Water becomes unstable if superheated in a microwave. Image: Shutterstock
Could your cup of coffee burn you?
There have been reports of people being injured when using a microwave to boil water. This is because water heated in a microwave oven can be heated above its normal boiling point (superheated).
In an average kitchen, water will boils at 100 degrees Celsius if there is a bubble of steam or air present. But in the absence of bubbles, the water can become superheated above 100 degrees. This occurs because, unlike a filament that heats the kettle or saucepan, microwaves pass through the cup and heat the water directly, causing it to become hotter than its container.
Heating the container will cause a small amount of localised superheating, initiating boiling (bubbling) in a small area and stirring the water. Microwave containers are usually smoother than kettles or saucepans, so bubbles of air do not cling to the sides and the water doesn’t boil, causing it to become unstable.
According to scientists at the University of New South Wales, one litre, superheated by one degree, can suddenly produce approximately three litres of steam. This will cause water to boil vigorously and explode out its container.
The water can also explode if you add a powder, such as coffee, or an object to stir it.
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