[vhfpmr]

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.

[hamdani yusuf (OP)]

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?

[chiralSPO]

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).

[Bored chemist]

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.

[hamdani yusuf (OP)]

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.

[Bored chemist]

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.

[hamdani yusuf (OP)]

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.

[hamdani yusuf (OP)]

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?

[hamdani yusuf (OP)]

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.

[chris]

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?

[hamdani yusuf (OP)]

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.

[Bored chemist]

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.

[Bored chemist]

Another one is trying to introduce air bubble into the superheated water. Let me know if you have more ideas

Do it carefully.

[hamdani yusuf (OP)]

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.

[hamdani yusuf (OP)]

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.

[Bored chemist]

How are you measuring the temperature of the water?

[hamdani yusuf (OP)]

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.

[hamdani yusuf (OP)]

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.

[hamdani yusuf (OP)]

Interestingly, I can use uncut nylon cable tie to induce boiling on superheated water, although it couldn't be used as boiling chip.

[hamdani yusuf (OP)]

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.