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When I boil water in the kettle, it makes a nice cup of tea. Sometimes I need to use a microwave because a kettle isn't available. I boil the water in the mug and it looks pretty normal, but when I drop in the teabag the water froths up and looks foamy. I don't see what the chemical difference is here, so I assume it must be some physical difference. I have noticed this with multiple types of tea and multiple microwaves, the results being consistent so it's not just a weird microwave or something like that.

What is the reaction here and how/why does it occur?

Here is a photo of the 'fizzy' looking tea just after dunking in the teabag.

enter image description here

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    $\begingroup$ Answered on Cooking SE: cooking.stackexchange.com/q/22262 $\endgroup$ – Dmitry Brant Aug 27 '13 at 20:20
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    $\begingroup$ Please don't do that, it's really dangerous! You're superheating the water, as user28161's answer says, and it's possible for this to result in all the water boiling at once when you put the tea bag in, sending boiling hot water up into the air. $\endgroup$ – Nathaniel Aug 28 '13 at 0:50
  • $\begingroup$ Tried, but I cannot reproduce the results. What type of water and tea bag you are using? It is not related to superheating, which is very dramatic and you would report it. I remember I have seen it few time before, but can't remember the exact condition. $\endgroup$ – unsym Aug 29 '13 at 6:27
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    $\begingroup$ @Nathaniel Froth is not a sign of superheating, in fact it is rather difficult to superheat water in a microwave. $\endgroup$ – user10851 Sep 4 '13 at 18:57
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    $\begingroup$ @DmitryBrant It has been asked on cooking, but the upvoted answer is wrong. If your water doesn't bubble in the microwave, you probably just never got it hot enough. $\endgroup$ – user10851 Sep 4 '13 at 18:58
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I doubt that it's superheating, as I understand superheating is a rather violent phenomenon. The most likely explanation is dissolved gases; gas solubility in water decreases with temperature. The kettle boiling process is very turbulent and so can release all the dissolved gases, resulting in pure hot water. The microwaved water is heated in a very gentle way, and so the gases do not yet have a chance to escape from the surface... not until they can start nucleating on your tea bag.

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A common problem with microwave is that you lack control of the temperature of the water. Second problem is that the water is heated from the top and the sides of the mug mostly. Result is, that the content of the mug is not really boiling hot everywhere, altogether (after mixing) it is well below 100 °C A side effect from this is, that the water is not dearated like it is when You boil it on a hot plate. So: You have some water close to 100 C at the surface and colder (containing air) water below. When You dip in the bag, all that mixes, the air bubbles out.

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    $\begingroup$ So the way to confirm the mechanism would be to boil water and then let it cool quickly in a closed container with little air in it (deaerate it and limit the amount of air dissolving back it). Then try to make your tea in the manner that resulting in the bubbling. This would result in no bubbles according to Georg. Please could someone confirm which is accurate as the superheated water explanation is common while this answer is intriguing. I can't find a ratio of 25C and 100C water that works using a solubility/T chart but the final system does not need to be saturated. $\endgroup$ – kaine Aug 28 '13 at 13:40
  • $\begingroup$ @user28161 There is no need to be anxious about reaerating the water. After conventional boiling just avoid stirring/swirling the water. It takes some hours before the air diffuses down through some cmters of water, $\endgroup$ – Georg Aug 28 '13 at 15:23
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    $\begingroup$ I've seen a science show demo of superheating in a microwave oven. To achieve the effect required a very clean, smooth container and relatively pure water; the sudden boiling was a difficult result to achieve, and while not exactly "explosive", was somewhat more vigourous than a little foaming. Perfectly safe, clean tap water still contains a lot of contaminants; together with an "ordinarily" clean cup would likely provide ample nucleation sites for "normal" boiling. I'm inclined to think that dissolved air in the water is (or is closer to) the correct explanation than superheating. $\endgroup$ – Anthony X Sep 4 '13 at 0:30
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http://en.wikipedia.org/wiki/Superheating

I think you are superheating the water and you provide nucleation sites (by means of the tea bag) so it starts to boil.

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  • $\begingroup$ In short: the water was still (undisturbed) enough to stay stable above its usual boiling point, until you created a disturbance and started the boiling process. The opposite also exist (see Supercooling) : On that one, I heard a story that some horses would get caught while trying to cross a lake as they hit a supercooled surface, causing it to start to freeze instantly and therefore catching the horses in ice (not sure that this story is true, I'd welcome references) $\endgroup$ – Olivier Dulac Aug 28 '13 at 11:18
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    $\begingroup$ If the water was superheated, it would "explode" when You dip the tea bag. Superheating by microwave is not as common as the wiki article suggests. $\endgroup$ – Georg Aug 28 '13 at 12:57
  • $\begingroup$ @Georg As stated in your explanation, the mug is heated unevenly. The top could be superheated which much of the rest is cooler which would not cause it to explode when the bag is added. A localised boiling could be observed before mixing causes it to stop. $\endgroup$ – kaine Aug 28 '13 at 13:44
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Another problem here could be the temperature of the water. If you pour boiling water into a cold mug, then the temperature of the water will drop due to heat transfer to the mug. Also as you are pouring some of the water turns to steam.

If you boil the water in the mug, then the water in the mug is at the boiling point.

Hotter seeping temperatures tend to bring out bitter flavors in the tea.

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protected by Qmechanic Sep 3 '13 at 22:20

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