It turns out that if you microwave eggs, they will explode. Here is a video showing it.
This does not surprise me in the least. A raw egg is a liquid and anyone should know that a liquid bought to the boil will create gases that occupy a much larger volume and if not allowed to escape because of the shell, that this added pressure will at some point cause it to explode.
What did surprise me was why anyone would microwave an egg in the first place. According to Charles Choi, it appears that some people, especially cooks in restaurants, use this method to cook hard-boiled eggs. I have always done this by boiling them in water and it never even occurred to me to try the microwave option. But even if the egg does not explode while being cooked, the danger of an exploding egg is not over. It can also explode when you dig into the egg itself.
In experiments, the scientists reheated nearly 100 hard-boiled eggs in carefully controlled conditions. First, the eggs were placed in a water bath and microwaved for three minutes, and the temperature of the water bath was measured both at the middle and end of the heating cycle. The eggs were then removed from the water bath, placed on the floor and pierced with a fast-acting meat thermometer to induce an explosion.
About a third of the reheated eggs exploded outside the oven. For those eggs that did explode, their peak sound pressure levels ranged from 86 to 133 decibels at a distance of 12 inches.
133 decibels is not a trivial level of sound, as this chart of the decibel levels of other sources indicate. (Note that decibels lie on a logarithmic scale, not a linear one, and so an increase of 10 decibels from (say) 80 to 90 corresponds to a ten-fold increase in the intensity of the sound.)
So why do the eggs explode even after being taken out of the microwave? It is due to the phenomenon of superheating.
The researchers suggested that microwaved egg yolks can develop many small pockets of superheated water much hotter than water’s normal boiling temperature. When this unstable water gets disturbed — say, when the egg is bitten into — it can boil very rapidly. “The release of steam can be explosive,” Nash said. Nash and his colleague Lauren von Blohn detailed their findings Dec. 6 at the annual meeting of the Acoustical Society of America in New Orleans, Louisiana.
The case was ultimately settled out of court. The researchers suggested their restaurant acoustics experiments should underscore the fact that “heating certain foods in a microwave oven can be dangerous,” Nash said.
This phenomenon of superheating occurs when a liquid is raised to a temperature above its boiling point without the usual creation of bubbles that release the pressure, take away the heat, and keep the liquid temperature at the boiling point. If you look closely at boiling water, you will see that bubbles form at specific places in the container corresponding to irregularities or impurities that serve as nuclei for bubble formation.
But when water is heated in a very smooth container that has few irregularities and impurities, then sufficient bubbles might not form and the temperature of the liquid can rise above the boiling point, though you may not notice it. As a result, the pressure builds up inside the liquid. Then if the liquid is disturbed (say by putting in salt or sugar or a spoon), the presence of the nuclei causes the pressure to be released suddenly in an explosion that can cause serious burns.
So you need to be careful when microwaving liquids. When I do so, I always set the time to a value that I know is safe, not enough to bring it to the boiling point. But I know someone who would simply set the timer to a pre-set value that was longer than necessary to bring it to a boil and then stop the heating at the time she wanted. That was fine, as long as she remembered to shut it off and did not get distracted by something else.
Something similar happens with supercooling, as this video shows.