That tends to happen dramatically only if the water has been superheated. If it is only at boiling point the bag still will bubble fairly vigorously, but that is caused by all the dry, hollow spots in the tealeaves and the fibres of the bag. The air in such spots expands, partly from heating and partly from taking up water vapour from the hot liquid. 

I have seen a lot of denials that the explosive boiling of superheated liquids in the microwave oven is possible, but I assure you from personal experience that it happens often enough to be realy dangerous. Smug denials are based on the fact that the conditions are not easy to reproduce to order and that quite trivial effects can prevent the superheating. For instance it commonly happens that an old glass vessel has microscopic surface cracks that nucleate boiling, which abolishes the effect pretty nearly absolutely. So can suitable particles of mineral dust, or even dense organic particles, such as some kinds of nutshells. Any sort of wooden spoon or stirrer that has one end in near the bottom of the fluid in the vessel prevents any "bumping" by inducing smooth boiling. Many metal objects have similar effects.

If however, you do have a microscopically smooth ceramic or glass or some kinds of plastic vessel, possibly with a trace of oil covering its inner surface, then the least hint of nucleation can make the liquid overheat and explode. A teabag is very full of nucleation sites, so it can cause anything from vigorous bubbling to explosion.

After a mug of water has been boiled in a microwave,

When the water boiled in the microwave (bubbling)

superheating is no longer possible.

A good (and reproducible) example of superheating

and explosive boiling is to place an egg in a microwave.

Dont try it, there are "tubes" enough on Youtube.

Georg

It is hazardous to make assertions about what is or is not physically possible without considering carefully all the relevant factors as well as the proper context of the question. Firstly, are we speaking about severe bubbling on adding the teabag, or an all out boiling explosion? If the former, then to assert that no superheating is possible is a rather large assumption; it requires that before all the process of boiling had completed, all the water containing any excess heat over and above what one simplistically might expect to be sufficient to cause boiling will have been convicted to any surviving nucleation centres in the liquid. This I am sure anyone in the forum will realise is a large assumption. There are all sorts of tests that one can try, but I am sure that the simplest would be to drop various sources of nucleation centres and see what happens at various moments after a large glass beaker of clean water has been given time to boil vigorously in a microwave oven, and then to stop boiling. Sprinkling half a teaspoon of dry salt or sugar as the final bubbles disappear might indicate where some un-nucleated vapour had been lurking.

Or of course, one could drop in a teabag, but I think you will agree that it would offer fewer surprises.

Conversely of course, there is the question of whether the water we had been boiling were sufficiently experienced or not; naive water has bad habits. So let us suppose that we start with a nice, large, clean beaker of water (distilled if you so prefer) in our microwave oven, and boil it. If it boils explosively, something that I am sure we all have experienced, top it up and continue until we have a nice, full beaker of boiling water.

All right so far?

Turn off the power.

Wait for a minute or so.

Turn on the power.

Now this experiment, if it is to be properly and cogently performed, will require some statistical expertise, but if cogency is not required, just repeating it a few times should do nicely. Which water would you expect to be likelier to boil explosively? The naive or the experienced?

As for the reproducible eggs, they have very little to do with it. They explode, not so much because of superheating, as because they are in their shells. If you do not believe this (and have the patience to clean up the resulting messes and have some eggs to spare) then put in eggs one at a time, standing up in non-metallic egg cups or some similar suitable support. Some eggs should have whole, sound shells. Others, remove the large end of the shell and rupture the membrane so that the liquid is open to the air. (It is not sufficient simply to puncture the shell).

Observe how each egg boils. You may find signs that some of the opened eggs superheat, though I should bet against it, but the reason that the entire eggs explode is partly that the yolks are fatty and can get hotter than the rest of the liquid, and partly that the shells and membranes permit enough pressure to build up for the eggs' contents to rise several degrees above ambient boiling point. When the shell finally yields, the result is naturally reproducibly messy. You could reproduce this effect with water in any kind of sealed container, preferably microwave-transparent, but I do not recommend it, although, come to think of it, it should be a lot less messy than the exploding eggs.

Hello Jon:

.- Microwave physics:

"Rotation" of molecules in liquid water is not possible.

The molecules are locked into a diamond-like structure

by hydrogen bonds. (Same structure as in ice, but there

is is rigid over the dimensions of the crystals.

Result: Ice is not heated by microwaves to an extent

one could realize in a household oven)

The structure in liquid water is dynamic and allows 

a strong orientation (DK=81 !) whose relaxation has

its maximum at 30 GHz. Microwave ovens with about

10 GHz work on a flange of this maximum where water is

still transparent enough. Higher frequencies would be

absobed on the suface.

All this is specific for liquid water! There is no substance which is

heated by 10 GHz in a similar extent.

Especially fat is not heated by this frequencies.

Try it, please.

http://books.google.com/books?id=bj1EnQPB0CMC&pg=PA184

Superheating:

Superheating is achieved more easy  by microwaves for a reason

not mentioned yet: If You heat from outside, by a flame or

a electric heating plate, You always have a thin boundary layer

with a temperature higher that the bulk in Your pot.

There the bubbles will start first. If You try to have less temperature

in that layer, You need more time, another thing which will  give

fate more time to start bubbles.

All this is less when heating with microwaves, because this is

mostly scalar, the heat beeing evolved nearly homogenous through the

volume.

Viscosity:

If You heat some viscous lqwuid from outside, some other phenomenon

takes place: film evaporation and "burning" of the viscous mass.

(Every cooks knows that an uses a bain marie  to heat gelatin

solution)

Same thing occures when heating eggs in a pan: You have to reduce heat

to avoid "black" eggs where they have contact to the pan.

This is what I assume You mix up with "not easily superheated".

(You can look up this things in textbooks chapters "standard operations"

for chemical/therrmal engeneering.

Georg

Hello Georg,

>"Rotation" of molecules in liquid water is not possible. The molecules are locked into a diamond-like structure by hydrogen bonds. <

I think you are being just a little bit free with your terminology. "Locked into a diamond like structure"??? That is a very loose "locking" if you will forgive my saying so! When I spoke of rotation I was referring to rotational forces, not necessarily spinning free, as might happen in a low-pressure uni-molecular gas. In an electromagnetic field, polar molecules are subjected to rotational forces that orient them to line up with the field. In a liquid, such as water, if the field is strong enough then any bonds, whether stable or transient, will be broken. So let us not talk about rotation being impossible. In particular hydrogen bonds and van der Waals forces in a fluid of small molecules, such as those of water, are rather transient for us to speak about "diamond-like" structures, don't you think? They are continuously breaking by the billion (or quadrillion for all I know, I have never thought of calculating it accurately) in every millilitre above freezing point, never mind boiling point!

But that is a total red herring anyway; I don't know why you raised it. More to the point, as you are well aware, the electromagnetic field in microwave radiation changes continuously (and rather rapidly). As a result, let us consider a particular molecule in liquid water, however repugnant the idea might seem to a quantum theorist, let's call him Maurice. Let us go further and assume that Maurice is well connected by hydrogen bonds and that our microwave field is too feeble to break those bonds. What happens? Does Maurice sit dead still? You know very well he does not. His hydrogen bonds become strained. As the field passes on, it will go through a phase in which Maurice with his strained bonds is aligned with the field and at that point he experiences zero rotational force from the field. However he does not experience zero rotational force from his strained bonds! He begins to correct his position to relax those bonds. The energy that the field had exerted cannot simply disappear, right? His recovery rotation is expressed as heat, even though it is not a full rotation, nor in the example of our beloved Maurice, even a half rotation.

Meanwhile of course the orientation of our microwave field continues changing, and the poor, dear, Maurice has a rather dizzy time ahead of him, whether he can complete a rotation or not.

I trust we are in agreement about the life history of Maurice?

I trust furthermore that we are in agreement that a sufficiently intense microwave field would make a mockery of his hydrogen bonds.

>(Same structure as in ice, but there is is rigid over the dimensions of the crystals. Result: Ice is not heated by microwaves to an extent one could realize in a household oven)<

Now here Georg, you positively dismay me. "Ice is not heated by microwaves to an extent one could realise in a household oven"???

It is fortunate for us that we never realised this when we bought our first microwave oven. Our ignorance enabled us as some of the first duties of that oven, to defrost some frozen meat. And as Dilbert might put it: "Now at this point a miracle happens..." Miraculously our meat thawed.

Georg, in my wife's absence I have on occasion been able to experiment with the microwave oven to a prudent extent, and I determined that Maurice would behave very much the same in liquid and solid water. In a solid crystal of course the probability of his rotating hard enough to break any bonds would be negligible, but he still can wobble about a little bit. The work he performs in doing that is less than he would achieve in liquid, let alone a gas, but it is work nonetheless, and produces heat. Let us not be unreasonable; I read your remarks, not to imply that they would be no heat generated in ice, simply that they would be negligible heat generated.

However it had occurred to me to wonder just how much, so I tried some experiments. It was rather fun actually. I prepared some microwave-transparent containers of water, which I brought to the boil, and then froze into solid blocks, all of the same mass, and all still in their transparent containers. Then I took two similar containers of water at ambient temperature and brought them to the boil in the oven. I noted the time that this took. I then put in a single container at ambient temperature and brought that to the boil. Not surprisingly, that took roughly half as long.

Now I put in a single container of water, this time with one of the blocks of ice, which I had frozen as deeply as my domestic equipment would permit. Guess what happened Georg?

The water boiled after an interval that suggested that the block of ice had absorbed the microwaves at roughly half the efficiency that an equivalent glass of water would have done. In my experiment each container held roughly 200 mL of water. I did several replications of course (wasn't it clever of me to remember that long word when so long out of the laboratory?) And some of my frozen glasses of water were just starting to melt at a few spots when the liquid water was starting to boil. Watching the behaviour of the melting ice was extremely informative. I strongly recommend that anyone using a microwave oven for thawing anything takes a good long thoughtful look at the behaviour of such a block of ice and carefully considers some of the implications of conduction, convection and positive feedback.

Now Georg, it is common cause that ice does not absorb microwaves, certainly not at the frequencies in domestic ovens, as efficiently as liquid water does. So far no surprises. However I do hope you will accept that absorbing microwaves at roughly half the efficiency is far, far from negligible! Agreed?

>Microwave ovens with about 10 GHz work on a flange of this maximum where water is still transparent enough. Higher frequencies would be absobed on the suface.<

I have no quarrel with that whatsoever, but may I point out that my domestic microwave oven is not one of your laboratory models with GT stripes. On the back of my oven it tells me that I can only expect on average 2450 MHz, less than a quarter of your frequency.

>All this is specific for liquid water! There is no substance which is heated by 10 GHz in a similar extent.

Especially fat is not heated by this frequencies.

Try it, please. <

Sorry Georg, but I cannot. As I said, I am limited to about a quarter of that frequency. For sure sloth I will take your word for it that water is uniquely responsive to that frequency, but forgive me my reservations in the light of your denial (as I understood it at any rate) that a domestic microwave oven could melt ice. Having also observed personally that it can heat fat, forgive me if I do not undertake to store fat in a 10GHz oven in the fond expectation that it will be secure against heating. If you have in fact such an oven and a suitable mass of dry fat at your disposal, and you feel moved to demonstrate your faith in the immunity of the fat to microwaves, then given your professional laboratory competence, it would not become me to advise you against trying some hands-on experimentation.

Please let us know the outcome.

I'll break off here and continue the next topic in another reply. I still have a few replies outstanding from the past, but I know that you will understand the constraints of available time.

All the best

Jon