It sounds like a classic urban myth: A guy puts a cup of water in the microwave to make coffee, takes the heated liquid out, drops in some Folgers crystals, and boiling water explodes out of the cup, burning his hands and arms. It’s no myth, says Anuradha Prakash, spokesperson for the Institute of Food Technologists and professor and program director of food science at Chapman University in Orange, California.
When clean water is microwaved in a very smooth container (like a really clean mug), it can superheat—meaning that it reaches a temperature above boiling without bubbling. Bubbles release heat from the water, but for bubbles to form, there need to be “nucleation sites.” These are either impurities in the liquid or imperfections on the surface of the container.
If you’ve superheated the water, then whatever you add to it, like instant coffee or sugar, creates nucleation sites for bubbles to form and expand. “The high temperature causes the bubbles to expand quickly, in some cases causing mild explosions,” says Prakash. Microwaving distilled water is riskier since it contains no impurities, although Prakash says explosions can occur in tap water too if the dissolved minerals are not large enough to act as nucleation sites for the bubbles.
Generally, microwaves will heat a cup of water to temperatures that are close to boiling within a minute or two, says Prakash. The FDA advises adding sugar or coffee before heating to “greatly reduce” the risk of burns from superheated water. Prakash says putting a wooden stirrer in your cup is also a good way to mitigate the risk.
Check out this MythBusters clip where they superheat water and drop in a sugar cube: