Ask a Physicist: Exploding Coffee

Nick, from the US, wants to know:
“After retrieving my medium sized ceramic cup of coffee from the microwave that had been in for 2 minutes, I quickly stirred the coffee with a thick straw. Immediately bubbles started to rise and overfilled the half full cup, unfortunately burning my thumb. Could you please tell me what reactions caused this to happen? Thank you very much.”

Sounds like you superheated your coffee! Ordinarily, water boils at 212°F, but under the right conditions, the molecules can get to a much higher temperature without vaporizing. Then, when the liquid DOES boil, it does it all at once!

Water transitions to its gas phase when its molecules hit the liquid’s surface with enough kinetic energy to overcome its surface tension, breaking free of the intermolecular bonds that keep them in the bulk of the fluid. That surface doesn’t have to be at the top of the cup, though—if there’s a gas bubble trapped at the bottom, even a tiny one, the water molecules can break into that bubble, helping to inflate it until it becomes buoyant and detaches from the bottom—this is the process we usually see as boiling. “Blowing bubbles” like this in water is a lot like blowing up a balloon, in that the hardest part is getting the process started—the smaller the bubble, the greater the elastic force compressing it.

Small imperfections and dimples at the bottom of the pot trap tiny air bubbles, serving as nucleation points for water vapor to boil around. Without these imperfections and hot spots, microwaves can superheat your drink with nasty results.

Usually, small imperfections in the container trap tiny air bubbles that act as starting points, jump-starting that “bubble-blowing” process through the most difficult part. But when they don’t have any “seed” points to collect around, the high-energy molecules end up staying dissolved in the water until they’re disturbed by someone poking a straw into the cup or jostling it, introducing a bunch of bubbles. When this happens, as you unfortunately discovered firsthand, the water can all boil at once in an explosive way.

Microwave ovens are really conducive to this kind of overheating, especially when you’re reheating something; gases have a harder time staying dissolved in hot liquids, so a previously-hot cup of coffee is a lot more likely to superheat than, for example, a glass of cold tap water thrown straight into the microwave. To avoid this in the future, tossing something like a wooden stir-stick in the cup ahead of time might help.

It turns out that there’s a similar, much less painful process at the other end of the temperature spectrum—supercooling! When water in a smooth-walled container is cooled without being jostled, it can reach temperatures below 32°F without crystalizing:

Stephen Skolnick

You can learn more about superheating and see it in action with the Mythbusters.

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