Boiling point of water heated via microwave

[ScepticAmatuer]

I took about 100 ml of distilled water, put a small, clean rock in it (to prevent "bumping" or explosion) and heated it repeatedly in a microwave oven on high for 45 seconds.

After each heating I checked the temperature of the water with a digital temperature probe. I did this 7 or 8 times. I saw bubbles coming off of the rock and the sides of the glass.

The highest reading I ever got was 93.5 degrees C. I gave up when the quantity of water become noticeably reduced (50%).

So, I thought, the thermometer must be off. So I boiled tap water in a large pot to roiling and checked the temperature with the probe. 99.5 degrees C was the highest value I could read. That's about right for my altitude.

My question is why was the boiling point in the microwave lower? Shouldn't it have actually been higher due to superheating? Does this have anything to do with hydrogen bonding (or the breaking of hydrogen bonds)?

 

[DrClaude]

Microwave ovens are notorious for producing uneven heating. You may be observing local hot spots where you have boiling water, while the average temperature of the water is lower.

 

[ScepticAmatuer]

Microwave ovens are notorious for producing uneven heating. You may be observing local hot spots where you have boiling water, while the average temperature of the water is lower.

Possible, but I stirred vigorously with the probe as I took the readings. I did the same when I checked the stove heated water (a much larger volume).

 

[ScepticAmatuer]

I see now several problems with my experiment.

1. I should have used distilled water in both tests.

2. I should have used the little rock in both tests.

3. I should have used the same container in both tests (difficult under the circumstances).

4. I should have removed the stove heated water from the heat source before taking the reading exactly as I did in microwave case.

5. I should have brought the stove heated water to a boil repeatedly (same number of times, but I doubt I can use the same time of heating exposure).

6. I should have recorded each individual temperature measurement.

Not too great a design I guess.

 

[russ_watters]

Possible, but I stirred vigorously with the probe as I took the readings. I did the same when I checked the stove heated water (a much larger volume).

But you didn't stir it while it was being heated! You stirred it after removing it from the microwave, resulting in evening-out the temperature to something just below boiling.

When on a stove, you heat water from the bottom and sides, which is nearly perfect for uniform temperatures because as the steam bubbles rise, they will condense back into water if the water above is too cool, heating it up and resulting in very uniform temperatures.

 

[gjonesy]

The rock...its the variable in your experiment that probably caused the anomalies in your experiment. The microwave produces heat via electromagnetic field oscillation of water molecules, the stove top creates heat through conduction. The rock in your experiment was added to prevent the distilled water from exploding when super heated but it also was acting like a (heat sink) absorbing heat from the water. Since the microwave is going to heat the rock slower if at all the rock is always going to be cooler than the surrounding water. So you should preheat your rock and repeat the experiment. That is if your goal is to get a higher temperature from the microwaved distilled water.

 

[ScepticAmatuer]

Ha! Very nice indeed. Thanks.

Totally irrelevant, but it was 50 years ago that my mother had me collect quarter-size gravel rocks to put in her tea kettle so it wouldn't "bump", as she called it, as the water began to boil. That was way before microwave ovens were in common use.

 

[gjonesy]

You still may not measure anything higher than 100c to observe a "super heating" effect, do to heat transfer. Either from the glass container or the heat probe.(Thermodynamic equilibrium)

 

[ScepticAmatuer]

I certainly hope someone smarter than me has performed this little experiment (and the right way). I'm having difficulty locating research papers, however.

I did read one post on another forum where someone claimed to have reached 105° C via the microwave.

 

[sophiecentaur]

Perhaps what is needed if a method of having the water stirred whilst it's actually in the microwave and also a method of measuring the temperature. If the oven has a turntable, perhaps a stirring paddle could be dipped in the water. The paddle could be on a rod, stuck to the roof, just off centre of the turntable axis. (That's something I would have expected in domestic microwaves, for stirring sauces and soups etc but never seen one so far) An alcohol in glass thermometer could be read through the door and should not affect the performance.

 

[russ_watters]

I certainly hope someone smarter than me has performed this little experiment (and the right way). I'm having difficulty locating research papers, however.

I'm not sure I follow. This seems like a very basic experiment: why would there be research papers about it?

 

[gjonesy]

I think the best way to preform this particular experiment would be to remove the rock and just heat the distilled water in a glass container and use an infrared thermometer to take your readings. By removing the rock and not disturbing the water with a heat probe there is less heat transfer. You could get a super heating result you could measure. But this method could be dangerous if you touch the glass container...if you take it out of the microwave and accidentally dropped anything in it.

 

[sophiecentaur]

But this method could be dangerous

Yes, agreed. Great care needs to be taken where superheated water is involved. Putting coffee granules in pre-heated milk can make a real mess and scald you in a volcano of white froth.

 

[ScepticAmatuer]

You still may not measure anything higher than 100c to observe a "super heating" effect, do to heat transfer. Either from the glass container or the heat probe.(Thermodynamic equilibrium)

I really wasn't trying to get higher than

I'm not sure I follow. This seems like a very basic experiment: why would there be research papers about it?

You could study water your entire life and never even scratch the surface. Start with its isotope effects and that will get you through the first 50 years. Why investigate the diameter of the earth? Everybody knows it's flat.

 

[russ_watters]

You could study water your entire life and never even scratch the surface. Start with its isotope effects and that will get you through the first 50 years. Why investigate the diameter of the earth? Everybody knows it's flat.

That's a really odd response that doesn't answer my question at all. Sure, you could study water your entire life and never scratch the surface - but this experiment and the issue it investigates are junior-high basic. "Isotope effects" would be a decade beyond it. So it still begs the question as to why you think there would be professional research papers about it and why you are investigating it. Your flat-earth comment is bizarre and wrong, but it suggests you think there is something wrong about our current understanding of how water works. If there is another question here that you are not asking, just ask it.

 

[gjonesy]

Your flat-earth comment is bizarre and wrongI think sceptic was just being facetious russ_watters. Personally I could care less if it for a high school science fair project or if it's serious work in fluid dynamics. I'm a novice lending my knowledge. Curiosity is the root of discovery. And if sceptic learns something from this discussion then it was worth the time to talk to him. Wouldn't you agree?

Reference https://www.physicsforums.com/threa...ter-heated-via-microwave.842368/#post-5285611

 

[russ_watters]

I think sceptic was just being facetious russ_watters. Personally I could care less if it for a high school science fair project or if it's serious work in fluid dynamics. I'm a novice lending my knowledge. Curiosity is the root of discovery. And if sceptic learns something from this discussion then it was worth the time to talk to him. Wouldn't you agree?

I'm not sure he was, but yes I certainly agree. And if the thread had ended after your post #8, that would have been great. But he asked for more and all I did was ask for a clarification of what he wanted.

 

[ScepticAmatuer]

I'm not sure he was, but yes I certainly agree. And if the thread had ended after your post #8, that would have been great. But he asked for more and all I did was ask for a clarification of what he wanted.

Okay I have a confession to make. Two confessions actually. The reason I did this was to check the calibration of my recently acquired digital thermometer probe, not to advance the state of our knowledge in fluid mechanics.

I thought a simple way would be to boil water in the microwave and check that it read somewhere very close to 100 degrees.

The first time I put the bottle in with 100 milliliters of water and the water exploded ( but not the bottle ) after about a minute and 30 seconds. That clearly didn't help.

So then I read that I had to add the rock or something to make it so that it wouldn't superheat and explode. So I added the rock.

Then, I heated again. I didn't want to put too much time on the microwave because I didn't want it to explode again. So I chose to do it repeatedly for 45 seconds on high.

But as often as I tried. I could not get the temperature above 93.5. And I thought maybe somebody on this forum would know why. Actually it should have taken one post to answer the question.

My question for those who object to the very asking of a question, or take offense, would be why? If you know the answer give it. If you don't, silence would be in order. Emotion and insult have nothing to do with it.

 

[ScepticAmatuer]

Yes, agreed. Great care needs to be taken where superheated water is involved. Putting coffee granules in pre-heated milk can make a real mess and scald you in a volcano of white froth.

Another idea for one of my "high school" level science experiments - superheat the water to explosion in the microwave then measure the temperature of the water left in the container and the temperature of the water splattered in various places and ways.

I'm going to make russ_watters really mad at me for being ignorant. But I don't think any temperature you'll measure in this situation will be above the boiling point. I don't think water anywhere in the container reached the boiling point.

 

[russ_watters]

But as often as I tried. I could not get the temperature above 93.5. And I thought maybe somebody on this forum would know why. Actually it should have taken one post to answer the question.

And it did: you were provided the correct answer in post #2 (and additional details in several other posts, particularly my answer to your follow-up question in the next post).

My question for those who object to the very asking of a question, or take offense, would be why?

Nobody has objected to asking the question or taken offense to the question itself. Any hint of offense would be the result confusion/frustration over your seeming unwillingness to accept the answer. We can only help people learn if they accept the answer they are given!

Emotion and insult have nothing to do with it.

Agreed. So please do not bring them into this discussion.

By the way, I've done a similar experiment because of the need for accurate temperature measurement for beer brewing. I choose to use water boiling on a stove because I knew it would provide more even heating than water boiled in a microwave and continuous application of heating while taking the reading. If you google "how to calibrate a thermometer boiling water" you will see that the technique requires you to insert the thermometer and take the reading while the water is at a rolling boil, which can only mean it is on the stove.

 

[ScepticAmatuer]

And it did: you were provided the correct answer in post #2 (and additional details in several other posts, particularly my answer to your follow-up question in the next post).

Nobody has objected to asking the question or taken offense to the question itself. Any hint of offense would be the result confusion/frustration over your seeming unwillingness to accept the answer. We can only help people learn if they accept the answer they are given!

Agreed. So please do not bring them into this discussion.

By the way, I've done a similar experiment because of the need for accurate temperature measurement for beer brewing. I choose to use water boiling on a stove because I knew it would provide more even heating than water boiled in a microwave and continuous application of heating while taking the reading. If you google "how to calibrate a thermometer boiling water" you will see that the technique requires you to insert the thermometer and take the reading while the water is at a rolling boil, which can only mean it is on the stove.

So you did the rolling boil part for the same reason,
but not the microwave part? I wish you had/would carry thru with the microwave part and prove out the assumption you made without checking!

 

[ScepticAmatuer]

I think the best way to preform this particular experiment would be to remove the rock and just heat the distilled water in a glass container and use an infrared thermometer to take your readings. By removing the rock and not disturbing the water with a heat probe there is less heat transfer. You could get a super heating result you could measure. But this method could be dangerous if you touch the glass container...if you take it out of the microwave and accidentally dropped anything in it.

This is the ideal solution definitely. Unfortunately, I don't have the resources to acquire one.

 

[russ_watters]

So you did the rolling boil part for the same reason,
but not the microwave part? I wish you had/would carry thru with the microwave part and prove out the assumption you made without checking!

It's not an assumption, it is a theoretical prediction based on previous (non-boiling) experimental results. Besides: I trust your experiment, so why would I need to perform it myself? Your experiment confirms the theory.

 

[ScepticAmatuer]

Another reason I was intrigued was that the consistent 93.5°C result behaved very much like the limit you see at a true boiling point.

It reached the temperature and would rise no further no matter how much additional heat I put into it.

The temperature at a real boiling point should remain constant until it either stopped boiling it or it all boils away, regardless of how much

 

[ScepticAmatuer]

It's not an assumption, it is a theoretical prediction based on previous (non-boiling) experimental results. Besides: I trust your experiment, so why would I need to perform it myself? Your experiment confirms the theory.

I don't really want to push it, and don't want you to take offense, please, but you said it was a stupid experiment that nobody would do it and now you say the answer is known from previous experiment.

 

[russ_watters]

Another reason I was intrigued was that the consistent 93.5°C result behaved very much like the limit you see at a true boiling point.

It reached the temperature and would rise no further no matter how much additional heat I put into it.

That just tells me that you created a repeatable experiment. That's a good thing! If you want different results, you'll need to change the setup.

The temperature at a real boiling point should remain constant until it either stopped boiling it or it all boils away, regardless of how much

Over the short term, the setup acts as a steady state where the hot spots remain hot and the cool spots remain cool. Over the long term, as water boils away, the shape changes, the temperature distribution changes and the results will vary. So have you actually tried the experiment with different amounts of water or let it boil for much longer until the amount of water is much smaller?

 

[russ_watters]

I don't really want to push it, and don't want you to take offense, please, but you said it was a stupid experiment...

I certainly never said any such thing!

...that nobody would do it and now you say the answer is known from previous experiment.

I didn't say "nobody", I said scientists. You asked for published papers! But millions of people perform experiments on the non-uniformity of microwave cooking every night when they cook their dinner.

Beyond that, you will find quite detailed heat transfer experiments online if you look for them -- far more complex than what you are doing, but the results still apply.

This subject is well settled scientifically, so most of what you will find is engineering of specific, complex systems. Try googling "boiler temperature uniformity".

 

[ScepticAmatuer]

That just tells me that you created a repeatable experiment. That's a good thing! If you want different results, you'll need to change the setup.

Over the short term, the setup acts as a steady state where the hot spots remain hot and the cool spots remain cool. Over the long term, as water boils away, the shape changes, the temperature distribution changes and the results will vary. So have you actually tried the experiment with different amounts of water or let it boil for much longer until the amount of water is much smaller?

No I have not. But I will. Is there anything that can lower the boiling point of water (at standard pressures, etc.)?

 

[nasu]

Okay I have a confession to make. Two confessions actually. The reason I did this was to check the calibration of my recently acquired digital thermometer probe, not to advance the state of our knowledge in fluid mechanics.

I thought a simple way would be to boil water in the microwave and check that it read somewhere very close to 100 degrees.

.

In order to calibrate a thermometer for the 100^o C point you have to put in in boiling water.
Not in water that was boiling some time ago.
So the idea with the microwave was not good to start with, if this is your goal.
Unless you put the thermometer in the microwave so it measures the temperature while water is boiling. But this won't be a good idea for most thermometers.
The temperature goes down quite quickly once the water stops receiving heat.

 

[ScepticAmatuer]

This is an interesting little science fair project. What do you think? http://pubs.acs.org/doi/abs/10.1021/la5019218

 

[ScepticAmatuer]

In order to calibrate a thermometer for the 100^o C point you have to put in in boiling water.
Not in water that was boiling some time ago.
So the idea with the microwave was not good to start with, if this is your goal.
Unless you put the thermometer in the microwave so it measures the temperature while water is boiling. But this won't be a good idea for most thermometers.
The temperature goes down quite quickly once the water stops receiving heat.

I believe this may be partially responsible for the effect. It probably takes me about 5 seconds to get the jar out and get the probe in the sample. Then probably 3 maybe 4 seconds for the thermometer reading to stabilize.

Would you expect the cooling rate to be linear? In other words, by subsequent measurements during the cool down period, could I extrapolate how far it dropped during the 7 seconds?

 

[nasu]

No, it is not linear.
But why would you do this in the microwave, if the purpose is to calibrate your thermometer?

 

[ScepticAmatuer]

No, it is not linear.
But why would you do this in the microwave, if the purpose is to calibrate your thermometer?

The thermometer is calibrated now. I did it the first time because I thought it would be quicker.

 

[sophiecentaur]

could I extrapolate how far it dropped during the 7 seconds?

If you plot the temperature against time, you would expect an exponential curve. If you plot log(temperature) against time (t=0 would be the time you stopped the heating and removed the container), from the earliest you manage to grab a reading, you would expect to get a straight line. If you extrapolate backwards to the time you removed the container, your straight line would go through the temperature it started with. As with all heat measurements, there are possible errors and it would only help to reduce them. Actually, if you do a linear plot of temperature against time and you find that you can draw a fairly good straight line through the first few points, extrapolating back to t=0 could probably be good enough. Why not use both methods and see which one takes you closer to 100°C?
If this is all done with digital sensors, a decent spreadsheet could take in all the data and do all the hard work for you. And if you are not familiar with the simple coding that you can do with Excel, now could be a good time to get stuck in. Excel is pretty wonderful and has been for nearly thirty years of my experience.

 

[ScepticAmatuer]

If you plot the temperature against time, you would expect an exponential curve. If you plot log(temperature) against time (t=0 would be the time you stopped the heating and removed the container), from the earliest you manage to grab a reading, you would expect to get a straight line. If you extrapolate backwards to the time you removed the container, your straight line would go through the temperature it started with. As with all heat measurements, there are possible errors and it would only help to reduce them. Actually, if you do a linear plot of temperature against time and you find that you can draw a fairly good straight line through the first few points, extrapolating back to t=0 could probably be good enough. Why not use both methods and see which one takes you closer to 100°C?
If this is all done with digital sensors, a decent spreadsheet could take in all the data and do all the hard work for you. And if you are not familiar with the simple coding that you can do with Excel, now could be a good time to get stuck in. Excel is pretty wonderful and has been for nearly thirty years of my experience.

That was a lot of work to write that post. Thanks for that. I know some Excel. I'll try it both ways. and post the results.