Boiling Water in Microwave Can Be Dangerous
Some of the advice floating around the Internet turns out to be far-fetched. But here's one tip experts say you'd do well to heed: Take care if you use a microwave to heat water. In some cases, boiling water can explode, causing serious burns to your face and hands.
"We don't realize how quickly water is heating when it happens behind the closed door of the microwave," says physicist Steven L. Snyder, Ph.D., vice president of exhibit and program development at the Franklin Institute Science Museum in Philadelphia.
Most people know a microwave oven heats foods and liquids faster than a stove. But they might not realize that it also heats more evenly, since a stove has hot spots. When you combine the even, fast heating of water with a new and clean cup, you've got a recipe for trouble.
An older vessel is more likely to be pitted, rough or otherwise imperfect. Those imperfections let bubbles of steam form. With fewer imperfections—or minerals such as salt—in the cup, the less likely there'll be bubbles that tell you the water is at the boiling point.
"If the cup is new, smooth and clean, there's a chance the water in it can be a good bit past the boiling point and there might be no bubbles at all to warn you about the temperature," says Dr. Snyder. Glass containers pose a special risk because they're so smooth.
When you take the cup filled with superheated water out of the microwave, you disturb the water by jostling the cup. With no warning, bubbles that did not previously have a chance to form can suddenly form en masse—potentially causing the water to blow up onto your hands and face.
"My advice would be to use a kettle or pot on the stove to heat water, not a microwave," Dr. Snyder says. "But if you use the microwave for this purpose, do not use excessive amounts of time to heat water, check the water frequently and absolutely never look closely at the cup to determine how hot your water is." Mixing other materials in the water before it is heated, such as instant coffee or sugar, can reduce the risk.