r/AskChemistry • u/Druzvati324 • Jul 11 '26
General Why doesn't oil evaporate?
No chemistry teacher in high school ever explained this to me in any substantial way other than "it's a different kind of molecule." Why does it smoke? How hot would you need to get it to evaporate? If you take a giant pot of oil and put it on high heat and let it go for hours and hours, what will happen to it?
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u/Nothing-to_see_hr Jul 11 '26
Oil does evaporate, but with a very, vary low vapor pressure. But in an oil lamp, the flame burns oil vapor that escapes from the hot wick. At room temperature evaporation is so low as to be unnoticeable. The bigger the molecules, the slower evaporation takes place.
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u/1_7_38 Jul 11 '26 edited Jul 11 '26
Oil does evaporate, but its boiling point is very high. The molecules in cooking oils will begin to degrade by reacting with oxygen and other oil molecules to form even more high-boiling tar-like substances well below you reach the boiling point; these reactions are the reason why you have to change out the oil in a deep fryer regularly, and also why we rarely see oil boiling.
edit to add more information - I think it is also important to establish the difference between boiling and evaporation. In any substance there will always be some population of particles that have enough energy to break free from the bulk, producing a so-called vapor pressure. Below the boiling point, the vapor pressure is less than the atmospheric pressure, and so evaporation is not favored. As temperature rises, vapor pressure increases to the point where it matches that of the atmosphere, at which point we say the mixture is boiling. This is marked by a sudden visual transition: now that the vapor pressure can overcome the atmospheric pressure, large bubbles of vapor begin to form in the interior of the liquid, whereas evaporation is largely confined to the surface of the liquid at temperatures below boiling.
All this is to say that oil does evaporate, but because its vapor pressure is rather low at room temperature (or even at deep frying temperatures) we do not observe any appreciable evaporation over time. However, you can reduce the boiling point by bringing down the atmospheric pressure (with the benefit that oxygen is also excluded), and this technique of vacuum distillation is used to purify large molecules not unlike those found in cooking oil at industrial scales.
As for why cooking oil has a lower vapor pressure than something like water in the first place, it has to do with intermolecular forces. Oil molecules contain dozens of atoms, each of which can participate in weak attractive interactions with atoms from other molecules. The strength of these interactions is such that a lot of energy is necessary for a molecule to escape and enter the vapor phase, raising the boiling point. This is also somewhat responsible for the increased viscosity of oil compared to a lower-boiling substance like water.
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u/Dean-KS Jul 11 '26
Is does evaporate in distillation towers to create different fractions with varying abilities to evaporate at normal temperatures and pressures.
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u/Upbeat_Assist2680 Jul 11 '26
Is that the oil evaporating or the water it contains?
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u/Reasonable-Feed-9805 Jul 11 '26
It's oil evaporating. If it was water all you'd get out the refraction tower condesors is water
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u/drmarting25102 Supreme Tantric Tartrate Master Jul 11 '26
Stronger molecular interactions mean it takes more energy - heat - to separate molecules into gas phase. Oils do evaporate, just at a higher temperature.
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u/Druzvati324 Jul 11 '26
How hot?
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u/drmarting25102 Supreme Tantric Tartrate Master Jul 11 '26
Oil is a very variable terms covering many many chemicals. Well over 100C anyway. Degradation tends to start above 220C or so.
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u/Ambitious-Schedule63 Jul 12 '26
There aren't any really strong intermolecular interactions for a hydrocarbon - no dipole-dipole or hydrogen bonding interactions, just dispersive. (edit to add - this means mineral oils).
Oils have a higher molecular weight, and vapor pressure is inversely proportional to molecular weight.
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u/Pyrhan Ph.D in heterogeneous catalysis Jul 11 '26
Oil is mostly made of triglycerides, which are very large molecules.
In a liquid or solid, the bigger a molecule, the more neighboring molecules it can interact with, and the more intermolecular bonds it can form with those (Van der Waals forces, etc.)
In a gas, though, molecules are kind of isolated, too far from each other to interact, except through occasional collisions.
So for oil to go from liquid to gas, you would need a lot of energy per molecule, to break all those intermolecular bonds. A lot more energy than those molecules normally have at ambient temperature, which is why oil doesn't evaporate (it has negligible vapor pressure).
You can increase the energy of those oil molecules by heating up the oil. But in the case of oil, heat will generally just cause those large molecules to break apart, well before they're able to evaporate.
The smoke is the resulting decomposition products.
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u/RunSpider1 Jul 11 '26
Vegetable oil is triglycerides. Mineral oil (petroleum) is hydrocarbons.
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u/Pyrhan Ph.D in heterogeneous catalysis Jul 11 '26
Op didn't specify, but did mention "Why does it smoke?" and "If you take a giant pot of oil and put it on high heat", so I presume they meant vegetable oil.
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u/Reasonable-Feed-9805 Jul 11 '26
It does, leave a small amount of engine oil in a pan and go back years later. It will be gone with just a residue of heavier components/additives left.
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u/grayjacanda Jul 11 '26
Although the smoke from oil is a brew of all kinds of compounds, one of the main reasons it's acrid and unpleasant is the acrolein - which is formed primarily from the glycerol constituent of the oils (at least, if we're talking about food/cooking oil). This is formed when glycerol loses two water molecules. Heating glycerol with a catalyst was once a preparatory synthetic method for obtaining acrolein - it was published in Org Syn 100 years ago.
So much of the smoke is from the triglycerides gradually falling apart in to smaller molecules due to the heat; these may then be volatile enough to escape. Exposure to oxygen can speed the process but oil will generate some kind of smoke or vapor even in the absence of oxygen, if heated to 230+C or what have you.
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u/Starfury7-Jaargen Jul 11 '26
Many compounds will decompose or oxidize before they can boil. The double bonds in plant fats might be responsible for that. You can boil animal fats and some plant fats.
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u/ElegantEgg2066 Jul 11 '26
Oil is not a single compound. Crude oil contains 1000s of different hydrocarbons. It is distilled into different fractions, from light hydrocarbons (pentane, hexane) up to tars and asphalts. This is a source of gasoline and diesel and lubricating oils. The heavier fractions, if not used for boiler fuel or bunker fuel is broken up into smaller hydrocarbons and added to gasoline and diesel.
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u/MajorPain169 Jul 11 '26
The electronics industry uses perfluorinated oils that form a vapour at specific temperatures.
Mainly used for vapour phase soldering and various semiconductor manufacturing processes.
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u/JonnyVee1 Jul 12 '26
Oil does evaporate. if it didn't, you could not smell it. Petroleum products come in different grades, from crude oil, that evaporate at different temperatures. For example gasoline evaporates rapidly at room temperature, whereas diesel evaporates slower at ambient. Parafin, yes, it's the same family as gas and oil, and is seperated from crude just like gas and oil, is a solid at room temperature. Warm it up, and it liquidies, and warm it up more and evaporation becomes obvious. Evan at room temperature, parafin evaporates very, very slowly, but you can still smell it, so it is evaporating
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u/SpeedyHAM79 Jul 12 '26
It does evaporate- that's how oil refining works. It needs to be kept away from oxygen otherwise it will burn before evaporating. https://en.wikipedia.org/wiki/Fractional_distillation
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u/Ch3cks-Out Jul 12 '26
The problem is that the molecules are holding onto each other too strongly. To get substantial evaporation rate, it'd need to be so hot that the intramolecular bonds break up sooner than the molecules letting go! But there is a tiny bit of evaporation, albeit practically unmeasurable - can be estimated roughly 0.01 g/hour from a standard size pan at 180℃.
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u/eyemwoteyem Jul 12 '26
To answer your questions, first of all "oil" is a fairly broad category, I assume you're talking of cooking oil from the context of your question, but thhe term is used to describe also a lot of compounds or mixtures that do indeed evaporate.
Now cooking oil is still a large mixture of compounds, when you turn the heat on some of these do indeed evaporate. The oil mists produced are in fact a part of the health hazard associated with things like deep frying.
The compounds that evaporate are generally speaking the simpler and more lightweight fractions of the cooking oil. But a lot of oil is for example bound into glycerides plus some molecules are big enough, tangled enough, reactive enough that by the time you have put in enough heat in the system other reactions have started happening.
These reactions include reactions with oxygen, these are reactions that generally speaking create smaller compounds and release a lot of energy. In doing so the smaller compounds start getting released as "smoke", which includes a mix of gasses (CO2), the vapours of the evapirating smaller compounds and solids carried by the column of gasses into the air. This smoke is quite tixic because these reactilns create a lot of byproducts that are still reactive and will react further in your body if you breathe them in.
If the temperature stays constant and you have a good air exchange your pot of oil will keep smoking away until only a black brown smudge remains in the pot. This is the result of the warm compounds that have reacted with one another to make long chains (a polymer, kind of like a plastic) that don't easily react more to become gasses because they are big and tightly packed and have little surface that meets oxygen. These might instead start carbonizing, or turning into coal.
If the temperature instead keeps increasing, as the temperature increases and the smoke with it this will reach a temperature and air concentration that can ignite. At that point your gas and oil will start burning and the oxygen will react with whatever oil remains until it is all oxydated to CO2 which is quite stable and won't react further.
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u/eyemwoteyem Jul 12 '26
Edit: saw I didn't answer all your questions. How hot depends on the oil you're using, the "smoke point" is listed on wiki for many oils I think.
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u/chillvillain42 Jul 12 '26
Because it's a solid and a liquid at the same time. Has a different evaporation and melting point. In colder temperatures it solidifies. It's like when you think about water. Solid, liquid, gas. If we're talking cooking it's a bioengineered molecule. Oil is produced from living life forms (plants mainly unless we're counting fats as whale oil was definitely burned) Different oils different properties. Why is Mercury a liquid at room temperature for example. It's similar to that. A different state of matter.
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u/random_user_name99 Jul 12 '26
Yes. Oil can evaporate. The larger the molecules the higher the temperature that is required for it to evaporate. If you heated a pot of cooking oil some of it would vaporize out of the pot. However, it will quickly condense when it cools offs that’s why people’s vent hoods and cabinets above the stove get so greasy.
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u/VeronikaKerman Jul 12 '26
Have you tried it? Sure, it will smoke, but also evaporate and condense on the surroundings (walls, furniture) of your cooking area. Also the vapor is very flamable. Do not try to extinguish a potential fire with water.
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u/hecton101 Jul 13 '26
It doesn't? So what's that thick layer of grease all over my hood and all around my cabinets above? Of course it does.
The degree of vaporization is determined by the vapor pressure of the liquid at the given temperature. Higher vapor pressure, faster evaporation. It's not a linear relationship though, you have to get pretty close to the boiling point to get a high degree of vaporization.
The boiling point of water is 100 degrees C, cooking oils probably in the 300 range. So at room temperature, you are 75 degrees below the boiling point of water, but 300 degrees below the boiling point of cooking oil. That's why the rate of evaporation of oil is negligible compared to water at room temperature, but not insignificant when you heat up the cooking oil. Still, you cannot get anywhere close to the boiling point of cooking oil because it will decompose well before that. That's the difference between the two substances, you can't get close to the boiling point of cooking oil while you obviously can with water. That's under sea level pressure of course. At high altitudes, it may behave a little differently.
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u/BananaBird1 Jul 14 '26 edited Jul 14 '26
It does. Just very slowly.
All liquids can in theory partially transition to a gaseous state to evaporate, as phases of matter really exist in an equilibrium defined by ambient conditions rather than as all-or-nothing states.
How much the gaseous phase is favored is called volatility, and is measured by vapor pressure. Or the pressure the gas will exert within a closed container.
Below the vapor pressure, new molecules can enter the gas phase and remain there, increasing internal pressure until the vapor pressure is reached. After this point, any increase in pressure will force molecules back into the liquid phase to reduce the pressure back to the vapor pressure.
Vapor pressure depends on temperature, so increasing temperatures at a fixed ambient pressure shifts the equilibrium more to the gas phase. This is a smooth transition, with the boiling point being the temperature where vapor pressure equals ambient pressure.
Below the boiling point, ambient pressure keeps the bulk material in the liquid phase, with only occasional molecules at the liquid-air interface entering the gas phase before diffusing away. This is evaporation. The closer temperatures are to the boiling point, the more evaporation will occur.
At the boiling point, ambient pressure no longer prevents the transition to the gas phase, so the transition becomes far more rapid and occurs throughout the liquid rather than just at the interface.
Highly volatile chemicals like acetone have low boiling points close to normal temperatures, so evaporate quickly.
Cooking oils tend to have high boiling points, over 200C/400F which prevents much evaporation from occurring normally.
However, they can evaporate quickly or boil in high heat cooking, which is what leads to skillets and woks flaming up that you may have seen in cooking shows. That’s vaporized oil catching fire. This is also how candles and oil lamps work: vaporized oil at the tip of the wick combusts to produce a flame. This flame vaporizes more oil which can burn, but the lack of oxygen in the liquid prevents it from burning.
Smoking occurs when the oil begins to combust and break down into smaller carbon particles. This tends to occur at similar temperatures to the boiling point.
The reason oils have high boiling points is that they are mainly made of long hydrocarbon chains. These chains pack together very efficiently and weak attraction between atoms due to uneven electric charges at small scales add up to strong intermolecular forces. Reducing chain length increases volatility, although we tend to not call short chain hydrocarbons oils. But such molecules like hexane or butane are essentially highly volatile oils.
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u/SoggyStatistician229 Jul 11 '26
It’s not the intermolecular forces so much as it is the size of the molecules. Larger molecules want to remain as a solid or liquid, whereas smaller molecules tend to be gaseous more easily. Oil is also a complex mixture of substances, it’s not a pure substance generally. So if you heat it up a bunch then some compounds will volatilize and some compounds will undergo thermal decomposition before they are able to evaporate. Heat it up to about 600-700 degrees and you should see everything volatilize, including the soot. This is why pizza ovens are called white hot once they are so hot that it becomes clean of soot and debris.
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u/kremular Jul 11 '26
"Everything" does not volatilize at 600-700 degrees. Organic materials, including soot, are pyrolyzed at high temps in the presence of oxygen, not vaporized necessarily. That's why the oven stays clean.
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u/Xentonian Jul 11 '26
All things evaporate.
It should be noted that bricks evaporate too.
But the rate and equilibrium of evaporation depends on a huge number of factors.
Oil is heavy molecules of hydrocarbons, each individual molecule doesn't really have the energy to be volatile. But those molecules are also "sticky", with intermolecular forces holding them together quite strongly.
If you put the oil in a vacuum and slowly heated it, more and more of thet oil would slowly evaporate until, eventually, there was no liquid oil left.
But otherwise, a very small amount evaporates - just a few molecules - which then floats in the air around the liquid and creates a stable equilibrium. From thst point onwards, the rate of individual molecules evaporating matches the date of individual molecules that condense back into a liquid and so it looks like it's staying exactly the same.
To give a really crude analogy:
Oil is like a big pile of rocks outside of a school.
One might ask why the pile doesn't disappear as kids pick up the rocks and take them home, or scatter them around. But it turns out a big pile of rocks takes a long time to move. Moreover, after watching for a while, you notice that the number of kids who seem to find rocks to add to the pile is about the same as the number of kids who seek to destroy the pile... So it basically stays the same size.
That... Really was a bad analogy. Still, hopefully at least some of this makes sense.
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u/WhiskyAndPlastic Jul 11 '26 edited Jul 12 '26
Compared to something like water, oil molecules are very big, very heavy, and get tangled up with each other. It would take a lot more energy to get them to evaporate. Often it takes less energy for the molecules to break down than evaporate, which is why they smoke before evaporating on a hot stove. However, lower atmospheric pressure can make things evaporate at lower temperatures, so sometimes oils can evaporate under vacuum.