287056 Thu Mar 29 17:15:34 -0800 2001 10 27 General Chowhounding Topics 0 1542001 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: http://www.eskimo.com/~billb/weird/microexp.html ______ CrackMonkey: Non-sequitur arguments and ad-hominem personal attacks http://crackmonkey.org/mailman/listinfo/crackmonkey Thu Mar 29 17:15:34 -0800 2001 0 Christine DiBona 1 1542120 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. Link: http://www.snopes.com/spoons/fracture/microwav.htm Tue Apr 03 15:02:47 -0700 2001 1542001 0 Lisa Z 2 1542136 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. Tue Apr 03 19:34:03 -0700 2001 1542120 0 Greg Spence 3 1542179 YOU, Greg, might want desist with the condescending sexism, and look at: http://rabi.phys.virginia.edu/HTW/water_steam_ice.html 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." Link: http://rabi.phys.virginia.edu/HTW/water_steam_ice.html Thu Apr 05 05:21:44 -0700 2001 1542136 0 Grouchy Guy 4 1542201 Calling women ladies is not sexism in my neighborhood. My science may be out of date, but I'm no sexist. Thu Apr 05 14:48:54 -0700 2001 1542179 0 Greg Spence 5 1542204 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. Thu Apr 05 17:25:22 -0700 2001 1542201 0 Formerly grouchy guy 1 1542121 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. Tue Apr 03 15:11:49 -0700 2001 1542001 0 Greg Spence 2 1542127 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? Tue Apr 03 16:03:11 -0700 2001 1542121 0 Christine DiBona 3 1542135 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. ALL ON THE INTERNET IS NOT TRUE!! Tue Apr 03 19:18:56 -0700 2001 1542127 0 Greg Spence 4 1542168 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. Link: http://www.fda.gov/cdrh/consumer/erupted.html Wed Apr 04 16:24:50 -0700 2001 1542135 0 Christine DiBona 5 1542188 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. Thu Apr 05 10:59:12 -0700 2001 1542168 0 Greg Spence