Don’t be a Superheater

This cautionary tale recently landed in our inbox and we thought we’d share it.

A guy I know took a cup of  water and put  it in the microwave to heat it up (something that he  had done numerous times before). I  am not sure how long he set the  timer for,  but he wanted to bring the water to a boil. When the  timer shut off, he removed the cup from the microwave. As he looked into the cup, he noted that the water was not boiling, but suddenly it ‘blew up’ into his face. The cup remained intact until he threw it out of his hand, but all the water had flown out into his face due to the buildup of energy. His whole face is blistered and he has 1st and 2nd degree burns which may  leave scarring.

While at the hospital, the doctor who was attending to him said that this is a fairly common occurrence and water (alone) should never be heated in a microwave. If water is heated in this manner, something should  be  placed in the cup to diffuse the energy  such as a wooden stir stick or a tea bag.

Thank you, dear reader. This is not an urban legend, but is actually true, and here’s how it happens: microwaved water and other liquids  do not always bubble when they reach the boiling point. They can actually get superheated and not bubble at all. The superheated liquid will then bubble up out of  the cup when it is moved or something is put into  it.

What  happens is that the  water heats faster  than the vapor bubbles (which release heat energy) can form. If the cup is new it is unlikely to have small  surface scratches inside it that provide a  place for the bubbles to form. Since the bubbles cannot form and release some of  the heat, the  liquid does not  boil, and the liquid continues to heat up well past its boiling point.

Then, when the liquid is bumped or jarred, it is just enough of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why  a carbonated beverage spews when opened after having been shaken.

To avoid this disaster, never heat liquid in the microwave for more than 2 minutes per cup, and let it sit for 30 seconds before moving it. A wooden stick or teabag in the cup will work too, but make sure there are no metal components. Avoid putting infant formula or other children’s drinks in the microwave.