When oil is heated – say, when I’m cooking with olive oil in a skillet – does any of the oil evaporate? And if so, how fast does it evaporate compared to water?
Yes, oil can evaporate. Any oil is a mixture of a bunch of different substances which evaporate at different temperatures-- In general, the oils with smaller molecules will evaporate more easily than those with larger molecules. This process is used in oil refineries to separate out the different components, which are used for different purposes.
That said, biological oils like you’d use in cooking consist mostly of components that are very difficult to evaporate, so you will get very little loss from your frying pan.
you might find evidence of oil evaporation at home by finding an oily film/residue away from the splatter distance from the cook top. if you have a range hood you will find residue on the fan blades and on the far side of the filter where oil splatter can not reach.
some people (mostly real old farts) might refer to a oil refinery as an oil distillery which provides a clearer linkage to evaporation which is a big part of the refining process.
Oil generally consists of a mixture of molecules; an assortment of chains of carbon of various lengths from 1 carbon (methane, methaonol, etc. depending on the pieces attached) to incredibly long chain molecules (guess how many carbons in an octane molecule). The more longer molecules, the thicker and gummier the oil.
When exposed to air, these can evaporate. The much longer carbon molecules can take an awful lot longer to evaporate, and some may be so long they are more solid than liquid and don’t evaporate, have a much higher melting and boiling temperature. So for example, tar may smell “oily” when first applied to a roof or street, but after a few weeks, has no appreciable smell unless heated quite a bit. So oil may never completely evaporate, but the resulting spot becomes thicker and harder as the short chains disappear.
(Heat also “cracks” or breaks long carbon chains into shorter pieces, thus aiding the evaporation process).