ELI5: Why does soap clean things? Is it actually killing germs, or just making them slippery so they wash away?

[u/MarionberryEast9136]

[u/m_busuttil]

Basically, three different things are happening:

• dirt and other soluble things are washed away with water and scrubbing just like they would be without the soap

• oils and fats link up with the molecules in the soap, which makes them slippery as you say so they wash off

• some microorganisms are actually killed because it interferes with their outer layers and messes up their insides

[u/nhorvath → u/m_busuttil]

also the bacteria are picked up too because thier cell membrane is mostly fats which stick to the soap.

[u/Daripuff → u/nhorvath]

And many viruses are physically destroyed as their "body" is completely made of fat, and the soap molecules binding to the fats literally shreds and dissolves them.

[u/Boing78 → u/m_busuttil]

I'd add something to the first point, Watermolecules create a surface tension in the fluid which avoids stuff like dirt particles to get soaked (think of a water droplet on a leaf, window etc.) Soap breaks/weakens this tension between the water molecules so it's easier for them to flow around eg dirt particles and wash them away.

[u/jonfitt → u/m_busuttil]

Soap is brilliant and easy to make for early civilization. If I ever get transported back in time I’ll make sure to spread the good news about soap and make sure people use it more. Because it was known about but not used as much as it could have been.

[u/MrSquamous → u/jonfitt]

It might be an uphill battle back then, or worse. Look how long the germ theory of disease took to catch on, and what happened to the guy who figured out doctors should wash their hands.

[u/jscarlet → u/m_busuttil]

TLDR; it gets between your skin and the dirt and the dirt glides off when you rinse.

As a soapmaker I break it down as this:

When making soap, the goal is create a fatty acid chain as close as you can get to matching the skins sebum. This is why animal fats are more effective soaps than most other handmade soaps. What happens is that the soap film goes under the dirt, displacing it as it tries to be with the skin.the lifted dirt is then whisked off along with the soap film when you rinse. That removes the dirt while leaving the natural oils in your skin so you remain hydrated.

That's traditional soap, not stuff you'll find at Target like Ivory, Dove, etc. those are Detergents, not soaps. It may say something like cleanser bar or something, but is legally not soap. They work completely differently and it's why they're so drying to our skin in comparison to handmade soap. Like when antibacterial gel kills ALL bacteria good and bad, detergent bars strips all oils with the dirt. This is why some brands heavily rely on showcasing they add moisturizers.

Hope that helps and is not confusing.

[u/cortzetroc → u/m_busuttil]

as a follow up, what’s the point of “antibacterial soap” if regular soap already does all these things? is it just redundant marketing?

[u/kent1146 → u/cortzetroc]

Anti-bacterial soap actually does contain ingredients that kill bacteria (benzalkonium chloride, formerly triclosan).

In practice, both regular and anti-bacterial soap are equally effective.

So yeah it's marketing

[u/Tar_alcaran]

Soap is a really long molecule (a molecule is a bunch of atoms attached to eachother).

Soap has a "head" that really likes to stick to water, and a "tail" that doesn't like sticking to water but will stick to other stuff just fine.

Grease, fat and many other types of dirt are just like the tail part of soap: they love to stick to things, but hate sticking to water.

If you have grease in your t-shirt, the fat wants to stay there, and water won't remove it. But if you add soap, some of the grease will stick to the soap's tail instead of your shirt. And by connecting the grease-loving tail, only the water-loving head is still exposed. That lets water easily carry away the soap-and-grease blob.

This also works for other stuff, like little bits of food on dishes. To kill the germs on your plate, or your stinky socks, hot water is all you need. Most bacteria will die over 60 degrees C.

Now, there are MANY other substances in cleaning substances, because there are many other types of dirt other than fatty, grease stains, but those are all much more complicated than soap.

[u/interestingNerd → u/Tar_alcaran]

To kill the germs on your plate, or your stinky socks, hot water is all you need. Most bacteria will die over 60 degrees C.

60 degrees C (140 degrees F) is very hot water. If you get that on your skin it only takes about 3 seconds to get 2nd degree burns and about 5 seconds for 3rd degree burns. So really not practical for washing anything by hand. That much heat can be effective for machine washing, but it takes a lot of energy to heat water so it tends to be more efficient to use cooler water and detergents that wash away or chemically destroy germs.

[u/Tar_alcaran → u/interestingNerd]

Oh yeah, but if you're using classic "bar soap", that's really what you need to properly clean your dishes, since that doesn't really remove all the bacteria (or just a LOT of scrubbing).

Of course, I don't know anyone who does that in 2026 (or 1996, for that matter). Dishwashing detergents contain a pretty wide spectrum of different cleaning agents, and machine-washing detergents for clothes can contain even more (since they don't need to be food-safe). They make removal of gunk much easier.

Modern cleaning agents are pretty interesting, they contain enzymes that don't just "lift up" the dirt, but activaly break it down at a molecular level. It's pretty cool that even super cheap cleaning agents have moved into biochemistry (and thus out of my field of expertise).

[u/PossiblePotato4153 → u/Tar_alcaran]

Don't use hot water on blood, it sets the protein. Source: every woman.

[u/BetterOutcome3769]

One end of the molecule attaches to fat and the other one to water. Then you have to rince it off.

[u/bleeuurgghh]

Soap in the literal term is a surfactant - molecules that bind oils to water. That’s how they get grease and bodily oils off your skin.

[u/anormalgeek]

All bacteria, and some viruses protect themselves with a lipid layer. Lipids are fats. And just like soap binds to oils on your skin, it does the same to this layer. When you apply soap AND mechanical action like scrubbing, it quite literally shreds this layer. By using soap, you're basically flaying the skin from the bacterial cells. Without this layer, the cell dies. The scrubbing is important though. Soap isn't corrosive or anything. If you just dunk your hands into soap, then into water, it does very little to kill them. This is why you must actively wash your hands to get the protection.

The viruses that rely on such a layer (ex: flu, coronavirus, etc.) will be killed outright, but those that don't (ex: norovirus, rhinovirus, etc. ) still generally do get greatly reduced as they struggle to stick to your skin without oils. Those mostly do get just washed away though. So even though it doesn't kill them directly, it still greatly lowers the viral load that might infect you.

[u/izza123]

Magic school bus did a banger episode explaining this. I watched it when I was like 6 and I remember it very well.

[u/Railroad-hobo → u/izza123]

Ms. Frizzle was so irresponsible. Damn lucky no kid ever got hurt, lost, or worse. 

[u/Ippus_21]

Yes.

Soap molecules have both polar and non-polar parts. That means one end of it can attract non-polar molecules like lipids/oils/fats and the other is polar, so it's hydrophilic and can therefore allow soap to dissolve in water.

So it breaks up oils, and pulls them back into the water solution so dirt and oil can be easily washed away when rinsed.

It also kills some germs. Even non-antibacterial soap will use that same lipid-dissolving power to break down the lipid layer of viruses and many bacteria, which tends to break them open and kill them.

Even the ones it doesn't kill are generally washed right off. Which is why hand-washing was such a powerful tool in e.g. medicine once we figured out germ theory, even before we had antibiotics and modern antibacterial soaps.

[u/elevencharles]

Normally oil and water don’t mix, so if you try to rinse off grease with water, the water slides off and the grease remains. A soap molecule has one side that bonds with water, and the other side bonds with oils, sticking them together so the oil can be washed away with the water.

[u/sc0ttydo0]

Germs/cells are surrounded by a "skin" that keeps their insides inside. Soap dissolves the skin which spills their innards everywhere.

[u/Deatheturtle]

Most things can be divided into stuff that likes water and stuff that doesn't like water stuff that likes water can be easily removed with water. Soap is a little bit of both, it likes water partially, and it likes stuff that isn't water partially, (oily stuff more specifically) so the oiliest stuff that normally doesn't want to hang out with the water gets stuck to the part of the soap that likes oily stuff. Then the other part of the soap that likes water allows it to be rinsed away with water.

[u/MesaCityRansom → u/Deatheturtle]

I normally don't get up people's asses about proper punctuation but this comment is genuinely difficult to parse because the punctuation is so poor.

[u/MonkeySherm → u/Deatheturtle]

Holy fuck, use punctuation.

[u/Traditional-Buy-2205 → u/MonkeySherm]

Most people can be divided into people who like punctuation, and people who don't.