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I put a pot of water in the oven at $\mathrm{500^\circ F}$ ($\mathrm{260^\circ C}$ , $\mathrm{533 K}$). Over time most of the water evaporated away but it never boiled. Why doesn't it boil?

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    $\begingroup$ Was it a nonstick pot? boiling happens more readily when there are "pointy" places to start bubble nucleation $\endgroup$ Jun 9 '20 at 13:00
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The "roiling boil" is a mechanism for moving heat from the bottom of the pot to the top. You see it on the stovetop because most of the heat generally enters the liquid from a superheated surface below the pot. But in a convection oven, whether the heat enters from above, from below, or from both equally depends on how much material you are cooking and the thermal conductivity of its container.

I had an argument about this fifteen years ago which I settled with a great kitchen experiment. I put equal amounts of water in a black cast-iron skillet and a glass baking dish with similar horizontal areas, and put them in the same oven. (Glass is a pretty good thermal insulator; the relative thermal conductivities and heat capacities of aluminum, stainless steel, and cast iron surprise me whenever I look them up.) After some time, the water in the iron skillet was boiling like gangbusters, but the water in the glass was totally still. A slight tilt of the glass dish, so that the water touched a dry surface, was met with a vigorous sizzle: the water was keeping the glass temperature below the boiling point where there was contact, but couldn't do the same for the iron.

When I pulled the two pans out of the oven, the glass pan was missing about half as much water as the iron skillet. I interpreted this to mean that boiling had taken place from the top surface only of the glass pan, but from both the top and bottom surfaces of the iron skillet.

Note that it is totally possible to get a bubbling boil from an insulating glass dish in a hot oven; the bubbles are how you know when the lasagna is ready.

(A commenter reminds me that I used the "broiler" element at the top of the oven rather than the "baking" element at the bottom of the oven, to increase the degree to which the heat came "from above." That's probably why I chose black cast iron, was to capture more of the radiant heat.)

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    $\begingroup$ I would interpret your experiment thus: The water at greater depths of a glass vessel never reaches boiling temperature, even though the outside is >100 deg. C. This implies cooling from the inside which can only be effected by convection (which is also why the non-convecting Lasagne does bubble). The only area which can shed excess heat though is the upper surface. This means that the evaporation keeps the water surface well below 100 deg. C! It never "boils", not even on top. The "cold" water from the surface sinks to the bottom, where it is heated and rises again... $\endgroup$ Jun 8 '20 at 9:37
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    $\begingroup$ @fishinear "Evaporating water does not cool it down" -- excuse me? For example in a Sauna the evaporating sweat cools the skin down to manageable levels even though the air is up to 110 centigrade. $\endgroup$ Jun 8 '20 at 16:13
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    $\begingroup$ When you say "the glass pan was missing about half as much water as the iron skillet" do you mean that you had started with equal volumes of water, but at the end of the experiment glass was e.g. ~75% full compared to the skillet's ~50% full? $\endgroup$
    – Caius Jard
    Jun 8 '20 at 17:04
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    $\begingroup$ @CaiusJard Yes, just so. I measured the initial and final volumes using an ordinary measuring cup. I probably started with about a liter in each pan. I don't remember the final volumes, but half-ish of the water boiled away sounds like a stopping point that I could have chosen while peeking through the oven glass. $\endgroup$
    – rob
    Jun 8 '20 at 17:15
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    $\begingroup$ @rob As far as I know it should be the same for the boiling transition. Microwaved water is taken to be dangerous because when superheated it tends to vigorously boil (that is, explode) when slightly perturbed. Your experience seems indeed different, and, from what you describe, I don't think it was just a matter of superheat. I apologise for presuming too much. It seems something else is at play. Again let me thank you for doing the experiment and sharing, this is most interesting, that is how real life experiments are not as easy as textbook descriptions may imply, $\endgroup$ Jun 9 '20 at 2:18
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The water didn't evaporate. It boiled. If you could look closely at the water in the pot in the oven you would see small bubbles rising within the liquid, which would indicate boiling. But you wouldn't necessarily observe what is sometimes referred to as a "rolling boil, i.e., large bubbles rising in the water indicating a high rate of boiling.

You get a faster boiling rate when the rate of heat transfer to the water is higher, as when you boil water on a range top set on high heat. The heat transfer rate in the oven when set on bake is much slower because it is heat transfer primarily by convection (contact with naturally moving air) as opposed to conduction (contact with a solid high temperature surface), which is a generally higher rate.

Evaporation is a different phenomenon that occurs at temperatures less than the boiling point and occurs only at the surface of the liquid.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – tpg2114
    Jun 9 '20 at 17:18
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Water boils both in the oven and on the stovetop. But one is called simmer and the other is called rolling boil. What you are asking about is the visual effect that is called rolling boil, and your question is basically why does it happen on the stovetop and not in the oven.

The answer is that the oven heats up the metal pot to some lower level, not even close to the air's $\mathrm{500^\circ F}$ in your case through direct contact with the air, while the stovetop is able (through direct contact to fire) to heat up the metal pot to around $\mathrm{900^\circ F}$, which leads to faster boiling and the effect of the visible rolling boil.

https://cooking.stackexchange.com/questions/21432/what-temperatures-do-low-medium-high-on-the-stove-correspond-to

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  • $\begingroup$ @cmaster-reinstatemonica correct, that is the max, but I will edit. $\endgroup$ Jun 9 '20 at 21:40
  • $\begingroup$ Why the dowvote? $\endgroup$ Jun 9 '20 at 21:41
  • $\begingroup$ Thanks for the edit. That was indeed the reason why I downvoted. I've removed that now. $\endgroup$ Jun 9 '20 at 22:15
  • $\begingroup$ @cmaster-reinstatemonica thank you so much! $\endgroup$ Jun 9 '20 at 22:26
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Short answer is that it boils, but it boils differently because it's either evaporation from a liquid surface in low temperature or "bulk evaporation" aka. boiling, due to temperature gradient. Now because oven heats more or less uniformly all sides of pot is heated the same, thus eliminating clear temperature gradient. Without temperature gradient "strong visual" bulk evaporation is impossible. Besides in oven air is heated to high degree, thus producing higher pressure to escaping water vapor molecules, so water becomes a little-bit super-heated, which may raise water's boiling point a bit. This is second reason why you don't see standard boiling effects as with boiling kettle.

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It might also be prudent to consider the environment of the oven itself. The "atmosphere" in the oven is already at a temperature > 100°C and this means that water in the "air" is in the gaseous state. As water evaporates at the surface at temperatures nominally > 40°C this would mean that the water vapour is "immediately" absorped into the gaseous state (immediately in the sense that the energy transfer occurs far fast because of the molecular energy) and the remaining water is nominally cooling the surface of the vessel via thermal convection. I believe this is thermo-dynamics in action :)

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If you include 'microwave' in the OP's ovens, the water in glass containers boils throughout its volume, not just the surface, indicating heat transfer barriers in conventional ovens.

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