, and because people scrub more thoroughly with soap.
Both, depending on the germ. Some viruses and bacteria are wrapped in a fatty coat, and soap molecules can pry that coat apart, which breaks them. Germs without a fatty coat aren't broken this way, but scrubbing with soap lifts them off your skin so water can rinse them away.
The CDC recommends scrubbing for at least 20 seconds (about two rounds of 'Happy Birthday'), because studies show it removes more germs than washing for a shorter time. Scrubbing creates friction that helps soap get into dirt, grease and germs all over your hands, including between your fingers and under your nails.
Soap is made by a reaction called saponification: fats or oils are mixed with a strong base called lye. The fat molecules split into glycerol and fatty-acid pieces, and those pieces become soap molecules. Lye is caustic and can burn skin and eyes, so soap-making with lye needs adult supervision, gloves and goggles.
Soap is ancient. As early as about 2800 BC, people in Babylon (in today's Iraq) boiled fats with ashes to make a soap-like substance, and clay cylinders from that time describe the method. Ancient Egyptians also mixed oils with alkaline salts for washing and treating skin.
Not for everyday use at home. The FDA and CDC say there is no evidence that consumer antibacterial soaps prevent illness better than plain soap and water, and in 2016 the FDA stopped 19 antibacterial ingredients, including triclosan, from being sold in consumer soaps.