Can Microwaving Water Cause an Explosion?

From the Urban Legends Mailbag

Microwave Oven
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Dear Urban Legends:

I received this email that had been forwarded by many companies' safety departments. Apparently, there is actual science fact behind this, but the email is worded with all the normal Internet legend warning signs (e.g. the tragedy happening to the author's son, the doctor and hospital not being named, and the duration of heating not being specified).

Is there a legitimate concern for the average microwave user? Or is this occurrence so rare that the email is really just another scare-o-gram?

This is the text:

FW: Safety Alert - Microwaving Water

Importance: High

The following information came from a member of staff and is worthy of note -

Subject: Microwaving Water to Heat it Up

I feel that the following is information that any one who uses a microwave oven to heat water should be made aware of. About five days ago my 26-year old son decided to have a cup of instant 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 told me 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 instantly 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 flew out into his face due to the buildup 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 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. It is however a much safer choice to boil the water in a tea kettle. Please pass this information on to friends and family.


Dear Reader:

You're right, this narrative does display all the hallmarks of a typical viral "scare-o-gram." But, as you also observed, that doesn't mean it can't be true.

First, let's consider the specific story about a 26-year-old man who suffered first- and second-degree burns after microwaved water exploded in his face.

Is it factual word-for-word? We have no way of knowing. The author is anonymous, the alleged victim is anonymous, we are not told where or when the incident supposedly happened. We simply cannot verify its authenticity.

So let's consider the scenario in a more generic sense. Is it plausible to think that something like what was described in the message can happen?

The answer, evidently, is yes — under just the right circumstances.

The consequences of superheating water

This safety warning posted online by the Gurnee, Illinois Fire Department:

Liquids heated in a microwave oven may not turn into steam, even though they are very hot. Moving these containers of hot liquid, or putting a utensil or other object into them creates a "steam bubble" and the hot liquid may splash out, causing a scald burn. Liquids heated in a microwave are very hot, even if the container they are heated in is cool.

Scientists agree that this can happen, though they offer a somewhat more complicated explanation. As Richard Barton writes in New Scientist magazine, "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."

When water is heated on a conventional stove, the porous surface of the kettle and the convection caused by the hotter liquid rising from the bottom enable the water to convert to steam. It boils. But a stationary cup of water in a microwave oven can heat past the boiling point without actually boiling. If that happens, placing an object (like a teabag) in the water or jarring the cup could cause the sudden — and explosive — conversion of part of the water to steam.

"I imagine," adds Barton, "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."

Not an everyday occurrence

So, we know it's possible. But is it common? Does it happen frequently? No, says Louis Bloomfield, professor of physics at the University of Virginia.

"Fortunately, serious microwave superheating accidents are unusual — this is the first injury I've ever heard about." Which contradicts the email's claim that such injuries are a "fairly common occurrence."

A survey of the medical literature on microwave injuries also undermines that claim. There are references to trauma caused by everything from overheated pizza to exploding eggs, but no mentions I could find of serious injury due to surges or explosions of boiling water.

Lastly, is the warning useful, or just an alarmist rumor? It falls somewhere in between, it seems to me. Manufacturers do recommend that foods or liquids heated in a microwave oven be allowed to stand for a time before they're touched or consumed. Sounds like good advice to me.

Update:

FDA: Risk of Burns from Eruptions of Hot Water Overheated in Microwave Ovens - U.S. Food & Drug Administration, 4 April 2009

Sources and further reading:

Chapman, Murray, et al. "Microwave Madness." New Scientist. 19 Jan. 2000

Bloomfield, Louis A. How Things Work: Microwave Ovens. University of Virginia. 19 Jan. 2000

Microwave Oven Safety. Gurney, Illinois Fire Dept. 18 Jan. 2000

Superheating of Water (Video). Mythbusters. 19 Oct. 2007

Unwise Microwave Oven Experiments.

Science Hobbyist. 18 Jan. 2000

Microwaving Water. AFU & Urban Legends Archive: "Let's be careful out there..."

Last updated 11/29/15