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Why you should never heat hot drinks in the microwave

Lucy Devine

Published 
| Last updated 

Why you should never heat hot drinks in the microwave

If you've ever chucked a cup of tea or coffee in the microwave after it's gone cold, you might want to think again next time.

According to experts, reheating liquids in the microwave can actually have disastrous consequences – and it's all to do with a process called 'superheating'.

Do you reheat drinks in the microwave? Credit: MBI/Alamy Stock Photo
Do you reheat drinks in the microwave? Credit: MBI/Alamy Stock Photo

In a post shared on Facebook last week, one person recalled an incident in which a man tried to boil water using the microwave.

He took a mug of water and popped it in the microwave, before removing it once he thought it had reached boiling point.

"As he looked into the cup, he noted that the water was not boiling, but suddenly the water in the cup 'blew up' into his face," the person explained.

"All the water had flown out into his face due to the build-up of energy. His whole face is blistered and he has 1st and 2nd degree burns which may leave scarring."

Other users were horrified at the post, while some shared their own similar stories. Many had experienced the same after heating other liquids, such as gravy, soup and milk. One woman said she nearly lost her eyesight after making a hot chocolate in her microwave oven.

One man tried to use the microwave to boil water. Credit: Maksim Kostenko/Alamy Stock Photo
One man tried to use the microwave to boil water. Credit: Maksim Kostenko/Alamy Stock Photo

The reason for the 'explosion' is all down to a process called superheating – which occurs when a liquid reaches boiling point without actually boiling.

To understand how this happens, we chatted to Peter Barham, Emeritus Professor in the School of Physics at the University of Bristol.

"When water boils, it starts creating bubbles," Professor Barham explains.

"So if you put water in a pan, it starts to bubble long before it comes to a boil. The bubble itself is hard to make and requires a lot of energy.

"In a normal heating process – like a pan on a stove – water is being heated by an element underneath. That bit of metal gets very hot, so there's enough energy in the water just above it to start to boil.

"But in a microwave, it doesn't work like that. Microwaves heat water directly, they don't heat the outside of the container so there's nothing to start a bubble unless the bubble's already there.

"If you have nothing to start a bubble then the water will keep getting hotter and hotter. This is called superheating. Eventually it's so hot that it has to do something, and it starts to bubble explosively, throwing all the water up and out of the way."

Superheating can happen with any liquid. Credit: Shutterstock
Superheating can happen with any liquid. Credit: Shutterstock

Professor Barham explains that the explosion often happens when someone puts something into the cup such as a spoon – but can also occur once the cup is tipped up towards the mouth, leading to serious injuries.

"My best piece of advice would be don't heat liquids in a microwave, but it's also worth checking the manufacturer's guide for advice too," he adds.

While superheating can happen with any liquid, it's most likely to occur with previously boiled water. For example, reheating a hot drink.

On that note, we're off to finish our cup of tea before it gets cold...

Featured Image Credit: MI/Alamy Maksim Kostenko/Alamy

Topics: Science, Food And Drink

Lucy Devine
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