Coffee & Tea

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


Coffee & Tea 19

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

Christine DiBona | Mar 29, 2001 04:15 PM

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

Want to stay up to date with this post?