Why does Sweet ‘n Low make microwaved water fizz so strangely?

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Dear Cecil: A while ago I read your column concerning the effects of hot tea on styrofoam cups. (See The Straight Dope: Why does tea make holes in plastic foam cups?) Being an avid tea drinker myself, I was reminded of something. I work at the local public library where, in the staff lounge, there is an automatic dispenser for boiling water. When it broke recently, I had to start microwaving my water instead. But when I empty my packet of Sweet ‘n Low into the mug after microwaving the water, it bubbles and fizzes as if some strange chemical reaction were going on. This never occurs when the water comes from the dispenser or is boiled in a kettle on the stove. What are the microwaves doing to my Sweet ‘n Low? I’m worried! Fred K., Evanston, Illinois


Illustration by Slug Signorino

Cecil replies:

I never did trust that Sweet ‘n Low stuff. For that matter, anything with “‘n” in the title has got to be on the suspicious list.  But there’s nothing special about Sweet ‘n Low that causes it to effervesce, as we weens like to say. (“Bubble and fizz,” indeed. How do you expect to get anywhere in science with a vocabulary like that?) Sugar and salt produce equally vigorous reactions. So will just about any powder except flour, which just sort of lies there.

Here’s what happens. When you put a kettle on a conventional stove, some of the water at the bottom turns to steam, making bubbles that float to the surface. But the bubbles don’t form just anywhere. They start at crevices on the kettle’s bottom or sides where tiny pockets of air are trapped. The air pockets provide a surface where the steam can escape from the surrounding water and make bubbles. The tiny air pockets are called nucleation sites. (A related phenomenon is the condensation nuclei — dust particles, usually — that water vapor needs in order to form raindrops.)

In conventionally boiled water, there’s a certain amount of internal motion called convection. The hot stuff at the bottom becomes less dense and rises while the cooler stuff sinks. Convection brings more water into contact with the nucleation sites, so the bubbles get bigger faster. Microwaved water, however, heats up uniformly. Since there’s no convection, bubble formation is pretty limp. Without bubbles, the water heats as much as seven or eight degrees Celsius above boiling point without actually boiling, a process called superheating.

So what happens when you dump in the Sweet ‘n Low? Instead of bringing water to the nucleation sites, you bring nucleation sites to the water. It’s like seeding the clouds. All the superheated water that’s been sitting around in frustration, unable to vaporize, suddenly gloms onto the nearest particle, forms bubbles, and rises to the surface. Result: fizz galore. Play your cards right — you want to get the water as superheated as possible without actually boiling — and you can get the water to erupt over everything within a radius of three feet. Fun, educational, and maybe worth fifty grand if you settle out of court.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.