How soap removes grease — Chemistry, 7–10 years
Water alone runs over greasy dirt without mixing with it. Soap has one end that interacts with water and another that attaches to grease, so rubbing and rinsing can carry the dirt away.
Soap makes grease travel with water
Grease avoids water, so a splash does not remove it well. Soap molecules can hold onto grease with one part and water with another, breaking the grease into tiny droplets that water can carry away.
Why wash with soap?
People found that water was poor at removing oily dirt from skin, cloth and dishes. Soap had been made for a very long time, but chemistry explained its action: it helps two substances that normally avoid each other, water and grease, work together.
A washing-up test
Put one teaspoon of cooking oil in each of two cups. Add warm water to both, then add one drop of washing-up liquid to only one cup. After stirring, the oil in the soapy cup breaks into tiny droplets and stays mixed longer, making rinsing more effective.
Bubbles are not the whole job
It is reasonable to think that more foam means more cleaning, because foam is easy to see. But bubbles are mostly a sign that soap is present, not a measure of dirt removed. The soap’s contact with grease, rubbing and rinsing do the important work.
Cleaning hands and dishes
The same chemistry helps wash oily plates, greasy hands and some stains from clothes. Handwashing also removes microbes together with the dirt, especially when you rub all surfaces and rinse. Soap cannot remove every mark, so the material and the stain still matter.
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