Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: hamdani yusuf on 27/01/2020 01:12:18
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What is the current explanation on superheated water by microwave?
Is this superheated water in atmospheric pressure achievable using other heating methods, such as mechanical heating, induction heating, electric heating, magnetic heating, infrared radiation, etc?
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Superheated water can be produced in pure water, in clean vessels without nucleation sites, and as long as it is not too agitated.
- So mechanical heating/stirring would not be a good way to do it
- Pure water is not very magnetic, so magnetic heating would not work so well
- Pure water is not very conductive, so heating by electric conduction would not work so well
- Inductive heating of a clean metal vessel could work
- Infra-red radiation (eg inside an oven) would work well
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Try distilled water, smooth container and a big magnifying glass.
Edit: and a hot humid, calm day.
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You can demonstrate superheated water with a candle and a test tube.
There's nothing mysterious about microwaves here- except people often use them to heat water in ceramic mugs with nice smooth surfaces.
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Superheated water can be produced in pure water, in clean vessels without nucleation sites, and as long as it is not too agitated.
- So mechanical heating/stirring would not be a good way to do it
- Pure water is not very magnetic, so magnetic heating would not work so well
- Pure water is not very conductive, so heating by electric conduction would not work so well
- Inductive heating of a clean metal vessel could work
- Infra-red radiation (eg inside an oven) would work well
The information I gathered around this problem doesn't indicate the use of pure water.
The water can be regular drinking water but should not
contain many undissolved particles. It can be heated in any
domestic microwave oven; a research-grade microwave oven
such as used by Kennedy (1) to determine the enthalpy of
vaporization of liquids is unnecessary here. The recommended
heating vessel is a glass teapot sold to be used in the microwave
oven (no metal parts). Using such a teapot generally has several
advantages: its walls have relatively few nucleation
sites for bubble formation, yet enough to put a reasonable
limit on superheating; it has a plastic lid to prevent boiling
water from splashing out of it; and it has a plastic handle so
that it can easily be removed from the microwave oven once the
water is boiling.
Erné, B.H. & Snetsinger, P. (2000). Thermodynamics of water superheated in the microwave oven. Journal of Chemical Education
77(10), 1309-1310. doi: 10.1021/ed077p1309
And this is a comment by someone who can reproduce the phenomenon reliably.
Steven Spielman, PhD in Applied Physics; optical properties of solids
Updated Mar 4, 2019
I was able to accomplish superheating without much effort. I heated water in a small glass bowl, in a microwave oven. Before it reached boiling, tiny air bubbles adhered to the glass, the result of dissolved gases coming out of solution. When the boiling point was reached, (presuming here) water vapor entered these bubbles without bound and they grew quickly. The water boiled normally.
I shut off the microwave for a few seconds, and the boiling stopped. Then I turned it on again. The dissolved gases have already been driven out of the water, so there would have been no air bubbles to get things started. The water just sat there quietly for 10 seconds or so. Then there was a “PUNF!” sound. I opened the oven to find the bowl mostly empty and the walls and ceiling wet.
So, to avoid it, don’t microwave water (including coffee, etc.) which has recently boiled. Keep your face away when you first disturb it with a spoon or sugar.
https://www.quora.com/How-likely-and-how-dangerous-is-superheated-water-in-a-microwave-oven-and-what-everyday-precautions-should-be-used-to-avoid-injury
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You can demonstrate superheated water with a candle and a test tube.
There's nothing mysterious about microwaves here- except people often use them to heat water in ceramic mugs with nice smooth surfaces.
Do you have reference for this? Or have you experimented yourself?
What are minimum conditions to produce it?
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This phenomenon becomes my concern due to the danger such as shown in these reports.
A 26-year old man decided to have a cup of coffee. He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for, but he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup, he noted that the water was not boiling, but suddenly the water in the cup “blew up” into his face. The cup remained intact until he threw it out of his hand, but all the water had flown out into his face due to the build up of energy. His whole face is blistered and he has 1st and 2nd degree burns to his face which may leave scarring.He also may have lost partial sight in his left eye. While at the hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc., (nothing metal).It is however a much safer choice to boil the water in a tea kettle.General Electric’s Response:Thanks for contacting us, I will be happy to assist you. The e-mail that you received is correct. Microwaved water and other liquids do not always bubble when they reach the boiling point. They can actually get superheated and not bubble at all. The superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or tea bag is put into it.To! prevent this from happening and causing injury, do not heat any liqui d for more than two minutes per cup. After heating, let the cup stand in the microwave for thirty seconds! before moving it or adding anything into it.
Here is what our local science teacher had to say on the matter: “Thanks for the microwave warning. I have seen this happen before. It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new, or when heating a small amount of water (less than half a cup).
What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point.
What then usually happens is that the liquid is bumped or jarred, which is just eno! ugh of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken.”
America’s FDA has also warned about the issue, noting on its website:
The FDA has received reports of serious skin burns or scalding injuries around people’s hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Over-heating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil.
This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.
In spite of these factors, microwave users would certainly do well to heed the advice in the email. Hoax-Slayer visitor Al Chang relates the following experience illustrating just how dangerous superheating can be:
This just happened to me. I have a very smooth two-layer plastic tumbler. Imagine a cup suspended within a cup, so there’s an air pocket between the two cups. It’s designed to keep heat or cold from transferring from the inner surface to the outer surface.
I’ve heated water in it in the microwave before. There’s a certain amount of time around 2m50sec where it goes from being completely still to boiling over. Normally, I just keep it below that time and it’s fine. Up until today my only concern was the water itself boiling over while in the microwave.
Today, it must have been just under that time, because though it was still water, I took it out of the microwave, jiggled it a little and boom the water exploded all over my arms and hands. Very nasty scald. I suspect I could duplicate this consistently (which I might try under more controlled conditions once I heal up). It goes straight from still water with the tiniest of bubbles to overflowing in the microwave. I suspect it has a lot to do with the cup. Perhaps because there’s no way to transfer heat to the outside world other than the top of the water and the stillness keeps bubbles from forming. Movement then triggers the explosion. I have some first and second degree burns to show for it. I was lucky I was right near the sink and was able to get to cold water right away.
So word of warning to your readers!
https://www.hoax-slayer.net/true-circulated-warning-claims-superheated-water-microwave-can-explode/
The next victim could be one of our family, so we better make them informed. The correct explanation is needed to reliably, effectively, and efficiently prevent accidents. Until then, it is better to play safe and take preemptive actions although they may not really necessary.
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You can demonstrate superheated water with a candle and a test tube.
There's nothing mysterious about microwaves here- except people often use them to heat water in ceramic mugs with nice smooth surfaces.
Do you have reference for this? Or have you experimented yourself?
What are minimum conditions to produce it?
How have you come to the conclusion that it is a rare phenomenon?
It's so commonplace that there's a wiki page about it.
https://en.wikipedia.org/wiki/Bumping_(chemistry)
Yes, I have done it.
"What are minimum conditions to produce it?"
As I said;
"You can demonstrate superheated water with a candle and a test tube." (and water, of course- the candle has to be burning).
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The correct explanation is needed to reliably, effectively, and efficiently prevent accidents.
Not really.
Just learn to put the coffee or tea in the water, before you heat it.
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You can demonstrate superheated water with a candle and a test tube.
There's nothing mysterious about microwaves here- except people often use them to heat water in ceramic mugs with nice smooth surfaces.
Do you have reference for this? Or have you experimented yourself?
What are minimum conditions to produce it?
How have you come to the conclusion that it is a rare phenomenon?
It's so commonplace that there's a wiki page about it.
https://en.wikipedia.org/wiki/Bumping_(chemistry)
Here are the specific conditions lead to bumping.
Bumping occurs when a liquid is heated or has its pressure reduced very rapidly, typically in smooth, clean glassware. The hardest part of bubble formation is the formation of the small bubble; once a bubble has formed, it can grow quickly. Because the liquid is typically above its boiling point, when the liquid finally starts to boil, a large vapor bubble is formed that pushes the liquid out of the test tube, typically at high speed. This rapid expulsion of boiling liquid poses a serious hazard to others and oneself in the lab. Furthermore, if a liquid is boiled and cooled back down, the chance of bumping increases on each subsequent boil, because each heating cycle progressively de-gasses the liquid, reducing the number of remaining nucleation sites.
https://en.wikipedia.org/wiki/Bumping_(chemistry)
I boiled water numerous times without ever experienced a bumping.
Even in the case of microwave, it is not as common as normally boiling water, otherwise it won't become a myth worth busted in myth buster.
Furthermore, below statement is found in the conclusion of this article: https://digitalcommons.sacredheart.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1063&context=chem_fac
It shows why, for safety reasons, precautions
are taken to prevent superheating in the laboratory, where
smooth-walled glass vessels are generally used; in the kitchen,
such precautions are unnecessary, since cooking pans and their
contents are sufficiently (micro)rough that superheating is
seldom a problem.
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While searching for references, I found this interesting video showing superheated water without microwave.
Here is the description by the uploader.
First I put some water into the pot. Then I started the stove and set its power to the maximum level. Ten or twelve minutes later the water was not boiling and there were not bubbles at the bottom of the pot. I thought that something strange was happening and it came to my mind the superheating phenomenon, so I took my videocamera and started recording. In the next seven minutes there were no changes (no boiling, no bubbles) so I decided to hit the objects in order to make small vibrations, but the water remained the same. Finally I hit the pot directly and suddenly the water started boiling. I was cooking when I recorded this video. I don't know how I superheated the water, sorry.
And further description in the comment section.
1) The pot was on a student hall of residence where I lived for one year. I think that it was not a new pot...
2) I cleaned that pot with a normal sponge, without any special care...
3) I did not use tap water that day, I used bottled water (I always use bottled water for some recipes)
4) Yes, it was a electrical stove
5) Yes, I have tried to reproduce it many times, but I did not get any results...
And another comment from a viewer.
You got lucky! This just happened to me, but it exploded so powerfully that there was water on my ceiling on the other side of the kitchen! The craziest thing is that it was tap water. I put water on the stove to make some tea an took a shower. I came out and it wasn't boiling so I was like WTF. Just me bumping the stove caused the explosion. I got a little burned on my arm, but I'm lucky considering how powerful the explosion was. Only a few ounces left in the pot after the boom!!!
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I have also tried to reproduce the superheated water using microwave. For a start, I used commercial bottled drinking water which is readily available. To limit the danger, I used a small quantity of water in a small smooth glass container.
I repeatedly start the microwave until the water boiled, but it never get superheated. May be next time I'll try to use distilled water.
I'll try to explore if there are limiting conditions to reproduce the superheated water, such as ion content, dissolved gas content, volume/surface area ratio, heating time, heating power, etc.
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The correct explanation is needed to reliably, effectively, and efficiently prevent accidents.
Not really.
Just learn to put the coffee or tea in the water, before you heat it.
It can't be done if you have to prepare hot water for several people who haven't decided what kind of drink they want. It's also possible that they just want to have hot water.
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Use a kettle.
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The trick is to rid the water of nucleation centers. Bottled drinking water is no "purer" than the stuff that comes out of a tap - indeed whilst some mineral waters claim to be good for you because they contain no solutes, others claim exactly the opposite. But they all contain dissolved air (ask any fish!) and as pointed out in one of the above quotes, this can nucleate boiling. So you can maximise the chance of superheating by filtering (to remove solid nuclei) and degassing - slow heating with a little agitation, followed by cooling with no agitation.
There is a secondary consideration when using glazed pottery (coffee mugs). Salt glaze is quite conductive and some earthenware contains a fair bit of iron, so the pot actually gets hotter than the water inside it. If you bring the water quickly up to 99 deg C the pot may be at 110, so when you turn the microwaves off, the pot heats the water. For true superheating you need a smooth nonconductive vessel like laboratory glass.
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The trick is to rid the water of nucleation centers. Bottled drinking water is no "purer" than the stuff that comes out of a tap - indeed whilst some mineral waters claim to be good for you because they contain no solutes, others claim exactly the opposite. But they all contain dissolved air (ask any fish!) and as pointed out in one of the above quotes, this can nucleate boiling. So you can maximise the chance of superheating by filtering (to remove solid nuclei) and degassing - slow heating with a little agitation, followed by cooling with no agitation.
There is a secondary consideration when using glazed pottery (coffee mugs). Salt glaze is quite conductive and some earthenware contains a fair bit of iron, so the pot actually gets hotter than the water inside it. If you bring the water quickly up to 99 deg C the pot may be at 110, so when you turn the microwaves off, the pot heats the water. For true superheating you need a smooth nonconductive vessel like laboratory glass.
In my own experiment, the water was boiled several times, thus dissolved air should be already removed after the first boil. I used a clear glass with smooth surface.
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I found another video showing superheated water by microwave. Here the container is made of styrofoam.
Here we can see some bubbles are already formed on the container's surface even before the sugar is added. Thus nucleation site hypothesis is discredited by this video, and we need to find better explanation.
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I found another video showing superheated water by microwave. Here the container is made of styrofoam.
Here we can see some bubbles are already formed on the container's surface even before the sugar is added. Thus nucleation site hypothesis is discredited by this video, and we need to find better explanation.
It is remarkable what you can do with a bit of dishonesty, some vinegar ands some bicarbonate of soda in a sugar packet. (and a teabag).
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You can demonstrate superheated water with a candle and a test tube.
The chemistry teacher used to give us pieces of porous pot to put in Pyrex beakers "to make the water boil more easily".
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The information I gathered around this problem doesn't indicate the use of pure water.
It doesn't even need to be water at all, I once successfully re-decorated the kitchen with an egg.
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While searching for references, I found this interesting video showing superheated water without microwave.
What surprises me is not that water superheats in a microwave, but that it doesn't happen like this on a hob more often.
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I found another video showing superheated water by microwave. Here the container is made of styrofoam.
Here we can see some bubbles are already formed on the container's surface even before the sugar is added. Thus nucleation site hypothesis is discredited by this video, and we need to find better explanation.
It is remarkable what you can do with a bit of dishonesty, some vinegar ands some bicarbonate of soda in a sugar packet. (and a teabag).
What makes you think that the video uploader was dishonest?
I explored his channel and found no sign of prank or trolling.
I think the best method to resolve this dispute is by experimenting it ourselves. The question would be: is it possible to create superheated water using non-smooth container such as styrofoam or paper cup?
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The uploader might simply be mistaken. That doesn't look like superheated water to me. It could be that there was a significant amount of gas dissolved in the water--heating reduces the solubility of the gas, but off-gassing can be fairly slow without nucleation sites (think diet coke vs diet coke and mentos).
Sometimes I see a similar effect when adding the teabag to a cup of freshly heated water (it's not quite to boiling temp--usually 70–80°C).
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What makes you think that the video uploader was dishonest?
Exactly the same observation that you made.
Here we can see some bubbles are already formed on the container's surface even before the sugar is added.
You just can't do that with superheated water.
So, we know it isn't superheated water.
So, it must be something else.
It could be lemonade.
But it certainly isn't superheated water.
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What makes you think that the video uploader was dishonest?
Exactly the same observation that you made.
Here we can see some bubbles are already formed on the container's surface even before the sugar is added.
You just can't do that with superheated water.
So, we know it isn't superheated water.
So, it must be something else.
It could be lemonade.
But it certainly isn't superheated water.
When we get unexpected result, one or more of our assumptions must turn out to be false. I think I can try to replicate the experiment. I have all the required ingredients already. I'll post the result here ASAP.
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When we get unexpected result, one or more of our assumptions must turn out to be false
One assumption you are making is that YT videos are trustworthy.
We know that assumption to be false.
We also know that bubbles are impossible in superheated water (and we know why).
So, when we see bubbles, and someone tells us the water is superheated, we know that he is not telling the truth.
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When we get unexpected result, one or more of our assumptions must turn out to be false
One assumption you are making is that YT videos are trustworthy.
We know that assumption to be false.
We also know that bubbles are impossible in superheated water (and we know why).
So, when we see bubbles, and someone tells us the water is superheated, we know that he is not telling the truth.
Fortunately, the experiment is quite simple with standard household equipment. Anyone can replicate it to either verify or falsify the result.
I will know for sure which assumption is false when my experiment is done.
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You can demonstrate superheated water with a candle and a test tube.
There's nothing mysterious about microwaves here- except people often use them to heat water in ceramic mugs with nice smooth surfaces.
Do you have reference for this? Or have you experimented yourself?
What are minimum conditions to produce it?
How have you come to the conclusion that it is a rare phenomenon?
It's so commonplace that there's a wiki page about it.
https://en.wikipedia.org/wiki/Bumping_(chemistry)
Yes, I have done it.
"What are minimum conditions to produce it?"
As I said;
"You can demonstrate superheated water with a candle and a test tube." (and water, of course- the candle has to be burning).
Do you know if the phenomenon can happen when using a container with larger opening, such as a bowl or a plate?
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In my own experiment, the water was boiled several times, thus dissolved air should be already removed after the first boil. I used a clear glass with smooth surface.
My first attempts to produce microwaved superheated water using tap water never succeeded. The water always boiled before being superheated. Reheating the water didn't make any difference, although it should have been degassed by the prior boiling process.
My next attempt used demineralized water with conductivity around 2 microSiemens/cm. To minimize the risk, I used only small amount of demin water in a small glass. In my first attempt, I overheated the water, thus it exploded inside the microwave even before I added anything which can act as nucleation site.
For the next try, I became more cautious by pausing the microwave frequently to check if the water had became superheated. Instead of using salt grains which would provide a lot of nucleation sites at once, I used a solid stainless steel utensil to make the boiling process more controllable. Additionally, it doesn't contaminate the water like salt does, hence the water can be used multiple times. When the metal was dipped into the superheated water, steam bubbles were generated on its surface. It kept bubbling for a while until it stopped and became normal hot water.
I also tried to use a paper cup as the water container. Unfortunately I couldn't find a new one, thus I took a used paper cup previously used to drink tea. Even after I washed it using demin water, I couldn't produce superheated water in it. The water always boiled normally in the microwave.
It would take some time for me to edit the video before uploading it to my Youtube channel. Meanwhile, I'll try to continue the experiment to have an exhaustive investigation and get a firm conclusion. Getting a new paper cup is on my list. I also plan to use a metal container. Another one is trying to introduce air bubble into the superheated water. Let me know if you have more ideas to reduce uncertainty and give us a better understanding on the phenomenon.
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Yesterday an elderly gentleman called up the science phone-in show we do for BBC Radio Norfolk and asked why, when he made a cup of tea using the microwave, the inside of the cup looked like it had been painted with creosote.
On further probing, it turned out that his tea-making technique involved putting water, bag and mug into the microwave together from cold and nuking the lot until it looked hot. I didn't ask why the kettle would not suffice. He did also impart that he's never made a cup of tea in his life and usually drank beer, which might have something to do with it.
I speculated that that superheating of the water around the teabag, and within the substance of the teabag itself, led to the liberation from the tea of more of the heavier, tannin molecules; these then decorated the inside surface of the mug.
Am I right?
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Do you know if the phenomenon can happen when using a container with larger opening, such as a bowl or a plate?
I have recently repeated the experiment using glass bowl. At first I thought it was a failed attempt because the water always seemed to boil. After the microwave heating stopped, I checked if the water had became superheated by dipping a metal tool into it. No visible bubbling on the metal surface. The heating and checking were repeated several times with the same result until I thought that there was something wrong with the container.
Then I took a closer look at the bubble formation during boiling. The bubbles were mostly formed from the bottom center part of the bowl, which are the area I hadn't checked by metal dipping. Previously the metal tool was dipped near the surface and off center.
After some time of prolonged heating although it was already visibly boiling, I checked again by dipping the metal tool, this time at the bottom center of the bowl. To my surprise, and relieve, I saw bubbles formed on the surface of the tool. I then replace the metal tool with a plastic spoon. I still get some bubbles, but this time they are smaller in diameter and at lower forming rate. It shows that the water has became superheated inhomogenously.
I think that the bumping effect in chemistry is a different phenomenon than microwave superheating.
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Yesterday an elderly gentleman called up the science phone-in show we do for BBC Radio Norfolk and asked why, when he made a cup of tea using the microwave, the inside of the cup looked like it had been painted with creosote.
On further probing, it turned out that his tea-making technique involved putting water, bag and mug into the microwave together from cold and nuking the lot until it looked hot. I didn't ask why the kettle would not suffice. He did also impart that he's never made a cup of tea in his life and usually drank beer, which might have something to do with it.
I speculated that that superheating of the water around the teabag, and within the substance of the teabag itself, led to the liberation from the tea of more of the heavier, tannin molecules; these then decorated the inside surface of the mug.
Am I right?
Tea has been staining things since long before the invention of the microwave.
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Another one is trying to introduce air bubble into the superheated water. Let me know if you have more ideas
Do it carefully.
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Another one is trying to introduce air bubble into the superheated water. Let me know if you have more ideas
Do it carefully.
Thank you.
I have finished several tests using microwave oven. I think I start to understand how it works. Here are some of the results:
- the water can still be superheated even after it boils.
- Plastic drinking straw can be used to induce superheated water to boil.
- The part where it's already induced to boil will eventually lost superheating state, while the other parts may still be superheated. Hence, if the inducer is then moved, it will generate bubble again.
-Introducing air bubble is similar to other inducer. The water can still stay in superheated state.
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I found out why previously my metal tool didn't readily induce the superheated water to boil. Apparently, it absorbed so much heat that the water failed to boil. Only after the tool became hot enough, the superheated water still have enough energy to turn into steam.
Hence, using a solid copper wire can solve this issue. The superheated water becomes more easily induced to boil since the wire doesn't absorb much energy.
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How are you measuring the temperature of the water?
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I also tested some materials to use as boiling chip. Their effectiveness vary.
A shiny metal coin effectively induced boiling. The water can't get to superheated state even after prolonged heating. Steam bubble are visibly generated on its surface.
A plastic button turns out to be a good inducer too. Unfortunately I can't find out what kind of plastic it was made of.
I cut a cable tie which is made of nylon. It can't induce boiling. The water always get superheated when heated long enough.
I also cut a solid coper wire with similar length as the tie. It induced boiling pretty well.
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How are you measuring the temperature of the water?
I didn't.
I saw another video trying to measure the temperature of superheated water using a contact type thermometer, which predictably induced boiling. I think I better use a non-contact thermometer to measure the temperature after I better understand the induction of boiling on superheated water.
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Interestingly, I can use uncut nylon cable tie to induce boiling on superheated water, although it couldn't be used as boiling chip.
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How are you measuring the temperature of the water?
I didn't.
I saw another video trying to measure the temperature of superheated water using a contact type thermometer, which predictably induced boiling. I think I better use a non-contact thermometer to measure the temperature after I better understand the induction of boiling on superheated water.
I had repeated the experiment and measured the temperature using thermal camera. It didn't show significant difference between superheated water and normally boiled water. Using default setting of 0.95 emissivity, the measurement showed only around 90º C in both cases. To get the correct value, the correct setting of emissivity is needed, but for this experiment I only needed to compare the temperature difference between superheated water and normally boiled water.
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I also tried to use a paper cup as the water container. Unfortunately I couldn't find a new one, thus I took a used paper cup previously used to drink tea. Even after I washed it using demin water, I couldn't produce superheated water in it. The water always boiled normally in the microwave.
I bought new paper cup for the experiment, but I still couldn't produce superheated water in it. To make sure that it wasn't due to contaminating substances from the paper cup, I poured the water into a glass bowl and repeat the experiment. This time the water can be superheated, so I concluded that there are something physical in the paper cup that prevents the demineralized water from being superheated.
Other experiment I've done using microwave oven was placing the glass bowl containing demineralized water inside a metal bowl. When the microwave oven was turned on, the water heated up much slower than before. But after prolonged heating, I can still get superheated water.
The next step was removing the glass bowl from the experiment setup, so the water was in direct contact with the metal bowl. The heating up process was even slower this time. Unfortunately my microwave oven eventually stopped working after prolonged heating. It seems like the water only absorbed a small portion of microwave energy while the majority of it was reflected back to the magnetron.
It would take a while to fix it before I can continue the experiment. I hope it only broke the power supply unit since the control display went completely blank. If I was lucky, it might be just blown out fuse.
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I bought new paper cup for the experiment, but I still couldn't produce superheated water in it. To make sure that it wasn't due to contaminating substances from the paper cup, I poured the water into a glass bowl and repeat the experiment. This time the water can be superheated, so I concluded that there are something physical in the paper cup that prevents the demineralized water from being superheated.
Have you tried other types of insulating vessels?
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Have you tried other types of insulating vessels?
Not yet.
I guess that on the surface of the paper cup there are something that acts as nucleation site that makes the water boiled instead of superheated, just like what boiling chips do.
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I have found the
If I was lucky, it might be just blown out fuse.
It was just blown out fuse. The fuse is rated at 8 Amp 250V, although the microwave nameplate says it's 1400 W.
Previously, I can only get 5 Amp fuse, which instantly blown out when the heating is started, although it was set at low power. Apparently, the power setting only affects the resting time for the magnetron. Lower power only means longer resting time, but initial power stays the same. That's why the 5 Amp fuse couldn't stand a chance.
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You will find the same problem with an induction cooker - they run at full power with a variable mark/space ratio and rely on the thermal inertia of the pot to deliver reasonably constant power to the contents.
There was some mention of paper cups. Unlikely to produce superheating as the surface is never smooth and can trap microscopic air bubbles as you fill it. Essentially, anything that isn't water and a surface that isn't polished, will induce normal boiling. And any solute will increase the normal boiling point.
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And any solute will increase the normal boiling point.
What do you mean by that?
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I also have tried using a semi transparent plastic cup labeled "safe for microwave oven". The demineralized water always boil normally and shows bubble generated from the surface of the plastic cup. Repeated heating by microwave doesn't change the result, although there supposed to be no longer dissolved air in the water. When the plastic cup is replaced by a smooth and clear glass, I get superheated water. It even exploded inside the microwave when the magnetron is still on. Subsequent trials don't produce explosion, but I still got superheated water since it started boiling when I dip a metal wire into it.
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The plastic surface will probably have a different contact angle with the water: glass is slightly hydrophilic (water wets it) but most plastics are hydrophobic. So the interface chemistry and physics will be different - apparently sufficiently so that the plastic nucleates boiling.
"Solute" means anything dissolved in the water. Salt lowers the freezing point and raises the boiling point of water, but if you throw a handful of salt crystals into a hot pan it can nucleate boiling before it dissolves - chefs used this to promote "smooth" boiling rather than dissolve the salt in cold water which can then superheat and "bump".
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Is there any solute which can decrease the boiling water in atmospheric pressure? What is the theoretical justification?
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Silly of me to forget. "Constant boiling solutions" are worth investigating.
A well-known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by mass) boils at 78.2 °C.[7] Ethanol boils at 78.4 °C, water boils at 100 °C, but the azeotrope boils at 78.2 °C, which is lower than either of its constituents.[8] Indeed, 78.2 °C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures or pressure maximum azeotropes.
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Is there any solute which can decrease the boiling water in atmospheric pressure? What is the theoretical justification?
Some materials- like HCl can raise or lower the boiling point of the solution, depending on how much you add.
Ammonia reduces the boiling point of water, and forms no constant boiling mixture.