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When one heats oil in a pan, there comes a time when it starts boiling (bubbles coming up) just like water.

Now I know that water bubbles are formed to the the increase in the K.E of molecules and thus the breakage intermolecular forces. Does the same happen with oils?

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    Are you interested only in a practical kitchen sense, or something that could only be acheived in a lab? Technically you can boil oils, but most if not all used in cooking decompose in air to give smoke before you reach that point, so you'd have to work under an inert gas to demonstrate – Chris H Dec 1 '17 at 10:45
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    This is probably more of a chemistry or physics question, I don't see it as cooking related, if your oil is boiling in your kitchen it's time to evacuate. – GdD Dec 1 '17 at 14:44
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    It's plenty cooking related; look at the first paragraph. – Cascabel Dec 1 '17 at 18:32
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Oil does boil like water, at least in theory, but in practice you should never see oil boil during cooking. The temperatures cooking oils will boil at is much higher than their smoke points. You never want to heat oil to (or above) its smoke point as - aside from generating smoke - it also has a big negative effect on the flavour of the oil and will spoil whatever you're trying to cook.

Even worse, as you heat oil beyond its smoke point you run the risk of reaching its flash point, where the oil can catch fire. A fire which can easily get out of control and result in a disaster. So if you see oil smoking in your pan you want to immediately turn down the heat, not just so you don't ruin your meal, so you don't risk ruining your kitchen.

Finally the bubbling you see when frying things in oil isn't the oil boiling. It's water vapour from whatever you're cooking. Since when frying the temperature of the oil is significantly higher than the boiling point of water, any water the hot oil comes into contact will turn into vapour.

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Refineries boil billions of gallons of oil every day. They do it in an oxygen free, enclosed environment at various pressures. Depending on the unit , they may boil a little water in there too. Cooking oils can also be boiled away;although temperatures will be so high that it should be done in an oxygen free, enclosed environment.

  • This is a cooking site, and if you read the question, it's indeed asking about oil in a pan in the kitchen. I don't think that oxygen-free industrial setups are really answering the question. – Cascabel Dec 2 '17 at 2:11
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    I actually like this answer. The question itself is on the theoretical side, so knowing that there are circumstances under which it does boil is part of the answer, even if they are not easily achievable in a home kitchen. – rumtscho Dec 2 '17 at 15:55
  • Linoleic acid, a fairly common component in cooking oils, boils at 407.8212 °C at 1 atmo, - No O2: chemspider.com/Chemical-Structure.4444105.html Many oils have perfectly nice boiling points. They just don't hold up in the kitchen because the boiling point is higher than the smoke point in the usual kitchen atmosphere. It'd be interesting to see and taste the results of anaerobic high temperature frying of say, potatoes. Setup could prove a real challenge, but there are likely some unfound culinary delights to be discovered in O2 free cooking at very high temperature. – Wayfaring Stranger Dec 3 '17 at 4:24
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I would say no. There is some room for interpretation, but even if you count it as "boiling", then it is not the same way as water does.

First, oil is not a pure chemical compound, it is a mixture of fatty acids and other stuff extracted from the plant. Even you could define one exact mixture as "oil" and it were "boilable", you still probably wouldn't have a strictly defined boiling point. "Probably", because some mixtures do have a boiling point (as opposed to a temperature range in which they boil) - I am not enough of a chemist to know which have a point and which a range.

Second, oil is not even a well defined mixture. It varies from bottle to bottle. So some oils would have a different behavior during "boiling" than others.

Third, when water boils, you can condense it and it becomes water again. It just undergoes a phase change at its boiling point, no further changes. But for oil, this is not true. While still in liquid state, heating causes it to undergo pyrolysis and other changes - you see it smoking when you heat the pan, it polymerizes if thin enough (that's how you season iron pans), etc. So whatever mixture it was before heating, it is not the same mixture when it reaches its boiling temperature range, it has changed chemically. So if you can heat it to a temperature at which it does turn from liquid to gas (and it doesn't completely burn away, or polymerize into a lump, etc.), then condense it, whatever you condensed won't be the same thing you started out with. It is much more than just a phase change.

I guess people can still make the case that there is some temperature range at which whatever you have in your vessel (which is no longer the oil you started out with) could turn from a liquid to a gas, and this should be described as "the oil is boiling". So I am not saying a firm, emphatic no. But as you see, even if you find the use of the label "boiling" acceptable, it is actually very different from what most people think of when they think of boiling, or from how water boils.

  • I might make your third point the primary answer, and mention the rest as side notes. We wouldn't worry about the fact that fruit juice can be a lot of different mixtures if we were asking if it could boil; the real reason oil is different is what you mention about it breaking down/changing before it can just make a simple phase change. Also, mixtures do generally have a single boiling point. It does vary depending on the mixture, but e.g. ethanol/water mixtures boil at a single temperature. (The ethanol doesn't boil separately at a lower temperature.) – Cascabel Dec 1 '17 at 18:38
  • I guess everybody would place a different weight on each of the three points. I intepreted the question as not only "does it boil" but "does it boil like water" and from this point of view, it being a mixture with a boiling range instead of a boiling point is significant. And good point about some mixtures having a sharply defined boiling point. – rumtscho Dec 1 '17 at 19:06
  • Hm, as far as I know, all proper mixtures (i.e. like ethanol/water, not like blobs of oil and water "mixed") that are actually capable of a liquid to gas phase change will have a single boiling point. So I am skeptical about "probably wouldn't have a strictly defined boiling point" - if oil did boil before pyrolysis and other chemical reactions, it would almost certainly be a single boiling point. It'd be different for different oils, sure, but it'd boil. So that's why I said the third point is the important one - otherwise, it'd boil (maybe not at a kitchen-relevant temperature though). – Cascabel Dec 1 '17 at 19:25
  • @Jefromi interesting. I found ausetute.com.au/puresubs.html, stating "pure substances display a sharp melting and boiling point" and "Homogeneous mixtures do not display a sharp melting point, they melt over a range of temperatures.", nothing about the boiling point of mixtures. My intuition would be that the boiling point will behave similarly to the melting point, but this could be a wrong assumption. – rumtscho Dec 1 '17 at 19:40
  • One easy difference is that liquids can't have structure, while solids can have relatively complex structure, which can change as they begin to melt. It's very easy to find articles (e.g. Vapor-liquid equilibrium on Wikipedia) that discuss the boiling point of a mixture, which I think is a pretty nice indication that a single boiling point is a normal expectation. – Cascabel Dec 1 '17 at 20:35
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Like mentioned above, oil has a decomposition point that is lower than its boiling point. So when you heat it, it will decompose before turning into a gas. The gas you do see is smoke from the oils combusting / pyrolyzing. However, if you were to heat oil in an oxygen free environment (O2 is required for combustion) then you might be able to form gaseous. This will depend on how hot you have to heat it - if the heat required to form the gas is larger than the energy of the atomic bonds in the molecule, then the molecule will decompose (pyrolyze) before becoming a gas.

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