I think what you're asking is actually "Why does water boil and oil smokes
When you heat up water, it eventually gets to a temperature called its
'boiling point' where it starts to boil. Oil also has a boiling point,
but many kinds of oils will reach what's called a 'smoke point' first.
At the smoke point, the oil won't boil, but it will start making black
smoke, just like if it were burning. So why is that?
Oil is typically made up of a number of different kinds of
oils all mixed together. The molecules all have some things in common
(which is what makes them all oils), but they aren't exactly the same.
When you mix different compounds together like this, the mixture's
boiling temperature will be different than the boiling (and burning)
temperatures of each of the things in it.
The important part of this is that by the time some of these
molecules get up to the mixture's boiling point, it will be so hot that
they will start to burn (in other words, the boiling point of the
mixture is higher than the burning point of some of the components)
This is what makes the smoke. So it really is smoke from burning - it's
just that only a small part of the oil gets burnt.
(republished on 07/24/06)