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Q & A: Boiling Oil & Water

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Q:
What is the boiling temperature of cooking oil? Using the same amount of heat, cooking oil and water, which liquid will boil first? Why is one liquid boiled before the other? Thanks for your answers.
- Kevin Nguyen (age 10)
Fremont Christian School, Fremont, CA
A:

Kevin -

One question at a time. Your first question is actually the toughest. This is because it's hard to measure the boiling point of oil. The reason is that well before it reaches its boiling point, oil will start to smoke. This is called the ’smoke point’. The smoke points for some common cooking oils are here:

Safflower - 510 F (266 C)
Soybean - 495 F (257 C)
Corn - 475 F (246 C)
Peanut - 440 F (227 C)
Sesame - 420 F (216 C)
Olive - 375 F (191 C)
(from http://wywahoos.org/wahoos/cookbook/tools.htm)
The exact temperatures will also depend on how pure the oil is.

The boiling point estimates that I’ve found are pretty sketchy, but a fair estimate for soybean oil (most cheap cooking oil is soybean oil) is about 300 C (or 572 F).

You can compare this to the boiling point of water, which is 100 C (or 212 F). The boiling point of a liquid is the temperature where the liquid will change into a gas. The reason that different liquids boil at different temperatures is because of the chemical bonds that hold them together. So when I say that oil has a higher boiling point than water, what I am actually saying is that the chemical bonds that hold oil together are stronger than the ones holding water together - it takes more heat to break them apart. The main reason for this is that the oil molecules are much bigger, so each one has more surface to stick to the other ones.

So what does this mean in real life? Let’s say you took a pan of oil and a pan of water and put them both on the stove. Then you turn the stove on to heat them both up at the same rate. Once they get up to 100 degrees C, the water will start to boil. And around 257 degrees C, you’ll see the oil start to smoke. But you’ll have to get all the way to 300 degrees C before the oil will boil. So the water boils first and the oil last.

-Tamara


(republished on 07/24/06)

Follow-Up #1: boiling oil and LDL cholesterol

Q:
does it mean if I use the oil with the highest smoke point to fry my food it will not affect my LDL as long as the oil does not boil? I am just trying to establish a case as to why my cholesterol level has jumped quite a bit. My only conclusion was my fried food because I love my pan-fried fish. I watch what I eat.
- Donny Lim (age 44)
Gold Coast, Australia
A:

There’s no direct connection between the boiling point and the effect on LDL’s.  We’re no MD’s here, but can report the conventional wisdom about oils.  You do want to avoid smoking, because as oil starts to oxidize there are said to be nasty chemicals formed. You might want to use an oil with a high smoke point and with little saturated fat and no trans fat. Refined high oleic canola oil might be good.

Mike W.


(published on 06/07/07)

Follow-Up #2: hot oil

Q:
Why does oil result in deeper/ worse burns thatn water?
- Susanne (age 38)
California
A:

Water boils away at 100°C but oil can get much hotter.

Mike W.


(published on 09/04/09)

Follow-Up #3: boiling mixed oils

Q:
what would happen if you boiled all of the oils in one pot?
- bill gjhu
miami,flordia,america
A:
So long as the oils form a stable solution, you know that their chemical potentials (and hence vapor pressures) are lower in the mixture than they are separately. Hence the boiling point (the temperature where the total vapor pressure is 1 atmosphere) is raised above the value of the lowest of the constituents.

That's not terribly informative if you have a mix of oils with different boiling points. One standard approximation, which can often work pretty well, is to say that the vapor pressure is the weighted average of the vapor pressures of the constituents. The fraction of the average that comes from each oil would be proportional to the fraction of the molecules of that type, not the fraction of the weight. You then look at that total vapor pressure as a function of temperature. (The vapor-pressure vs. T curves are usually well described by the Clausius-Clapeyron relation, so if you have its parameters for each oil, you can estimate the total vapor pressure without having to look up values for each temperature.) The boiling point is where that total vapor pressure reaches atmospheric pressure.

Mike W.

(published on 10/14/09)

Follow-Up #4: Don"t drink boiling oil.

Q:
what would happen to you and the oils if you drank the mixture of oils from a glass at there boiling temperature?
- Mary (age 9)
New Jersey
A:
You would get severely burned.  Don't try this at home.

LeeH

or anywhere else. Mike W

(published on 02/09/10)

Follow-Up #5: Heating oil and water

Q:
will the same amount of heat applied to water and separately applied to oil cause either to reach a higher temperature (due to their individual innate properties)? or will the same amount of heat applied, result in the same end temperature?
- ari (age 30)
teaneck, nj,
A:
The oil will get hotter than the water for a given amount of added heat.  That is due to the difference in Specific Heat capacities.   The defining equation is ΔT = ΔQ/C where ΔT is the change in temperature,   ΔQ is the amount of added heat, and C is the specific heat capacity.   C for water is approximately twice that of typical oils so its temperature rise will be half of that for the oil.
The real question is 'Why is Cwater larger than Coil?'.  Roughly speaking, you pick up the same heat capacity for each available 'degree-of-freedom". Small molecules can water pack a lot of different modes of motion and of squashing into each other like springs into a small volume. Bigger molecules like most oils, rather stiff in some directions, tend to have fewer available ways of moving  and squashing per volume.

See:  http://en.wikipedia.org/wiki/Heat_capacity  for information on heat capacity.

LeeH

(published on 06/25/10)

Follow-Up #6: evaporating water through oil

Q:
If a layer of oil was placed on top of a layer of water and then heated would the water evaporate when the temperature reaches 100c?
- Omkar (age 12)
Auckland, New Zealand
A:

Actually, the water can evaporate at lower temperatures than 100°C, just as it could with no oil. However, the oil layer can slow the evaporation dramatically. If the water is  heated to boiling, with big bubbles of vapor popping up from inside the liquid, the oil will be less effective in slowing the evaporation.

Mike W.


(published on 06/27/10)

Follow-Up #7: olive oil boiling

Q:
I realize that unsaturated oils takes a lower temperature to boil as their bonds are easily broken, while saturated oils the opposite. Yet why does my olive oil needed a very high temperature, at around 90 c to boil while my palm oil is relatively lower than that temperature? Does using an Extra Light olive oil affect these odd results? (This is an experiment, and all of my three results for boiling olive oil were constant at that high temperature.) Thanks in advance.
- Dita (age 17)
Jakarta, Indonesia
A:

I'm puzzled by that 90°C figure for the olive oil boiling point. It seems way too low. Then palm oil was even lower? Was there perhaps a typographical error?

Mike W.


(published on 12/27/10)

Follow-Up #8: Experimenting with hot oil?

Q:
Would it be a good idea to heat oil for an experiment to about 45 degrees Celsius? This would not even begin to approach the smoking point for vegetable oils correct?
- Micah Cloward (age 16)
East Stroudsburg, PA, USA
A:
I'm not in favor in heating up oil for experiments except under carefully controlled conditions.  I agree 45oC is not that hot, my worry is how do you heat it.   If you use a flame like a Bunsen burner, an accidental spill could ignite it.  Likewise on a hot kitchen stove.  Please ask your chemistry class supervisor, or whomever, to give you advice on proper safety procedures.

LeeH

(published on 02/08/13)

Follow-Up #9: removing water from oil

Q:
Because water and oils have different boiling points, I should be able to separate the two using heat correct? I have fair amounts of heating oil saturated with desolved water so it fails to seperate on its own. If I understand, I should be able to heat the mix and the water should evaporate out leaving the oil behind, like distilling rigt?
- tony (age 30)
seattle wa usa
A:

Right, the water tends to evaporate faster than the oil, so that should help. We (sort of) answered a fairly close once before (http://van.physics.illinois.edu/qa/listing.php?id=2641) so you could have a look at that, with the link it gives. You won't be able to get rid of all the water,since the part in solution just evaporates, rather than boils, but you may be able to get rid of enough to make the oil usable. You might have a look at this link-  http://xhttp://www.machinerylubrication.com/Read/503/removing-water-in-oil- for a more professional discussion of your options.

Mike W.


(published on 01/09/14)

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