February 10, 2013

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

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.


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