The Homely Scientist

So this post is both a Jedi Knight and Sith Lords kind of science post.  David on ScienceBase is taking issue with the "superheated beverage" phenomenon that is oft reported:

There is an urban myth that has done the rounds for many years that it is possible to cause yourself serious injury by reheating a cup of coffee in a microwave oven and then giving it a stir as soon as you remove it from the oven, thus causing the supposedly “superheated” drink to release its pent up energy in a great scalding geyser. Who hasn’t received the spam-mail describing the 26-year old who was several disfigured by such an incident?

Now, one thing that concerns me about this video, aside from the fact that some viewers of a thrill-seeking bent may wish to emulate it, is that it is actually very hard not to have any nucleation points in a generic beaker of water, and it would be nigh on impossible to superheat an open beaker of water in this way. I suspect that what the perpetrators did instead was to heat the beaker of “water” (maybe with some clarified vinegar added) to near boiling point and then to simply drop in the coffee (with a dose of added some sodium bicarbonate). The rapid neutralization of the acetic acid in the vinegar with the bicarb would be adequate to produce the result you see in the video clip.

His post is both dangerous and wrong.  The thing is that, while it might not be "superheated", you can very easily heat coffee (or water as I did) to the boiling point, not see the indicative bubbles, grab it, and have it go FOOOMMM!!! in your hand.  Like I said, it happened to me so I know of which I speak.

So let’s get the Jedi Knight bit here.  First you need to understand a bit about microwaves (discovered by accident at Raytheon, btw).  When you heat something in the old nuker oven it doesn’t heat evenly.  This is really important to know.  You  should always stir food coming out of the nuke before serving—especially to kids.  Uneven heating, IMHO, is one of the factors that play into this FOOM phenomenon.

Next boiling.  Remember there is the boiling point of something and the activity of "boiling" (bubbles).  Everything has a boiling point.  Water, as you know, is 100 deg C/212 deg F.  The bubbles come from the lower pressure on the surface of the liquid causing the air to rise.  The bubbles form, as David rightly says, on nucleation sites.  Yes, same thing as the old Mentos - Diet Coke thing.  What can happen is that in a microwave a liquid, for some reason, doesn’t have enough or any nucleation sites.  Maybe the liquid is really hot at the top (which it usually is in a microwave) and all the particles are at the bottom.  Who knows, who cares.  The effect is that the bubbles don’t form.  That means there is an imbalance in the pressure (essentially).  When you remove the vessel from the microwave it jostles or when you put something in it (spoon, sugar, whatever) the bubbles just dying to form do.  All at once.  Explosively.

Yes, I burned the snot out of my hand when it happened to me.  It hurts.  A lot.  Is the liquid superheated?  Probably not.  Getting water above 100C is rather hard unless you put it under pressure (hence super-heated steam pipes).  Still 100C water will burn. So technically I don’t care.

What can you do to prevent this?  Well one thing is to be careful of how long you heat a liquid in the microwave.  Might not need as long as you think.  Can you tell if you have one of these coffee-geyers in your mug?  Nope.  That’s the problem.  If you aren’t sure there are two things I suggest you try.  First is to nudge the mug with a long spoon (you want to make sure your hand IS NOT in the microwave).  The other is to remove the mug with an oven mitt on.  That should protect your hand.  Then when you set the puppy down, just wait a min.  Then try a spoon or something in it and stir.  Keeping the oven mitt on might look goofy, but … well you know.

All the above said I’ve only ever had this happen to me once.  Once was enough though.  Now, I’ve come close when I was boiling water for hummingbird food (gotta have the water clean and hot to get all the sugar in).  But knowing ahead of time, I was careful.  No, I never got a geyser, but when I hurt myself the amount that came out of the top was enough.

Okay, my rant is over.  Be careful out there.

Tags: boiling water, microwave safety

Share This

This entry was posted on Wednesday, January 17th, 2007 at 5:41 pm and is filed under Jedi Knights of Science, Science at home, Science for the Dark Side. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.