[explanation: This is in response to a certain email forward I received which warned in sensational language of the dangers of heating water in a microwave oven. The author claimed that her son was severely burned while trying to prepare coffee in a microwave. The allegation was that the microwaved water exploded in his face.]
Unlike most sensational warnings distributed via email forwardings through the internet, most of which are clearly hoaxes, this one (a report of a mug of microwave-heated coffee exploding violently) is unusual because it can easily be confirmed or debunked, because it relies on simple physical phenomenon. I set out to determine whether this phenomenon actually occurs.
I selected a clean glass mug from our apartment's selection. The particular mug is quite thick and has a large glass handle, so it's easy to handle when it contains a hot liquid. The mug is 6.35 centimeters in (inner) diameter.
To begin, I thoroughly cleaned the mug to remove any particles or grease that might have been in it. It is also important to rinse thoroughly, because any remaining soap will weaken the surface tension of the water.
For a first try, I added 200 mL tap water to the glass and set it in the center of the microwave. After about three minutes the water began to boil. I let it boil for about a minute, then removed it and allowed the boiling to cease. I then began to microwave it some more, and again it began to boil. I took it out again, let the boiling stop, and then started the microwave again. Once again the water began to boil, so I took the glass out of the microwave.
There was something curious about how the water was boiling. It was not a rolling boil as one might expect, but it appeared that the water was only boiling near the surface, with very small bubbles. I plunged a fork into the water, and many large and small bubbles burst forth from all around fork. Clearly, something like the phenomenon described in the email was happening.
I decided to start over with a smaller volume of water. I dumped out the water I had been using, re-cleaned the mug, and added 100 mL new water and began heating it in the microwave. After a few minutes the water began to boil slightly -- although, again, the water appeared to boil only near the surface. I stopped the microwave just long enough for the boiling to cease, then restarted the heating. This process repeated twice more. After this, the water remained under heating in the microwave for several minutes. Then, suddenly, with a tremendous "thwump" noise, nearly all of water burst out of the mug in a very satisfying mini-explosion inside the microwave oven.
I immediately repeated this process with a new batch of 100 mL tap water and the same results were obtained, much to the glee of my now-spectating roommates.
When the mug contained boiling water, it was clearly at the proper temperature to boil (approximately 100 degrees Celsius). However, I was able to stop the microwave heating just long enough for the boiling to cease, then restart the heating and not have boiling immediately resume. This indicates that the water was becoming super-heated, heated to a temperature well above its normal boiling point. Such a state MUST be unstable. The slightest disturbance will cause very rapid conversion from liquid to gas, which causes the observed explosion.
I think this is very similar to the situation in super-saturated solutions. At a given temperature, there's only so much of a given solid that you can dissolve in a given volume of a given liquid. Beyond this amount, no additional solid dissolves. For instance, for a given amount of water at a given temperature, there is a finite and well-defined amount of sugar that may be dissolved. However, the amount of solid that may be dissolved (its solubility) usually increases with the temperature of the liquid (the solvent). You can dissolve more sugar in a constant volume of hot water than in cold. However, if you saturate a volume of hot water with sugar (eg, dissolve as much as possible) and then allow the liquid to cool, you will arrive at a state where there is more sugar dissolved in the liquid than is normally allowed. This is another unstable state, and any disturbance (such as dropping a single grain of crystalline sugar into the liquid) will cause the rapid formation of sugar crystals. Similarly, in a cloud, the air may be supersaturated with water, but no rain forms until a suitable nucleus (eg, a particle of dust) initiates the formation of droplets. In the case of our super-heated water, my theory is that some kind of stimulus is needed to provoke the phase transition in super-heated water. If small air bubbles are present, or if heating is uneven, traditional boiling will be encouraged. However, in the microwave oven, heating is relatively homogenous, so apparently this stimulus is missing. Water heated on a stovetop is heated from the bottom. This sets up convection currents, which encourages mixing. This steady-state flow could favor traditional boiling. Furthermore, any boiling that is initiated will begin at the bottom, and the bubbles will propagate upward through the liquid, encouraging more bubbles to form throughout the liquid. In the microwave, boiling is equally probable to begin anywhere. The bubbles still propagate upwards. Thus the lower layers of the liquid are more likely to remain undisturbed, promoting superheating. This may partially explain the surface-boiling effect.
The event described in the email may really have happened! Furthermore, in a controlled situation the phenomena can be quite entertaining.