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 risi...
@rob Right if it only enters from the top surface. But for the pot in the oven, heat transfer occurs not only at the top (open) surface by convection, but from the sides and bottom by a combination of convection and conduction. So I think bubbles can still be in the bulk of the liquid. Anyway, give me a link to our other answer as I'm interested in seeing it. Thanks for the feedback
The other answer is correct. This answer conflates two issues - slower boil rate due to convection (which is true, but irrelevant); and that the heat comes from all sides rather than only from the bottom, which is what causes the rolling bubbles. If you covered the pot and put a conduction heat-source on top of the pot, you'd see the same boil rate but significantly fewer bubbles.
This answer is incorrect in that it ignores that surface evaporation happens at lower temperatures as the "rolling boil": The creation of vapor bubbles takes a surprisingly large amount of energy, which is not necessary when there is already a surface to evaporate from. On the other hand, the water evaporating at the surface cools the liquid down. Thus, as long as the heat input into the liquid does not exceed a certain minimum value, and thus lifting the equilibrium temperature above the rolling boil limit, there will be exactly zero vapor bubbles forming at the bottom.
As I explained in a comment to rob's answer that afaics we can conclude from first principles that the water does not reach 100 centigrade anywhere in the glass vessel, including the top surface. On the contrary: The surface is probably the coldest place. Never underestimate latent heat.
@EricDuminil Certainly there is always some water loss due to evaporation but it is strictly a surface phenomenon and would be significantly less than due to boiling. My only point is the OP is primarily observing boiling not evaporation.
@cmaster-reinstatemonica Evaporation does not cool a liquid down. It simply keeps it longer at the same (boiling) temperature. That is, the incoming energy is used to convert the liquid into a vapour, instead of heating it up more. Converting liquid to a vapour takes a lot of energy. And that is the same amount of energy, independent of whether that liquid is located at the surface or deeper in the container. The main reason why it evaporates/boils slightly more at the surface, is because the creation of bubbles takes slightly more energy, because it needs to push the liquid up to make room.
@fishinear Let's clarify this point by point: 1) Evaporation takes energy, and happens at pretty much any temperature. It's only faster when it's hotter. 2) If you don't heat a liquid, but let it evaporate, it will cool. Your refrigerator does this. 3) Equilibrium temperature is determined by amount of heating minus heat loss through evaporation. This may well below 100°C, as any Tour-de-France rider can attest.
@fishinear 4) To form a vapor bubble, you must overcome the surface tension. Due to that surface tension, the pressure inside the bubble is higher than the pressure of the liquid, which is higher than the air pressure at the surface. That higher pressure means that evaporation takes more energy. And the smaller the bubble, the higher the surface curvature, and thus the higher the vapor pressure inside the bubble. That's why you need microscopic impurities on the vessel's surface to facilitate boiling. Without such impurities, you can superheat water to well above 100°C (dangerous!).
@cmaster-reinstatemonica If you don't heat a liquid, but let it evaporate, it will not cool. Your refrigerator works by using a different process: adiabatic expansion, that is, rapid reduction of pressure. That does not happen in a container in an oven.
@fishinear "If you don't heat a liquid, but let it evaporate, it will not cool": This is plain incorrect (and does not even make sense as a statement: The history (whether it was previoulsy heated or not) has no influence on the effect of present evaporation). There is a whole family of technical appliances called evaporative coolers..
@fishinear For pure evaporation (no external heating) it will definitely cool locally (at or near the surface) because evaporation involves higher (than average) kinetic energy water molecules escaping the surface (overcoming intermolecular hydrogen bonds), thereby reducing the average kinetic energy of those remaining at the surface for a localized temperature drop. The extent (depth) of the cooling will depend on other factors, such as the ratio of the exposed surface area to volume.
@cmaster-reinstatemonica: You are saying that you can "superheat (liquid) water to well above 100°C" in a pot of "pure" water in an oven? Sure, the pressure at the bottom of the pot will be slightly higher than at the top (just accounting for the head) and as a result, the boiling temperature will be slightly higher. You can superheat steam way above 100°C, but liquid water isn't going to get above its boiling temperature at the current pressure.
@Flydog57 No. What I mean is this effect: en.wikipedia.org/wiki/Superheating . You can heat water to 110°C at ambient pressure using this effect. It's dangerous because once bubbles are introduced into the liquid (shaking and/or introduction of a rough surface, a metal spoon is enough), the liquid starts boiling immediately until it's back down to 100°C (or whatever the boiling point of the liquid is), possibly shooting up out of the container. And over your hands if they happen to be close by.
@cmaster-reinstatemonica Just tried it out myself in my oven, and yes, the water does reached 100 degrees, and yes it does boil in the way that Bob D describes, and no it is not cooler at the surface than at the bottom, and no, there is no significant circulation of water from the top to the bottom. The cooling effect due to evaporation is minor, and stops when a balance with the water vapour above the liquid is reached. Remember this is in an oven where the water vapour cannot go away.
@fishinear I never assumed that there are any temperature differences from 100°C within the water that you can actually measure with normal thermometers. When I talk about the cooling via evaporation, I mean that the water looses heat energy due to the evaporation. That does not mean that the temperature actually falls significantly away from 100°C. As to water vapor above the liquid, at 100°C water's vapor pressure reaches the ambient pressure. If you have superheated water, the vapor that's produced at the surface will simply blow the air above it away, unless your over is air-tight.
Discussion on answer by Bob D: Why do…
Discussion on answer by Bob D: Why do…