Exploding Coffee from Superheating in the Microwave

exploding-coffeeI got a physics lesson today in a dramatic fashion about superheating and wanted to post a note about it so that it may serve as a warning to others.

Superheating is when a liquid is heated (in the microwave) to a temperature above its boiling point, but it doesn’t begin to boil. Boiling happens when vapor bubbles grow in the liquid, rising and breaking at the surface. If there are no trapped bubbles of air in the liquid (due to imperfections of the container) then the liquid might need extra few degrees to overcome the surface tension of the tiny vapor bubbles. But once these tiny guys grow, they grow fast and hence the explosion.

I encountered this when I was heating a cup of coffee this morning. I put the coffee in the microwave for 2 minutes, which is usually fine. Sometimes it begins boiling right at the end of the two minutes so I know it’s real hot. This time it wasn’t boiling or steaming at all and I (being sleepy) assumed that I accidentally set it for 1 minute, so I put it in for another minute and watched it, waiting for the boiling to begin. It didn’t. When the 3rd minute came to an end, I opened the microwave and looked at the coffee. As I picked it up, my first thought was: “That’s weird, it should definitely be boiling by now”. About halfway through that thought, the coffee exploded upwards in a directed beam of liquid, covering my face, my shirt, my whole kitchen, and most dramatically, the whole ceiling of my kitchen in black/brown Jackson Pollock painting.

I experienced the feeling that most of us do when we do something stupid that requires a lot of cleanup. In retrospect, I’m lucky that for some reason the coffee was not sufficiently hot to burn my face. It definitely hurt, but it didn’t actually burn the skin. My kitchen walls and ceiling are of the material that soaks that stuff up and so can’t be cleaned easily. So until I paint it, I will be left with a daily reminder of how I came face to face with Phantom of the Opera type of disfigurement due to my ignorance of a phenomena that I probably was told about in a high school Chemistry class long ago.

Let this be a warning to you. According to the internet, heat your coffee (or other liquids) with a wood stirrer in the mug, which helps the vapor bubbles form at normal boiling temperature.

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