Can a cup of coffee explode if heated in microwave?

Question

Hello this is a serious question, and I dont know if I am really in the right part of stack exchange:

A friend of mine was trying to make a coffee in her lab, she used the microwave to heat the water (I hope you enthusiast do not crucify us for this bewildering savagery). She set the timer to 2.5 min, but then had to run some work. After returning the water had already cooled beyond usability, thus she restarted the heating process for approx. 30s. Before reheating she realized that there was a lot of steam in the microwave cavity. After the reheating she takes out the cup, and after having taken it out, the water in the cup explodes in her face.

Some word on the used hardware:

The microwave is an old model from Carrefour home The cup is an ordinary ceramic cup with shape of a truncated cone.

This is not a joke, she did really burn herself and was lucky that she chose her glasses instead of her contacts. However more interesting is, how this explosion came about. There must have been some pressure build up inside the cup but how would such an inhomogeneity in such a small volume be explainable?

Answer 1

As others have stated, superheating can happen in a microwave, especially when using a smooth vessel, e.g. a new, unscratched cup. It is more likely in pure water, but not unheard of from other liquids.

But you need a hack:
The key to prevent superheating and a subsequent flash of steam when the cup is moved or so is to supply nucleation sites, or, simply put, some rough surfaces or gas bubbles where the water can turn to vapour.

• Many recommend using a teaspoon in your cup, but at least in some microwave models that can lead to "fireworks" of electric sparks. So questionable at best.
• If you can get your hand at some lab equipment, get one of those glass sticks with a hollow lower end or a little porous ceramic stone.
• But probably the simplest way is floating a toothpick in the mug.
• Or use an old, scratched mug, provided it is microwave-safe.

Answer 2

As far as I see, some explanation is already in the comments.

A short explanation may be helpful for the community, so I decided to take a note here.

The reason behind that is the super-heated water. In theory, it is possible to super-heat any liquid with polar molecules.

Temperature of an object is defined by the total kinetic energy of its molecules. The kinetic energy has two sources, the linear velocity of molecules, and spin of molecules.

Microwave ovens heat up objects by adding spin to the molecules of an object. Roughly speaking, molecules stay where they are, but spin around very rapidly under microwave to heat up. During this phase, they cannot find a way to vaporize (linear velocity).

When your friend take the cup out of the oven, the molecules are not under the effect of microwave (so they don't spin) anymore, so they find their way out to vaporize, and choose to vaporize (linear velocity) very quickly. That occurs as an explosion.

Answer 3

@MT_San hit the nail on the proverbially head. However, I thought this excerpt from an article I read recently might prove to be easier to understand.

When water reaches a boiling point, it doesn't necessarily turn to steam. It needs some kind of physical spot—a "nucleation site"—to form a bubble. This can be an imperfection or crevice in the surface of the container. That’s why a clean, scratch-free jug of water heated in the microwave can seem to be below boiling point, but explode as soon as you move it.

The water in there is super-heated beyond 212 degrees, but hasn’t found any nucleation sites to let it turn into steam. When you disturb it, swirls in the water can be enough to release all that pent-up energy into steam. Another example is throwing salt into boiling pasta water. It suddenly roils because you just tossed a handful of nucleation sites in there.

For safety's sake, purchase an electric kettle if you don't have an oven or hotplate at work to boil water with. Bonavita has an excellent gooseneck kettle with a built-in temperature sensor.