Question
A colleague of mine is in the habit of heating bottled water for his tea in a mug in a microwave oven. When the water is up to temperature he removes the mug. On several occasions, the water has started to bubble violently after he has added a tea bag. On one occasion, the boiling started when he was removing the mug. It was so violent that it blew 90 per cent of the water from the mug--which is obviously quite dangerous. What is happening?
Murray Chapman
I heard of this happening to someone one time and I've never shaken my fear of microwaves since. I'm always ready to drop a microwaved mug in an instant.
Answer
>> A portion of the water in the cup is becoming superheated--the 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.
This never occurs when boiling a kettle, for example, because the presence of the rough surface of the element, as well as the convective stirring from rising hot water, are sufficient to produce proper boiling. Turbulence in liquids is known to provide enhanced nucleation in other cases: when you pour a cola drink, for example.
In your colleague's case, the addition of a tea bag (and, in the other case, simple movement) sufficed to allow bubble formation. Even with a large proportion of the water superheated, only a little will convert to steam, as the amount of latent heat required for this phase change is very large. I imagine 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.
Richard Barton , Guildford Surrey
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