Water is very good at washing away many things. Grease is not one of them.
Oil and water do not mix well, so rinsing an oily pan or greasy hands with plain water often leaves the slippery material behind.
Soap solves the problem by interacting with both sides.
Soap molecules have two very different ends
Soap is a type of surfactant.
A typical soap molecule has a water-attracting, or hydrophilic, part and an oil-attracting, or hydrophobic, part.
That unusual structure lets the molecule sit at the boundary between water and grease.
The hydrophobic portion associates with oily material. The hydrophilic portion remains comfortable in water.
Soap is basically bilingual chemistry.
Soap helps break grease into tiny droplets
Add soap and agitation, and soap molecules can surround small portions of oil and grease.
Their oil-friendly ends point inward toward the greasy material while their water-friendly ends face outward into the surrounding water.
Groups of surfactant molecules can form structures called micelles.
Now the grease is no longer sitting as one stubborn layer on the surface. Tiny oil-containing structures can remain dispersed in the wash water long enough to be carried away during rinsing.
Rubbing helps the chemistry do its job
Mechanical action matters too.
Scrubbing hands, swishing laundry, or wiping a dish helps loosen dirt from surfaces and brings fresh soap solution into contact with it.
Soap reduces surface tension and helps water spread and penetrate more effectively.
The chemical and mechanical parts work together.
This is why a quick ceremonial wave under the faucet is not equivalent to actually washing something.
Why does soap help with microbes?
Soap does not need to kill every microbe to help clean your hands.
Washing loosens oils, dirt, and microorganisms from the skin so they can be rinsed away.
Some microbes have lipid-containing outer structures that can also be disrupted by surfactants, but physical removal is a major part of ordinary handwashing.
That is why soap, water, friction, and adequate washing time form such an effective combination.
Why does soap make bubbles?
Surfactants stabilize thin films of water around pockets of air. That makes bubbles and foam easier to form and longer lasting.
Foam can help spread cleaning solution, but the amount of foam is not a direct measurement of cleaning power.
A product can clean well without producing a mountain of bubbles.
The bubbles mostly have better marketing.
Why can soap work poorly in hard water?
Traditional soaps can react with calcium and magnesium ions in hard water to form insoluble material commonly called soap scum.
That removes some soap from the cleaning job and leaves deposits behind.
Many modern detergents use surfactants designed to work better under hard-water conditions.
Is soap the same as detergent?
They are related but not identical.
Traditional soap is generally made from fatty acids and an alkali. Detergents include a much broader range of synthetic or manufactured surfactants with properties tailored for laundry, dishes, shampoos, and other uses.
Both rely on the same useful trick: part of the molecule interacts well with water while another part interacts with oily material.
So soap does not make grease disappear.
It makes grease willing to leave with the water.
For something that spends most of its life sitting quietly beside a sink, that is a pretty good trick.
A related explanation is Why does soda fizz?, which looks at another familiar effect from the same general system.
And that’s the short explanation.
Related explanations: Why does soda fizz? and What is pH?