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Mar 29, 2001 04:15 PM

Boiling water in a microwave (or, Danger: Coffee Explosion)

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OK, I don't usually give in to the compulsion to forward random e-mails, but this was such a surreal intersection of mailing-list interests, thought I'd risk it

From: Jonathan Gilmour

Danger: Coffee Explosion

You warm up a mug of water for a few minutes in the microwave oven. You take it out, then you dump in some powdered coffee, tea, sugar, etc...

Dooosh! The water explodes in roiling foam, spraying boiling water all over your bare skin, and sending you to the emergency ward. I hate it when that happens.

It can be dangerous to heat pure water in a microwave. Coffee water sometimes "explodes" because the microwave heats it to a temperature that's far hotter than the normal boiling point. When this occurs, any tiny disturbance can trigger some violent boiling. This DOESN'T happen when water is boiled in a pot on the stove. The difference: a stove creates hot spots on the bottom of the pot which are far hotter than 100C, and this continuously triggers a roiling boil which cools the water down to 100C. Whenever there are bubbles of steam zipping up through the water, this provides some surfaces which allow the water to make more steam and
inflate the bubbles, which also colls it down to 100C. In fact, water can only "boil" at places where the water touches a gas. If there are no bubbles, then "boiling" only occurs at the top of the water and not down within it. So, when you heat water on the stove, the heat triggers bubbles at the bottom of the pot. The roiling bubbles act to cool the water and keep its temperature at or below 100C/212F degrees.

Things are different in a microwave oven. The water gets hot but the container usually does not. There are no "boiling-bubbles" triggered by a hot metal pot. Without those bubbles to cool it, the temperature of the water rises far higher than 100C. We call this "superheated water." Superheated water is just waiting for some sort of trigger which will let bubbles form and allow boiling to commence. If the water becomes hot
enough, a few bubbles will appear, but these quickly rise and burst, and the water isn't cooled much at all. In the microwave oven, even if your mug of water is bubbling slightly, don't trust it, since it's temperature has risen so high above 100C that bubbles are appearing spontaneously. If some unwitting victim should pour a soluble powder into the superheated
water, this will carry thousands of tiny air bubbles into the water. Each of these micro bubbles expands into a 1cm steam bubble, and the result is a huge "explosion" of hot froth. It's just like dumping icecream into rootbeer, but the froth can be so violent that the hot water sprays into the air.

Even more dangerous is to boil water TWICE in a microwave oven. Most containers have tiny scratches in their surfaces, and these scratches contain air. When you heat water, these tiny air bubbles will provide the
"seed bubbles" and normal boiling can occur. However, the air in these tiny bubbles quickly gets replaced by steam. If you turn off the oven and let the water cool, the steam bubbles collapse and vanish. The seed bubbles are gone. If you now turn the oven on again, the water will superheat. If your luck is bad, the water will superheat to a very high temperature, then explode violently when a single huge steam bubble bursts
into being.

Here's a simple, HAZARDOUS experiment to try. Wear safety goggles, and don't heat the water for an excessive amount of time.

Fill a clean mug about 1/3 full of clean water (DON'T FILL IT TO THE TOP!), then heat it for about three minutes in the microwave oven. Now carefully take it out and plunk it firmly onto the tabletop (whack it
hard, but not so hard that it breaks.) The boiling water will burst into froth. DON'T BURN YOURSELF! The superheated water acts almost like warm carbonated cola: if you strike the container, it will foam up instantly.

Another trick: heat up the water, then stick a dry wooden coffee-stirrer or a wooden popcicle stick into the water. Foosh! The water boils violently. The dry wood contributes a layer of air to the water, and the
air fills with steam and expands into a mass of hot foam.

Hmmmm. I wonder if de-ionized distilled water in a REALLY CLEAN container will superheat even more than normal? (DANGER, SUPERHEATED WATER CAN BURST
OUT OF THE MUG AND SCALD YOU!) I wonder what would happen if we put some dishwashing soap in the water...

SAFETY WARNING: Treat microwave-boiled water with respect. It can "explode" without warning. Don't dump any sugar in a mug of superheated water, or the spewing foam *really* gets violent. Don't try to boil water
twice. Also, don't heat your water for an extended time in the microwave, or the temperature will climb so high above boiling that any small "trigger" can lead to unexpected violent burst of scalding water. And frothing isn't the only danger: sometimes single huge bubble can appear at the bottom of the mug and launch all of the water upwards in an explosion. Allow "boiling" liquids to cool for several minutes before adding anything to them (or perhaps poke them carefully with a dry stick to force them into normal boiling mode.)

I got this from this site:

CrackMonkey: Non-sequitur arguments and ad-hominem personal attacks

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  1. Was really surprised to find that this is actually true.

    Apparently if you leave a wooden spoon or stirrer in the cup, it reduces the risk.


    4 Replies
    1. re: Lisa Z

      Ladies, please go to Amazon and order the book "Fear Of Physics: A Guide For the Perplexed" by Lawrence M. Krauss. It really is good to understand these types of things.

      1. re: Greg Spence

        YOU, Greg, might want desist with the condescending
        sexism, and look at:

        The piece is written by a Stanford educated prof
        at University of Virginia. I think he has a vested
        interest in NOT publishing misinformation as he needs
        to preserve academic credibility to maintain his
        career. Hard science profs who embarrass themselves
        don't generally have long careers 'cos they gotta
        publish, and journal referees don't suffer fools.

        Cite (emphasis added): "Above 100° C, liquid water
        can't exist IN EQUILIBRIUM with atmospheric pressure
        gas, even if that gas is pure water vapor.

        So how can you superheat water? Don't wait for
        equilibrium! The ROAD TO EQUILIBRIUM MAY BE SLOW;
        it may take MINUTES or HOURS for the liquid water
        to evaporate away to nothing. In the meantime, the
        system will be out of equilibrium, but that's ok."


        1. re: Grouchy Guy

          Calling women ladies is not sexism in my neighborhood. My science may be out of date, but I'm no sexist.

          1. re: Greg Spence
            Formerly grouchy guy

            Sorry. I read the address as a stereotyping women
            as needing dumbed-down physics. I know enough sharp
            women in math and hard science, that I tend to bridle
            at the suggestion or words resembling it.

    2. It's been a while since college but as a firefighter I do keep up with some of the science and I have to tell you that the laws of physics state that once water is boiling, it cannot be heated to above 212F in an open container.

      What happens is that the microwave heats the water very rapidly, resulting in its conversion to water vapor (steam) very rapidly, which produces a large bubble that breaks the surface tension of the water rapidly and sprays water everywhere. If you heat the water to the boiling point but do not exceed it, thumping the container on the counter or poking a stick in the water will distrupt the surface tension and allow vapor to be released.

      Try your experiment with a thermometer instead of a stick and you'll see what I mean.

      Conclusion: Publishing untrue things on the internet doesn't automatically transform them into the truth.

      5 Replies
      1. re: Greg Spence
        Christine DiBona

        Did you follow the links in Lisa Z's post? It seems that under certain circumstances water CAN be super-heated. I began using a microwave only about two years ago when I started working in an office, and was tickled to see an explaination of why pouring what appeared to be water at less-than 212 degrees into my french press coffee maker would cause such foamy mess.
        Like you, I'd always believed that 212 degree water was accompanied by bubbles, but it seems appearances are deceiving. I've no longer got a microwave to experiment with, so can't try the thermometer test, but have you tried it?

        1. re: Christine DiBona

          Yes, I tried it in high school physics class. Please, ditch the internet links and go to a library or other reliable source. When water boils, it stays at 212F as long as you're at sea level and not using a pressure vessel. When you apply more energy, you accelerate the process that converts the water into steam but you do not superheat the water, no matter what the energy source. In microwaves, the microwave energy creates friction between the water molecules as it excites them and causes them to rub against each other. With gas, you're applying the heat of combustion directly. Either way, it's the same result.


          1. re: Greg Spence
            Christine DiBona

            Dude, I'm going to have to write to the fda RIGHT AWAY and tell them that they've got some totally BOGUS internet CRAP on their web site, and that their scientists are ALL WRONG when they say that water can be superheated without appearing to boil. Link provided below to FDA - and if you don't like that source, lots of the hoaxbuster-type places ( which _are_ internet sites, by and large, and thus in your eyes highly suspect) have also agreed on the basic truth of it being possible to heat water in a microwave past 212 degrees. You may be reacting to a please-forward-this-message that made it's way around a while ago, that personalized and exaggerated this premise into an unverified incident. That doesn't negate the scientific fact of the superheated water any more than the discussion of it a book would lend it credibility.


            1. re: Christine DiBona

              You are right. In the absence of convective action (as in a microwave), water can be heated to slightly above 212F without boiling. Sorry for being obnoxiously wrong.

              1. re: Greg Spence

                Thank you.

                Superheated water does not bubble and so cannot release water vapor in a closed environment (microwave). Not releasing water vapor means it's easy for the temp to climb above 212 F. When the environment is open (you open the door), all the trapped water vapor is released at once, in an upward jet, so the water appears to explode.

                Looking at the water in the mug through the microwave door, the water doesn't appear to be boiling. Therein lies the danger. It's actually past 212 degrees, or superheated.

                The phenomenon is called bumping. Look it up.

                This happens with teakettles too. The water boils but the spout is closed.
                When the spout is opened, water lurches forth -- jumps out of the spout.

                The cup has to be smooth, like the stainless steel teakettle spout.

                I've been burned twice this way. Once by water in the microwave, the other time by the teakettle. The fireman would be wise to know.

      2. this happened to me this morning, fortunately the explosion was inside the microwave oven. I heard a large muffled bang, like a firecracker at least, maybe. I first thought the wife had fallen upstairs, it was that loud and strong. I looked into the oven and my 8 oz of water was now about 2oz and boiling, and there was water everywhere in the oven. I boil 8oz of water in a pyrex cup for tea like this everyday for 3 minutes and this is a first. this time it had not boiled by 3 minutes AT ALL, so I hit another instant minute, at about 3:15 BOOOM!!!!

        2 Replies
        1. re: oldorange1

          Oh yeah, I've seen it happen. Rule of thumb, if it hasn't boiled in the time it usually takes to boil, be very afraid! I had half the cup end up splattered all over the microwave when I jostled it slightly. Fortunately it missed me.

          1. re: oldorange1

            It happened to me years ago, and I was lucky to escape maiming. I'd nuked water to near boiling (3 min) and was pouring it away from me when suddenly it exploded all over the wall, cabinets with whumpf! If I'd been holding it straight up, it would've exploded all over my face. When I nuke water now, I always stand a porcelain Chinese soup spoon in the cup. Like many, I'd assumed the warnings were an urban myth because I'd gotten away with it no probs for a few years before the explosion.

            There's nothing like a little Dumb Luck now and then.

          2. (Someone may have mentioned this already and I probably missed it/glossed over it) Just curious, but doesn't the type of water and type of container play into this? I seem to recall seeing somewhere ('Mythbusters' or 'Good Eats', maybe both?) that to purposely make it happen you want to use water that contains little or no impurities (I don't remember if it was distilled water or just bottled water) and heat it in a container that has very smooth sides. Something about the level of impurities in the water not usually allowing the superheating process to take place. Thanks.

            1. In the several years that I've been reading CH, this has to be the most ancient thread that I've seen resurrected ....

              1 Reply
              1. re: jgg13

                I've recently seen one resurrected from 1999 - which feels like about a million years ago, doesn't it.