Heating water in the microwave can actually be dangerous. A man decided to have a cup of instant coffee, so he heated a cup of water in the microwave. When the timer went off, he removed the cup from the microwave and noticed that the water had not boiled. Just then, the water literally “blew up” in his face. His whole face was blistered with first and second degree burns, which left some permanent scaring and damage to his left eye. While at the hospital, the doctor attending him stated that his is a fairly common occurrence. Water (alone) should never be heated in a microwave oven. Why?
This phenomenon is known as superheating. It can occur anytime water is heated – especially if the cup or bowl is new. What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new, then it is unlikely to have small surface scratches in it that provide a place for the bubbles to form (called nucleation sites). Without bubbles, the water cannot release the heat that has built up, the liquid does not boil and it continues to heat up past its boiling point. If the water is bumped or jarred, it’s enough of a shock to cause the bubbles to rapidly form and the result is an exploding liquid that is scalding hot. One solution is to place a wooden stir stick or something non-metallic in the water to help diffuse the energy as it’s heating in the microwave.
While sharing this story with audiences, I’ve learned that many people have had similar experiences but never understood why the water just blew up. It’s an important piece of science to share with others in hopes of preventing a potential disaster.