Soap helps water carry grease away — Chemistry, 4–6 years
Soap is useful because water alone does not easily take oily dirt with it. Chemistry, 4–6 years.
Soap joins water and grease
Water and grease usually stay apart, like two people who will not hold hands. Soap can grip the grease on one side and mix with water on the other, so rinsing can carry the greasy dirt away.
The problem of oily dirt
People noticed that water washed away loose dust but left oily marks on skin, clothes and plates. Soap was made to solve that problem: it helps water surround greasy dirt instead of letting it slide away from the water.
Cleaning an oily plate
1. Put a drop of oil on a plate and add water: the oil stays in a slippery patch. 2. Add washing-up liquid and rub. 3. Soap breaks the patch into tiny bits that water can surround. 4. Rinse the plate clean.
More foam is not always better
It is reasonable to think that more foam means more cleaning, because foam is easy to see. But foam is mostly trapped gas; the soap must touch the greasy dirt, and rinsing must carry the loosened dirt away.
Hands, clothes and dishes
This is why soap and washing-up liquid help clean hands, clothes and dishes after oily food or play. They do not make dirt vanish: they help water take it away, so dirty water should be rinsed off.
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