Can simply heating a cup of water in a microwave pose a hazard? Yes, when the water becomes superheated and boiling bubbles can’t form. Superheated water can “explode” due to the buildup of energy when the cup is moved, or a spoon of sugar, coffee, or other substance is poured in. Here are the facts:
Water can overheat above the boiling point when it cannot release thermal heat through the gas bubbles that would cool it. (How Everything Works) The situation becomes worse if the water is in a clean cup or glass container, because boiling can be hindered by the lack of “nucleation sites” to form the bubbles. (Richard Barton, New Scientist). Adding a soluble powder to the water can produce an explosion of steam bubbles.
The simple solution is to put something in the water such as a stirrer, a wooden spoon, or a tea bag. Don’t use water that has been already heated or sat out overnight. And avoid excessive time heating water in the microwave. Learn how long it takes to boil a liquid in your microwave, and don’t heat it to that point. Sounds simple enough, but we use microwaves in offices and public areas where we are not familiar with the heating capacity of the unit and may not have access to the manufacturer’s user manual. But even reading the manual may not help (“do not overheat liquids”).
Taking time to explain the potential for danger — and posting the recommended settings for heating water near your office microwave — can avoid unpleasant surprises. For additional information, see the FDA’s consumer advisory on microwaves.