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If I warmed two cups of water from 20 °C to 90 °C, one in a microwave and one over a flame. Is there a test that could be performed to determine how each cup was warmed? Are there residual effects from different types of "warming"? We can assume 30 seconds pass after removing from the heat sources before testing.

Do not take the containers into account. They both get poured into identical new containers before testing.

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    $\begingroup$ Just adding my thoughts here: the way heat is distributed in the beginning is different. When heating over a flame, the heat is mostly originating from the bottom with a gradient towards cold at the top. Heating in a microwave is more uniform, unless the microwave has weird hotspots. The heating should at least not come from the bottom. You should be able to pick this up with a thermal camera, but it might be hard to already after 30 seconds. There could be soot from the flame deposited on the glass, but I don't think you mean this kind of 'residual effect'? $\endgroup$ Jan 11 at 12:28
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    $\begingroup$ Energy is energy there is no difference in it based on source. $\endgroup$
    – Qwerty
    Jan 11 at 13:14
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    $\begingroup$ I guess that misses the point of the question, but you could check for chemical differences: The fire could introduce soot particles or CO₂ into the water. $\endgroup$
    – A. P.
    Jan 11 at 22:06
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    $\begingroup$ You may think you've "solved" the container problem, but you've actually added a new problem which invalidates most answers. All answers consider the effects of heating that remain in a static, undisturbed liquid due to local hotspots and similar features of microwaving. If you pour this liquid into another container, you've mixed up the liquid and trashed that. You need to remove the edit about changing containers, and simply say "not including examining the container", otherwise the answers so far no longer answer the edited question. $\endgroup$
    – Graham
    Jan 12 at 11:44
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    $\begingroup$ @AccidentalTaylorExpansion It's probably hard to measure when heating water in a microwave, but when heating something more solid it's apparent that yes, microwave do have "weird hot spots". That's also one of the reasons why we usually have a rotating plate inside the microwave. $\endgroup$
    – Stef
    Jan 13 at 11:09

9 Answers 9

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No, and this is one of the important facts about thermodynamics: Once a system has reached equilibrium, its "thermal history" (how it got heated in the first place) has been erased and cannot be reconstructed.

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  • $\begingroup$ We shouldn't only consider thermal properties of the water. The paper Blackhole references about changing the water refractive index of microwaved water looks promising as a way to determine how the water was heated. $\endgroup$
    – Joe
    Jan 12 at 1:07
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    $\begingroup$ @joe, beware. I see at least one of those authors has published before on "nanoscale water clusters", which appears to be generally dismissed due to lack of replication. There are a large number of non-replicable papers on this subject, perhaps due to its intersection with homeopathy. I would want want to see replication here before assuming such effects are "promising". $\endgroup$
    – BowlOfRed
    Jan 12 at 1:38
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    $\begingroup$ Fair, I worded my answer poorly. I meant it was promising to see a response that approached the question from a direction other than thermal differences. I didn't review that paper and thought it was a good science (e.g. reproducible). Another angle would be electrical differences maybe, either would bump an electron out somehow? Another factor of course would be how pure the water samples are, could find side effects perhaps of any contaminants in the water getting heated differently. I was thinking tap water getting heated which of course has all kinds of things in it that might change. $\endgroup$
    – Joe
    Jan 12 at 12:00
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    $\begingroup$ @AfterShave, that effect is not unique to microwaved water. it is called bump boiling and happens in test tubes and flasks being heated by a gas flame.. Bump boiling is the exact mechanism upon which thermal inkjet printheads squirt ink out of their nozzles. the heater in this case is a thin-film electrical resistor. $\endgroup$ Jan 12 at 17:04
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    $\begingroup$ It is far from equilibrium after 30 seconds. There will still be significant macroscopic movement. $\endgroup$ Jan 12 at 22:36
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I agree with niels nielsen's answer at the spherical cow level, but I can see two factors that will be different in the real world. Whether they can be measured or not I do not know. Both come down to how even the heating was.

  1. The fire-heated water took longer to heat. This means more evaporation, the impurities will be slightly more concentrated in the fire-heated water. And, if you started from the same initial amount of water there's a tiny bit less of the fire-heated water left.

  2. The fire-heated water heated more unevenly, thus more of the water went above 90C during the heating process. The warmer the water the less dissolved oxygen (more was driven out in the heating.)

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    $\begingroup$ Where did you get the "took longer to heat" from? One could run the microwave oven on a low power setting and use a high-temperature torch for the flame version, to have an extreme example. $\endgroup$ Jan 12 at 0:23
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    $\begingroup$ What is spherical cow level ? $\endgroup$ Jan 12 at 4:02
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    $\begingroup$ @An_Elephant Extreme abstraction. Google 'spherical cow'. $\endgroup$
    – philipxy
    Jan 12 at 4:16
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    $\begingroup$ It was funny at first and I laughed hard. $\endgroup$ Jan 12 at 4:48
  • $\begingroup$ @PaŭloEbermann True--if you had a low enough power in the microwave you could defeat test #1. I was looking at typical power levels, though. $\endgroup$ Jan 13 at 18:21
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According to Yakunov, A. V., Biliy, M. M., & Naumenko, A. P. (2017):

There are so-called specific effects of microwaves. The first of them, although they have a thermal nature, cannot be reproduced in the common convection heater. These are (a) the effect of overheating, (b) selective heating, (c) the temperature gradients on the border, and so forth.

I'm not sure if these thermal effects have any detectable impact on water, though.

The possibility of nonthermal effect of microwaves on the different processes and substances, despite the considerable amount of experimental work, is the subject of lively debate recently. The mechanism of this influence is not entirely clear.

The existence of nonthermal effects of microwave seems to still be a lively debate. However, for instance, Asakuma, Y., Maeda, T., Takai, T., Hyde, A., Phan, C., Ito, S., & Taue, S. (2022) recently show that:

The phase velocity of light in water increases up to ~ 5% (RI of 1.27) during microwave irradiation. Upon stopping irradiation, the return to the equilibrium RI was delayed by up to 30 min. Our measurement shows that microwaves have a profound non-thermal and long-lasting effect on the properties of water.

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    $\begingroup$ Overheating is pretty much an issue in everyday microwave use. You heat pure water all the way to almost boiling, you put a tea bag in, you get a small steam explosion and burns. The OP asks about heating to 90C so this should not be an issue. The RI thing smells like a bad science, but this is an oppinion only. $\endgroup$
    – fraxinus
    Jan 14 at 13:44
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If you leave the water samples inside the container used to heat them up, then you might check the temperature distribution and try to determine which way they were heated.

The water temperature will be more evenly distributed when heated over a flame due to convective currents in the liquid. The heat source is at the bottom so the very hot water in contact with the bottom of the container will rise to the top and the colder water at the top will fall down, be heated, rise up and so on. This self-stirring mechanism makes the water temperature more uninform.

Conversely, the microwave oven heats throughout the volume and so the water will tend to naturally stratify according to temperature (because above 4°C water density decreases with temperature). In other words there's no mechanism moving the colder water towards the heat source, like in the case of convective heating.

Here's a paper discussing CFD simulations and actual experiments: https://doi.org/10.3969/j.issn.1002-4956.2008.12.009

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  • $\begingroup$ How is that relevant after the water is "poured into identical new containers before testing"? $\endgroup$
    – PM 2Ring
    Jan 13 at 2:20
  • $\begingroup$ You're right. I added an introductory paragraph. I mostly wanted to answer because from other answers and comments it looks like people tend to assume the opposite temperature behaviour for the two heating methods. $\endgroup$
    – Luca Citi
    Jan 13 at 4:13
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If you've ever tried to microwave water for tea in a smooth mug rather than heating the water first in a kettle over a flame or electrically you'll probably have noticed the unpleasant frothing when a teabag is immersed.

I suspect the froth is from the gentle distributed heating in the microwave creating a supersaturated solution of air in the liquid, followed by the bag creating cavitation sites, and it would go away if you allowed the water to boil vigorously in the microwave for some time, so it's not a foolproof test.

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  • $\begingroup$ This. The frothing also disappears if you stir vigorously the water before putting the tea bag in. The uniform warming could be the cause because I have an induction stovetop, and it did not happen with it. (But I suspect that after pouring the water to another cup, the effect will disappear also). $\endgroup$
    – Rmano
    Jan 14 at 19:50
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If you wait until thermal equilibrium is reached in the water, no.

But, you said:

We can assume 30 seconds pass after removing from the heat sources before testing.

In which case, if you put enough temperature sensors into the water to create a 2D or 3D profile of the water, perhaps, or maybe even probably, because maybe there will be a temperature gradient throughout the water that differs for microwave heating vs flame heating. The flame being applied to the bottom would result in the hottest water on the bottom, whereas the microwave might be more evenly heated, or perhaps have hot spots interspersed in the water. This would be interesting to test.

See also this comment. A thermal camera might be used too.

And with the update:

They both get poured into identical new containers before testing.

Back to "no" again, once you've disturbed the original heat gradient, unless a unique heat distribution "signature" resulted from each type of heating.

Note that this signature might be in the form of a thermal camera video of the water after being poured, rather than in the form of a picture, because the pouring process would be very dynamic.

This would be a fun thesis to work on. AI and machine learning might be trained on thermal video samples of this pouring being done for both heating types and then be tested to see if it can accurately determine the way the water was heated by looking at the thermal video of the water during or just after being poured. Even if humans can't pick up clear thermal patterns, maybe AI and computers can.

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    $\begingroup$ I think it's the other way around. Microwave heats evenly but the hotter water rises to the top so you end up with a temperature gradient. If you want to heat a liquid in a microwave you need to stir every minute or so. On the contrary, the flame heats at the bottom, thus inducing convective flows in the water that tend to make the water temperature more uniform. It's sort of a self-stirring heating process. $\endgroup$
    – Luca Citi
    Jan 12 at 23:43
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    $\begingroup$ See here: yahoo.com/lifestyle/… and here doi.org/10.3969/j.issn.1002-4956.2008.12.009 $\endgroup$
    – Luca Citi
    Jan 12 at 23:46
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Yes you can. Water heated by a microwave causes all of the molecules to vibrate evenly throughout the liquid and this removes most of the oxygen from the water very efficiently. Water heated on a stove does not remove the same amount of oxygen as done by microwave action.

You can perform a simple oxygen level test to determine the level of oxygen in each heated sample. The sample with the lowest oxygen level is the sample most likely heated by a microwave.

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    $\begingroup$ Hm, judging from Loren Pechtel's answer, it might be the other way around (more hot water in the flame-heating case, which effectively pushes out dissolved gases). But unless the effects cancel out completely (degassing evenly everyhwere or concentrated strong degassing), this effect could be measurable and still be measurable after 30 seconds (though there will be further loss of dissolved gases during that time, and there is a saturation effect for the flame case). $\endgroup$
    – Chieron
    Jan 12 at 9:18
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    $\begingroup$ Then we should to the experiment to test this conjecture. $\endgroup$
    – steveK
    Jan 12 at 14:37
  • $\begingroup$ see also this answer to an equivalent question: physics.stackexchange.com/a/76188/96088 $\endgroup$
    – Chieron
    Jan 12 at 15:25
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    $\begingroup$ Re "vibrate evenly throughout the liquid": No, there are standing waves in there (constructive and destructive interference). That is why there is a rotating plate to try to even it out (but it is still not even heating). Otherwise, the rotating plate wouldn't be needed. $\endgroup$ Jan 12 at 22:39
  • $\begingroup$ I would agree that I may stand to be corrected. These standing waves would also create a moving barrier that would prevent O2 from evaporating, by reaching the surface. Is O2 size > H2O size? Wouldn't the force created by of mass O2 overcome the resistance of H2O trying to block it from reaching the top to evaporate? Until we test O2 levels, this remains an open question. $\endgroup$
    – steveK
    Jan 13 at 1:41
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If you leave the water in the same containers as when they were heated, then yes, but otherwise no.

If you have a very sensitive thermometer, you might be able to tell that the water in one container (the microwaved one) is slightly more uniformly-heated than the one heated by flame. But when you pour the water out, the flow would cause the heat to be pretty much identically distributed in both containers after the fact.

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Yes, there can be a measurable difference if the measurements are performed immediately after heating (no stirring).

Microwaves heat liquids unevenly, making the liquid at the top of the container much hotter than the liquid at the bottom as compared with conventional stove top heating.

https://publishing.aip.org/publications/latest-content/the-problem-with-microwaving-tea/

https://www.cnn.com/2020/08/04/world/tea-boil-water-microwave-trnd-scn/index.html

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