Monday, October 20, 2008

A water question. Any takers?

Water is weird.

I love the story of Erasto Mpemba, a high school student in Tanzania who (back in 1963) trusted his eyes more than his teacher. Mpemba noticed that his warm ice cream mix froze faster than cooler batches.

Aristotle apparently knew this already, and even up to Descartes' time, the cognoscenti accepted it, but part of being modern is being sensible, and Mpemaba was ridiculed by his teacher.

This phenomenon is now known as the Mpemba effect, but it's still not well understood. ("Not well understood" in sciencese means "not a clue.") That's not my question, but if you want to tackle that one, feel free.


In biology we've been playing with water. Today's opening question was "why is water wet?" ("Wet" does not mean "liquid.") That's also not my question--I can handle that one.

We looked at Brownian motion, watched a charged glass rod make a stream of water bend (and leaves of wheat dance), played with dropping water on a penny, and used wet yarn to transfer water from one beaker to the next. So far, I'm fine with my chemistry and grasp of hydrogen bonds and polar molecules--I'm feeling like a philosopher king, a rare moment in teaching.

What's not so rare, however, is the crash that follows 17 seconds after assuming the role of philosopher king.

We floated paper clips on water--very easy to do with dry paper clips, pretty much impossible if the paper clip is wet. When you float a paper clip, you can see the water dimple around it, like a thin film. I know about surface tension (though not in its florid post graduate physics form).

So here's my question. Does the paper clip remain "dry" while floating? That is, do the water molecules only cohere to each other and not adhere (at all) to the paper clip?

If you stare at that floating paper clip, it looks like it is resting on a film of water without actually getting immersed. My chemistry and brief observations say it's staying dry, my students think I'm nuts.

Any thoughts?


Anonymous said...

I always thought that warm water freezes faster than cold because there is less oxygen in warm water. It has started to break down and separate.

Mr. D said...

fellow NJ science teacher here...I do all those water tricks too...I have a "water day" to show all the cool things that can be done with the ubiquitous substance. I would guess that it is somewhere between total cohesion and a mix of the two forces...A few (relatively, of course) molecules would probably adhere to the metal, but the layer of water molecules underneath could still probably cohere to each other, thereby preserving the overall force. Darn good question, though...

Another water trick - get a piece of window screen and cover a jar with it (use a rubber band to make it tight) Fill it and quickly turn it over...make sure to hold it parallel to the ground. The water will stay in the jar. They love that one...

doyle said...

I will have to try the jar trick!

Any chance, Mr. D., that you're related to the Mr. D who once owned a local bookstore?

Mr. D said...

no chance...Did you go to the science convention? Flinn had a setup for that one...anyway, keep up the good work - you really have a way with words.

doyle said...

Mr. D,

I missed the convention--I hope to start going next year.

I'm still a fairly new teacher--this is my third year--and the few days I have missed for conferences have left me dizzy.

And thanks for the kind words.