Is it dangerous to heat water in the microwave?
March 21, 2011 8:23 AM   Subscribe

MythFilter: I get these chain emails every once in a while about how dangerous it is to heat water in the microwave. I can't seem to get a straight answer from anybody! Is microwaving water really that dangerous?

Here's the text of the latest email I got in my inbox this morning:

I know this to be true...when I worked at UL the engineers warned us of this phenomena.

Microwaving Water!

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 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 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).

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 liquid 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 vapour 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 enough 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.
posted by JMB1138 to Food & Drink (18 answers total) 2 users marked this as a favorite
posted by Burhanistan at 8:24 AM on March 21, 2011 [2 favorites]

For what it's worth I seem to recall Alton Brown mentioning on Good Eats that you should put a wooden chopstick in water you're microwaving for this reason. I don't think there's a super high risk though.
posted by ghharr at 8:25 AM on March 21, 2011

Yeah, this can happen, though it's unusual. You can see it happen on this clip from Mythbusters.
posted by mr_roboto at 8:27 AM on March 21, 2011

Alton Brown recommended putting a wooden stick in the cup you're boiling, because then the bubbles have something to form around and you'll see the bubbles before you pick up the cup.
posted by lilac girl at 8:28 AM on March 21, 2011 [1 favorite]

I've had one incident I could ascribe to this (or, since I was heating water to clean off some conductive gel off of some electrodes, could have been the gel interacting with the water when I dropped the electrodes in it). In either case, yeah, there was a large splash of boiling water that I didn't expect, but it wasn't like I was ever tempted to put my face over the cup. So I go with the Snopes answer.
posted by straw at 8:29 AM on March 21, 2011 [1 favorite]

Unless you're boiling distilled water, this won't happen. Virtually any impurity in the water will act in the same way as a flaw in the cup. Ie, it'll induce boiling at a temp below 100 degrees C. If you need to try it for yourself, heat water on the stovetop until almost boiling, then add salt. Watch it come to the boil all of a sudden.

(Happens everytime I make pasta..)
posted by Ahab at 8:30 AM on March 21, 2011

The Straight Dope answers this one as well, eleven years ago.
posted by Grither at 8:40 AM on March 21, 2011


I've seen this happen heating a variety of solutions and pure solvents (including water) in glass vessels, even roughed-up scraggy undergraduate labware with plentiful scrub marks. Boiling chips (and sticks, etc) are widely used to prevent the sudden "bump."

Ceramic mugs with smooth surfaces are even worse.

OP: use a chopstick or a skewer or a toothpick as a bubble-nucleation site and you'll be fine.
posted by janell at 9:01 AM on March 21, 2011 [1 favorite]

Unless you're boiling distilled water, this won't happen.

Never say never. At an old job I would heat tap water for tea in a microwave using old and not pristinely clean mug. Often, when I removed the mug from the micro the water was still (not boiling), but as soon as I dropped in a tea bag it would boil over. The first time it happened, the water boiled over and I got 2nd degree burns on the hand I was using to hold the cup. Subsequently, I would put the mug down before adding the tea bag. Sometimes it would boil over, sometimes not.

I didn't think that water would superheat in this mug given the many nucleation sites and impurity of the tap water, but it did. So, yes, it can happen. I've seen it and I've suffered burns from it. Never thought of the chopstick, maybe that would have helped.
posted by kuujjuarapik at 9:02 AM on March 21, 2011 [1 favorite]

I thought part of the problem with (older, non-rotating) microwaves is that they may superheat parts of the water, not the whole thing, so while nucleation sites (imperfections) on the mug might exist, the water near them wasn't being zapped at full power, but other parts (that aren't touching anything) may become superheated, and when they come in contact with nucleation sites(like a teabag): BUBBLE-SPLOSION!
posted by Grither at 9:06 AM on March 21, 2011

I've done this a hundred times in lab. It's a fun party trick for scaring undergrads, since there's a way to make it look like it's boiling all over your hand.

What I was taught as a student is that if you kind of gently and quickly tap the side of the vessel while it's still sitting in the microwave, you'll trigger the boil over before you pick it up and have it in your hand.
posted by arabelladragon at 9:13 AM on March 21, 2011

MetaFilter's own Matthew Baldwin had a close call with microwaved water.
posted by mbrubeck at 9:18 AM on March 21, 2011

Yup. My aunt was boiling filtered water in the microwave and got a face full - a trip to the ER ensued. I use a bamboo skewer or chopstick.
posted by pointystick at 9:21 AM on March 21, 2011

Yup, seen it happen myself. A friend microwaved a mug of water and then dropped a baby bottle into it. Since the displacement was pretty great, so was the explosion. I was glad we were in a big kitchen!
posted by wallaby at 10:09 AM on March 21, 2011

It's happened to me, with regular tap water and a regular rotating microwave - 2nd degree burns on my forearms and hand. I highly recommend using a chopstick or investment in a kettle.
posted by goo at 10:23 AM on March 21, 2011

You can also take the same principle the other direction and under the right conditions, supercool liquids in your freezer. Then on a hot hot day, you carefully take out one of the below-freezing beverages, and the moment you start drinking it, the entire thing starts instantly turning to ice!

So naturally you have to drink as much as you can as quickly as you can, because within a few second no fluid will remain to pour - the entire thing is a solid frozen lump!

Denied your thirst quenching, you put down the frozen lump and reflect on how you can no-longer feel your throat...

Well, you might still be thirsty, but at least you feel refreshed and cooled!

Physics can be fun for everyone. :)
posted by -harlequin- at 4:54 PM on March 21, 2011

I have had this happen, before, once in the last 2 decades of me microwaving things. Yes, it's possible. No, it's not something you should worry about.
posted by carlh at 6:03 PM on March 21, 2011

It happened to one of my mother's friends when she was re-heating a mug of coffee. The hot coffee burned her eyeball. Fortunately it hit the sclera rather than the cornea, so it didn't affect her vision once the burn was healed.
posted by JaneL at 6:07 PM on March 21, 2011

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