About five days ago, my 26-year old son decided to have a cup of instant coffee. He took a cup of water and put it in the microwave to heat (something that he had done numerous times before).
I am not sure how long he set the timer for, but he told me he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup. As he looked into the cup he noted that the water was not boiling, but instantly the water blew up into his face. The cup remained intact until he threw it out of his hand, but the water flew out due to the build-up of energy. His whole face is now blistered with first and second degree burns to his face which may leave scarring. He also may have lost partial sight in his left eye.
While at the hospital, the doctor who was attending him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something (such as a wooden stir stick or tea bag) should be placed in the cup to diffuse the energy. Its a much safer choice to boil the water in a teakettle.
On further research into the story we determined that scientists agree that this can happen, though they offer a somewhat complicated explanation. Richard Barton writes in New Scientist magazine:
A portion of the water in the cup is becoming superheatedthe liquid temperature is actually slightly above the boiling point, where it would normally form a gas. In this case, the boiling is hindered by a lack of nucleation sites needed to form the bubbles.
When water is heated on a conventional stove, the porous surface of the kettle and the convection caused by the hotter liquid rising from the bottom enable the water to convert to steam. It boils. But a stationary cup of water in a microwave oven can heat past the boiling point without actually boiling. If that happens, placing an object (like a teabag) in the water or jarring the cup could cause the suddenand explosiveconversion of part of the water to steam.
I imagine, adds Barton, that by keeping the cup still and microwaving for a long time, one could blow the entire contents of the cup into the interior of the microwave as soon as you introduced any nucleation sites. It is this sometimes explosive rate of steam production that means you should take great care when using a microwave oven.
The US Food and Drug Administration advises consumers to avoid superheated water. When heating water in the microwave you should:
Follow precautions and recommendations of the microwave regarding heating times.
- Do not use excessive amounts of time when heating water in microwaves.
- Determine the best time setting to heat the water, just to the desired temperature and use that time setting regularly.