Careful when heating water in microwave ovens

Anyone who is in the habit of warming up water in a microwave oven should do so with extreme caution, chemistry professor Alfred Vella has warned.

Prof. Vella was contacted to check the veracity of an e-mail being sent to people, warning about boiling water in a microwave.

The e-mail claims that a person suffered first and second degree burns and lost partial sight in one of his eyes after water he had heated in a microwave oven "blew up in his face".

Prof. Vella explained that when water was brought to boil over a fire the heat built up slowly and bubbles formed as the water's temperature rose, thus releasing some of the energy.

However, when water is heated in a microwave oven, super-heating can take place as water heats up so fast that vapour bubbles do not have time to form and the water continues to heat up past its boiling point.

"Then when one moves the water container, this provokes the bubbles to form with an explosive force that impels the hot liquid," Prof. Vella said.

As people would be looking at the water, to make sure they did not spill it, or to see why it did not seem to be boiling, the result would all too often be burns to their faces.

Prof. Vella pointed out that if a container in which the water was heated was new, the chances of super-heating occurring were even greater. This was because in used cups the small surface scratches helped bubbles to form, while new containers were smoother.

The risk of super-heating occurs only with water, as milk, tea, coffee or water having any other additives would reach boiling point and bubbles would then form.