My wife put a cup of milk into the microwave oven to heat it. After the bell rang, she took the cup out (the milk was not boiling). Whn she dropped a spoon into it, it immediately boiled over! How does this work? If you heat a fluid past the boiling point, how can it stay in the liquid state? Where is the extra energy stored?
Alton Brown explained that without a nucleation site for bubbles to form, liquids can become superheated. The milk didn’t boil because the inside of the glass was too smooth. The introduction of the spoon introduced said nucleation sites and allowed the milk to rapidly boil. The extra energy is stored in the liquid. If you had taken its temperature, you would have noticed that it was in excess of its boiling point (whatever that is for milk).
You can prevent superheating in the microwave by putting a chopstick into the milk.
FWIW,
Rob
What Beowolf said.
I’m surprised, though, that this could happen with milk. Milk is a colloid, not a solution, and I would have expected that the suspended particles of milk solids could provide nucleation sites.
As I understand it, liquids often have tensile strengths on the order of a hundred psi or so. The reason we generally treat them as if they don’t is that they have almost no tear resistance, or notch strength. As Chronos points out, it is surprising that a suspension of particles (fat droplets I mean) would have much of this effect, but perhaps the way milk production evolved to make the suspension as stable as it is explains some details of why it still does.
I remember seeing a NBC Dateline show about this like 10 years ago. They were interviewing some older women who suffered severe burns because they microwaved water in a glass container. I remember because I felt bad at first, then they mentioned that they ladies were boiling like 8 oz. of water for 12+ minutes.
If the liquid is in a container that can’t provide spots for the liquid to form bubbles and let off steam, the liquid becomes super-heated. The instant you put in a piece of silverware, all those bubbles that should have been forming in the container and releasing steam, happens all at once around the fork/spoon. So you get 8 minutes worth of boiling happening in about .5 seconds, which can obviously be quite forceful/violent.
Were they heating, not boiling, for 12+ minutes, perhaps because the water wouldn’t boil? It’s pretty easy to forgive an old lady for not understanding superheating and microwaves.