Actually, soap doesn't really do much with dirt (although scrubbing
helps!). What soap gets at is oils. To explain this, I'll have to start
by giving you a little bit of chemistry...
Nearly all compounds fall into one of two categories: hydrophilic
('water-loving') and hydrophobic ('water-hating'). Water and anything
that will mix with water are hydrophilic. Oil and anything that will
mix with oil are hydrophobic. But if you've ever tried mixing oil with
water, you've probably noticed that it doesn't work very well.
Hydrophilic and hydrophobic compounds just don't mix. (Ok, there's more
to the chemistry than just this, but this is the short version.)
This is why it's so hard to clean grease off of dirty dishes. The
grease won't mix with the water in your sink, so it won't come off.
This is where soap comes in. Soap molecules are kind of special in that
they have a hydrophilic part and a hydrophobic part. So one end of the
soap molecule can mix with water and the other end can mix with oil.
When you mix soap into the water in your sink, the soap molecules
arrange themselves into tiny clusters (called 'micelles'). The
water-loving (hydrophilic) part of the soap molecules points outwards,
forming the outer surface of the micelle. The oil-loving (hydrophobic)
parts group together on the inside, where they don't come into contact
with the water at all.
Now you add the greasy dishes. The hydrophobic oil molecules on the
dishes still don't mix with the water. Instead, it gets trapped in the
center of the soap micelles, where all of the molecules around it are
hydrophobic too. Then you can rinse away the soapy-water, taking all of
the trapped oil molecules with it.
If you want a neat way of seeing this at home, try taking a clear
glass of water and adding a thin layer of oil. You should be able to
see them separate pretty clearly. Now add a squirt of liquid dishsoap
and stir it up. The oil doesn't separate at all any more! It's all
trapped in little clusters of soap molecules, which can dissolve in
water just fine.
(republished on 07/13/06)