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This question already has an answer here:

When I am in a rush, I will heat up water in a cup in the microwave for tea. I usually put the cup on high for 2 mins.

When I put the teabag in, the water starts to fizz almost like it is carbonated, but the bubbles are finer.

I tested with boiled water and there was no fizzing.

I know water molecules are heated by absorbing the microwave radiation, I am assuming this has something to do with it?

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marked as duplicate by Floris, ACuriousMind, Kyle Oman, Martin, Emilio Pisanty Apr 8 '15 at 19:03

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Interesting Question. you mean it doesn't fizz when you put the water alone in it? $\endgroup$ – Mobin Apr 8 '15 at 14:27
  • $\begingroup$ "I don't believe this happens with water that it is heated over a stovetop."...have you tested whether that happens? $\endgroup$ – ACuriousMind Apr 8 '15 at 14:27
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    $\begingroup$ I have water boiling now, actually. $\endgroup$ – picus Apr 8 '15 at 14:28
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    $\begingroup$ I tested the boiled water and.... NO FIZZ! $\endgroup$ – picus Apr 8 '15 at 14:33
  • $\begingroup$ @picus put another object, anything like a pen, and and put it in the microwave and see what happens. $\endgroup$ – Mobin Apr 8 '15 at 14:34
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What you see is the escape of dissolved gases in the liquid.

Water in a cup in a microwave heats fairly uniformly; by contrast, if you heat on a stove, you hear the water "sing" because you get local boiling of the water that is right next to the wall being heated. During the "singing", dissolved gases are driven out (and they don't really re-dissolve - but the water vapor does, to a large extent). In this sense, conventional boiling is an outgassing operation.

The hot water is supersaturated; the presence of the tea bag creates nucleation points so the dissolved air can escape. That's the fizzing you see.

If you really overdo it with the heating of the water, adding a tea bag can cause full scale boiling - and result in nasty burns. It is usually recommended to heat the water with the bag in it to prevent this.

UPDATE

I decided to do the experiment. I took some tap water and boiled it. After cooling it down, I put it in a glass. I then put fresh tap water in another glass. I put the two glasses in the microwave (at the same time, on a turn table) and ran it at 1100 W for about 1:30 min. I then added a fresh green tea bag to each. The result is shown:

enter image description here

I would say that my tap water doesn't have a whole lot of dissolved gas, but the difference in the amount of little bubbles that show up at the rim of the glass is very visible. Note that the boiled water was still slightly warmer than the tap water when I started (maybe 25 C) and the tea came out a little darker. But if anything, that should have meant more bubbles, not fewer. This little experiment supports my hypothesis. Obviously it's possible to design a more careful experiment (mixing boiled and tap water in different ratios, cooling them to the exact same temperature, and measuring the resulting scum line width and plotting it. A high school science fair experiment waiting to happen...)

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  • $\begingroup$ @picus Have you tested this? Can you make the microwaved water fizz with something else than a tea bag? Did you carefully try full scale boiling? $\endgroup$ – Steven Mathey Apr 8 '15 at 15:32
  • $\begingroup$ Another experiment to try: boil water on the stove. Cool it down. Heat in the microwave. Add a tea bag. I predict there will be no fizz. $\endgroup$ – Floris Apr 8 '15 at 15:33
  • $\begingroup$ As a comment for fun: a) any type of "clean" liquid in a "clean" recipient can be superheated in a microwave, nucleation sites or sometimes simply touching/moving the cup results in explosive like boiling, which is, as you mentioned, potentially dangerous. b) Against common believe on metal inside a microwave, one can actually put a metal teaspoon inside the liquid to avoid this (provided the spoon does not have super pointy edges). $\endgroup$ – mikuszefski Apr 8 '15 at 15:43
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    $\begingroup$ Be VERY CAREFUL about trying to boil water in the microwave. We can be patient and wait until you try this at home. I don't want your next post to be from the burns unit. $\endgroup$ – Floris Apr 8 '15 at 19:07
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    $\begingroup$ re : being careful -- animations.physics.unsw.edu.au/jw/superheating.htm $\endgroup$ – J... Apr 8 '15 at 19:27

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