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Circulated Warning Claims That Superheated Water In Microwave Can Explode

Outline
Message claims that a cup of water boiled in a microwave oven can "blow up" and cause injury.



Brief Analysis
The information contained in this message is true. Water superheated in a microwave oven can indeed "blow-up" under certain conditions. That is, the superheated liquid can be explosively ejected from its container and potentially cause injury to a person in close proximity.

Detailed analysis and references below example.
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Last updated:12th April 2012
First published: 17th April 2005
Article written by Brett M. Christensen
About Brett Christensen and Hoax-Slayer


Example:(Submitted April, 2005)
A 26-year old man decided to have a cup of coffee. He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for, but he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. 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 cup remained intact until he threw it out of his hand, but 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 to his face which may leave scarring.

He also may have lost partial sight in his left eye. While at the hospital, the doctor who was attending to him stated that this is a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc., (nothing metal).

It is however a much safer choice to boil the water in a tea kettle.

General Electric's Response:

Thanks for contacting us, I will be happy to assist you. The e-mail that you received is correct. Microwaved water and other liquids do not always bubble when they reach the boiling point. They can actually get superheated and not bubble at all. The superheated liquid will bubble up out of the cup when it is moved or when something like a spoon or tea bag is put into it.

To! prevent this from happening and causing injury, do not heat any liqui d for more than two minutes per cup. After heating, let the cup stand in the microwave for thirty seconds! before moving it or adding anything into it.

Here is what our local science teacher had to say on the matter: "Thanks for the microwave warning. I have seen this happen before. It is caused by a phenomenon known as super heating. It can occur anytime water is heated and will particularly occur if the vessel that the water is heated in is new, or when heating a small amount of water (less than half a cup).

What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat has built up, the liquid does not boil, and the liquid continues to heat up well past its boiling point.

What then usually happens is that the liquid is bumped or jarred, which is just eno! ugh of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken."

If you pass this on you could very well save someone from a lot of pain and suffering.

Thank You Very Much.




Detailed Analysis
The core information contained in this message is true. Water superheated in a microwave oven can indeed "blow-up" under certain conditions. That is, the superheated liquid can be explosively ejected from its container and potentially cause injury to a person in close proximity.

As noted in the email, a specific set of circumstances is necessary to cause microwaved water to explode in this manner. In an article about superheating and microwave ovens, Professor Joe Wolfe of the University of New South Wales notes that such events can take place when the following conditions are present:
  • Using a container with a very smooth surface, such as an unscratched glass or glazed container.

  • Heating for too long.

  • Quickly adding a powder, such as instant coffee (or sometimes even an object to stir it).

  • Standing with one's face above the container makes injury more likely.

Professor Wolfe's article graphically illustrates the result of superheating water in a microwave oven in the form of a video of such an event occurring.

Superheating occurs when the liquid is heated to a temperature greater than its normal boiling point. Wolfe explains that the "superheated state is unstable, and it can very rapidly turn into liquid at the boiling point, plus a substantial quantity of vapour." Thankfully, some simple precautions can minimize the chance of injury due to superheating. Professor Wolfe suggests the following strategy for avoiding superheating related injuries:
  • Before putting the water into the oven, insert a non-metal object with a surface that is not smooth. (e.g. a wooden stirrer. A wooden skewer or icecream stick will do.)

  • Use a container whose surface is at least a little scratched.

  • Do not heat for longer than the recommended time for the quantity of water used.

  • Tap the outside of the container a few times with a solid object while it is still in the oven. Use a long object so that your hand remains outside the oven. Alternatively, and still keeping your hand outside the oven, insert a stirrer while the container is still in the oven. (Thus, if vigorous boiling occurs, most of the boiling water will strike the inside of the oven.)

  • Keep your face well away from the open oven door and from the container.
America's FDA has also warned about the issue, noting on its website:
The FDA has received reports of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Over-heating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil.

This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.
Thus, the information in this message is a valid warning for microwave oven users. However, like many email forwards of this nature, it also contains anecdotal information that lessens its credibility. The "26-year old man" is not identified, nor is the "local science teacher". There is no way of verifying if these are real people or just fictional constructs added to drive home the point. Also, there is no way of confirming if someone from General Electric actually responded in the manner outlined since no name or contact details are supplied. Unfortunately, this unverifiable information, along with the exhortation to "pass on" the message, may convince some recipients that the email is just another bogus warning.

In spite of these factors, microwave users would certainly do well to heed the advice in the email. Hoax-Slayer visitor Al Chang relates the following experience illustrating just how dangerous superheating can be:
This just happened to me. I have a very smooth two-layer plastic tumbler. Imagine a cup suspended within a cup, so there's an air pocket between the two cups. It's designed to keep heat or cold from transferring from the inner surface to the outer surface.

I've heated water in it in the microwave before. There's a certain amount of time around 2m50sec where it goes from being completely still to boiling over. Normally, I just keep it below that time and it's fine. Up until today my only concern was the water itself boiling over while in the microwave.

Today, it must have been just under that time, because though it was still water, I took it out of the microwave, jiggled it a little and boom the water exploded all over my arms and hands. Very nasty scald. I suspect I could duplicate this consistently (which I might try under more controlled conditions once I heal up). It goes straight from still water with the tiniest of bubbles to overflowing in the microwave. I suspect it has a lot to do with the cup. Perhaps because there's no way to transfer heat to the outside world other than the top of the water and the stillness keeps bubbles from forming. Movement then triggers the explosion. I have some first and second degree burns to show for it. I was lucky I was right near the sink and was able to get to cold water right away.

So word of warning to your readers!
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References
Superheating and microwave ovens
Professor Joe Wolfe
FDA Article: Risk of Burns from Eruptions of Hot Water Overheated in Microwave Ovens


Last updated: 12th April 2012
First published: 17th April 2005
Article written by Brett M. Christensen
About Brett Christensen and Hoax-Slayer