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The oil is said to be evaporate at very high temperatures but while cooking something in pan with oil, is the smoke that goes upwards because of oil evaporation? If not why does the chambers of chimney get greasy after some time?

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Looking at olive oil you can see that atleast some of it's constituents (oleic and palmitic acid) has a boiling point of about 355 degrees celsius. This just shows that it depends on the oil, and various elements of the oil will turn to vapor at different speeds. –  Max Jan 18 '12 at 10:39
    
@Max: Unfortunately, that's not how the world works. A mixture has a single boiling point. Its components do not boil separately. So it doesn't matter what the boiling points of the component fats in olive oil are; if it's possible for olive oil to boil, it will all do so together. (Note that this misconception is why so many people mistakenly think that you can simply boil away alcohol.) –  Jefromi Jan 19 '12 at 19:25
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@Jefromi My error seems to be that the oil burns instead of evaporate. While it is true that a mixture of liquids will have a single boiling point, it is also true that this boiling point is somewhat of an average of the involved liquids. Also, I merely said that the various elements will turn to vapor at different speeds, which is the foundation of ethanol destillation. You can cook away the alcohol, atleast down to a very low percentage, but it's achieved by temperatures lower than boiling (or a good reflux system). –  Max Jan 19 '12 at 20:57
    
@Max: Okay, fair enough, sorry. –  Jefromi Jan 19 '12 at 21:05

4 Answers 4

I am afraid Chef Flambe's answer is wrong. Not everything has a melting point and a boiling point.

Oil is made of big organic molecules, containing long carbon chains*. Unlike anorganic substances with small molecules (like water), heating oil does not lead to a point where the molecules stop attracting each other (that would be the boiling point). Instead, the big, fragile molecules just break up. Which means that oil has no boiling point at all, and it is impossible to produce oil in a gas phase. (You can produce something similar to "oil vapor" with a mister, but this consists of tiny droplets of liquid oil, not a real gas).

As oil breaks up before it boils, there is no oil evaporation. You can destroy oil by heating it, because it will turn into something different than oil. You can also burn it by heating it in the presence of oxygen, and this is what happens when you see smoke coming from your pan. (This is chemically different from simple breaking up of molecules). But no, it does not evaporate.

The chambers of the chimney get a greasy film because: 1) the particles in the smoke from smoking oil can feel somewhat greasy (pure soot feels greasy too) 2) when your oil breaks down under heat, some of the new molecules (pieces of oil molecules) can be light enough to become air borne and go up and build a film. While technically not an edible oil any more, they can have a greasy feeling to them. 3) When you fry, oil droplets fly through the air. You notice it on the stove around your pan, but I bet some droplets are small enough to be carried by the upward draft of hot air into the chimney.

*I simplified here a bit, because the oils we cook with are not made from a single chemical compound, they are a mix of different compounds. But the explanation still works for the mix, because it is always the same type of compound.

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+1! I do wonder if, under low pressure, oil might be able to evaporate. But most of us don't cook in vacuum chambers. –  Jefromi Jan 18 '12 at 15:04
    
@Jefromi Probably there is a pressure for which the oil would not be solid, or liquid. The effects reported by rumtscho are caused by the heat, which causes the long molecule of oil to break up before it can evaporate. That is the reason why using oil at too high temperature, or re-using it is not suggested: Those broken molecules are toxic for us. –  kiamlaluno Feb 6 '12 at 22:48

Yes, every substance theoretically has a boiling point, depending on pressure too (hydrogen at 0K in atmospheric pressure is still a gas).

Still, quite a few substances are flammable - with flash point far below their boiling point. Oil, for example, will first start smoking, then go up in flames long before reaching its boiling point in our atmosphere with ~20% oxygen.

Moreover, some substances undergo significant chemical reactions at certain temperatures, meaning whatever would eventually reach the boiling point will no longer be the original substance (thus the theoretical boiling point - the substance can't reach it because it will cease to exist and become something entirely different before reaching it.) I'm not entirely sure, but I'm fairly convinced thermal cracking temperature of oil is still below its boiling point, meaning no, even if you remove oxygen, oil will first separate into simple hydrocarbons, before they begin boiling.

OTOH, vegetable oil dries up - gets thick and sticky (although very slowly), meaning it shouldn't be used for bearings, hinges and the like. But that's not really on-topic.

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Everything has a melting point and a vapour point but oil needs the extra heat added to get it to the vapour point.
The smoke you see is the oil breaking down and turning into vapour. However, when you get oil build up in the oven hoods what is usually happening is a combination of vapourized oils and regular oil droplets which have been carried up with help from steam.

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I'm pretty sure that the smokepoint is before the boiling point. The oil do break down, but I'm not sure that smoke is classified as 'vapor'. It's just a by-product of the 'burning' of the oil. –  Max Jan 18 '12 at 10:44

While mixtures do have a specific boiling point, the amount of each component that boils off is not the same. If one of the components has a low boiling point compared to the others, then it is said to be more volatile, and so more of this component will boil off than the others when the boiling point is reached. Also, when you get above this volatile component's boiling point, it will be evaporating a fair amount even if the entire mixture isn't boiling.

However, in the case of a mixture that interacts a lot on a molecular level, the situation is different. Water and alcohol are both very polar, and hold to each other relatively strongly. When this happens, once you boil off a certain amount of alcohol, you won't decrease the concentration, because the small amount left is held as tightly as the water is.

However, in the case of oil, because air is present, the high temperature causes the oil to break down to the same components as you would get if you burnt it, even though it isn't technically burning (i.e. with a flame). When it burns, ideally, carbon dioxide and water is formed. However, since the temperature isn't as high as in a furnace for example, there is a lot of residual carbon left over. The airflow from the hot surface of the frying pan etc. pushes the carbon (Smoke), which is still very hot, onto the steel or brick, where it binds to imperfections in the steel's surface. Likewise, low boiling point oils can pass to the surfaces above, and very small droplets of more high boiling point oils can be carried upward in the bulk flow of hot air rising from the pan also.

All this is from a chemical engineering perspective, but I hope you can read between the lines.

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