The Great Hot Cocoa Mystery!

August 27, 2010

Last night (Thursday) was our first set of labs: 6-7:40PM and 8-9:40PM. That second one gets a bit late.

I’ve been doing this lab for a couple of semesters now. I’ve always thought it was a good introduction to the Scientific Method, and a nice way to fit in a lab when they haven’t really learned enough content do do much of anything else.

So just what is the Great Hot Coco Mystery? Well, make yourself a cup of instant hot cocoa: hot water in a ceramic mug, packet of cocoa poured in on top, metal spoon to stir. Stir it up a bit, then stop. Begin tapping your spoon on the bottom of the mug and listen carefully…as you continue to tap, the pitch of the tapping will rise. Stir a bit more, and when you resume tapping the pitch will have dropped (though not as low as at the start), and then proceed with rising some more. This process can be continued for quite some time, and the rise in pitch becomes quite obvious.

So what is the “experiment” part of the experiment? The students (who have brought their own mugs and spoons. Yay! No washing for me!) are given a largish number of cocoa packets (8/group of 4 seems to be enough), and access to a number of other products: Sugar Free Cocoa; Fat Free Cocoa; Cocoa with Marshmallows; instant coffee crystals; instant apple cider; lemonade; instant cappuccino mix; water heating on hot-plates; hot and cold tap water; ice. They must design at least three investigations, with at least three variations within that investigation (still working on some of the terminology here…bear with me). For example, say they want to investigate whether the effect (rising pitch) is related to water temperature. They could then make a cup of cocoa with boiling water, hot tap water, and cold tap water. They could add icy water for a fourth variation. They are allowed to collaborate with other groups to pool or share resources; though I don’t think anyone did last night.

With 20 minutes left in class, we share the “results”. I stand at the board and ask each group to tell me one of their investigations, and its results. Then the other groups are asked whether they can confirm or contradict these findings. We talk about experimental procedure…are you really changing just one variable at a time, or did you (intentionally or unknowingly) change multiple initial conditions? Why is that maybe not the best idea?
Given what the class has found, can we make any definitive statements as to what the *cause* of the effect might be?

In the last 5 or so minutes, I ask what would they want to do next…given more resources, more tools, more time, how would they set up the next investigation. Some want thermometers, or a way to measure the temperature change of the mug. Many want microphones to record the pitch change. Something that dissolves in cold water, in addition to the lemonade.

I like this as an intro lab for a number of reasons. It’s accessible…nearly everyone has made themselves a cup of hot cocoa. If not cocoa specifically, then something similar. But few people have ever tapped their spoon on the bottom. The effect is not too subtle…it doesn’t take motion detectors or infrared thermometers to detect (not that there’s anything wrong with those things. However, I have a strongish belief that computer-assisted labs are over-used in introductory physics classes). And it’s weird! I mean, who doesn’t want to know *why* this happens? But it’s also still mysterious. I don’t know the answer. The source from which I developed the lab didn’t know the answer. The students are not going to determine the answer in one night during lab. And that’s OK! That’s how Science is done! You don’t get The Answer at the end of a two hour period…hopefully you just get a bit of the way there.

End Note: I modified this experiment from a presentation I’d found on the internet. Someone (AAPT???) doing a demonstration for other instructors. I can’t find it with a quick google search, but I have the original material at school. I’ll update with references when I’m back on Tuesday.

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