Boiling Oil & Water
Most recent answer: 10/22/2007
- Kevin Nguyen (age 10)
Fremont Christian School, Fremont, CA
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)
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.
(published on 10/22/2007)
Follow-Up #1: boiling oil and LDL cholesterol
- Donny Lim (age 44)
Gold Coast, Australia
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 sunflower oil might be good.
(published on 10/22/2007)
Follow-Up #2: hot oil
- Susanne (age 38)
Water boils away at 100°C but oil can get much hotter.
(published on 09/04/2009)
Follow-Up #3: boiling mixed oils
- bill gjhu
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.
(published on 10/13/2009)
Follow-Up #4: Don"t drink boiling oil.
- Mary (age 9)
or anywhere else. Mike W
(published on 02/09/2010)
Follow-Up #5: Heating oil and water
- ari (age 30)
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: for information on heat capacity.
(published on 06/16/2010)
Follow-Up #6: evaporating water through oil
- Omkar (age 12)
Auckland, New Zealand
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.
(published on 06/26/2010)
Follow-Up #7: olive oil boiling
- Dita (age 17)
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?
(published on 12/26/2010)
Follow-Up #8: Experimenting with hot oil?
- Micah Cloward (age 16)
East Stroudsburg, PA, USA
(published on 02/06/2013)
Follow-Up #9: removing water from oil
- tony (age 30)
seattle wa usa
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
for a more professional discussion of your options.
(published on 01/07/2014)
Follow-Up #10: How much oil left in pot?
- Thomas LaCroix (age 26)
Encinitas, CA, US
First of all, smoke point is the temperature at which your the macromolecules in the oil start to break down. That is, there is a chemical change, rather than a simple physical transition. The parameter that we are concerned with would most likely be the vapour pressure of the oil and water at the temperature within the pot. Estimation of evaporation rates is discussed in here: http://hubpages.com/sports/Determine-Evaporation-Rate-for-Swimming-Pool. Dependence on the material (oil vs water) comes in two places: mass transfer coefficient and vapour pressure. Also note that boiling is not same as evaporation.
Still, I think what you are asking is quite hard to estimate. That is because oil-water is not a homogeneous mixture, but the oil rather floats as a layer between the water and air. The evaporation rate is highly dependent on the accessible surface area, this is why you need to hang your laundry rather than putting them altogether in a basket. This means oil and water will have different effective surface areas. It also depends on the velocity of the breeze and the temperature of the air in the immediate surroundings, since there is also the condensation process.
(published on 12/15/2015)
Follow-Up #11: temperature of oil and water
- Gary (age 52)
No, very basic thermal physics says that things come to the same temperature, not different temperatures. The deep reaon for that applies whether or not the things are made of the same stuff.
(published on 01/05/2017)
Follow-Up #12: heating kettle to 300°F?
- Zach (age 12)
Arvada, CO, USA
A standard electric kettle will shut off if its coils get too hot. They aren't designed to heat to 300°F. An ordinary pot on a stove is the stndard way of heating oil. It requires a lot of caution, since hot oil can give really bad burns.
(published on 02/15/2018)