Friday, January 9, 2009

Colligative properties

So I was working at the high school yesterday, talking to the kids about boiling point....someone had asked what happens to the boiling point of water when you dissolve something in it. That got into the topic of colligative since we had a flask of water boiling, I thought I would demonstrate how addition of a solute raises the boiling point, and temporarily stops boiling. So after getting permission from the regular teacher, I added a little salt to the solution, thinking that the boiling would suddenly stop and then pick up again after the temperature rose a tiny bit.

BAD IDEA. Do not EVER try this unless you are standing well back from the water. It made a little volcano, suddenly fizzing up and overflowing the container for a second or two. Then it stopped, as one would expect. Fortunately, I and the kids jumped back in time, so no one was splashed- it wasn't as if the flask exploded. But still, not the best idea.

In retrospect, of course, I should have expected that would happen. Before the salt grains go into solution (i.e. for a second or two) they provide a great array of nucleation sites for bubbles, so they provide an immediate acceleration of boiling. As soon as some appreciable quantity has dissolved, boiling stops since the temperature is now just below the new boiling point of the salt water. I'd never seen this happen but I should have remembered that salt is an excellent promoter of bubble formation....I remember once in a bar, getting a Corona (I almost never drink beer, and Corona is pretty much the only variety I ever drink) and thinking it would be cool to put salt around the neck, like a tequila. It overflowed all over the table, quite the little volcano....foam and bubbles everywhere. The waitress said she'd never seen anything quite like it.

In future, always remember the importance of nucleation sites for phase changes! (Indeed, this is related to how you can have 'supercooled' liquids, i.e. liquids below their freezing point, in very pure water- there aren't enough nucleation points so the solid cannot form.)

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