Inquiry in Action Chapter 5, Chemical Change.
Wow, I was SO EXCITED about this week's lesson.
We started off by discussing the difference between a chemical change and a physical change. I gave examples of each (eg, cutting up a piece of steel wool vs. spritzing it with water and letting it rust), and also wrote out a couple of examples on the board (methane plus oxygen becomes carbon dioxide plus water).
Then, the FUN stuff. We looked at a couple of plain white powders -- baking soda and baking powder. They look pretty much the same, but when we added vinegar they reacted somewhat differently. We discussed that CO2 had been created during the reaction, and that the reaction rate had been different for the 2 powders.
Next I put some red cabbage indicator, which I made using the same process we'd used here, in a couple of clear plastic cups. (The curriculum has a different way of making the indicator, but I thought the idea of giving out ziplocs, water, and cabbage to the kids was downright scary -- I had visions of this stuff being blasted everywhere.) I put a little cream of tartar (an acid) in one, and a little powdered detergent (a base) in the other. Of course, they turned 2 different colors. Only one of the students had ever messed around with this sort of thing before -- not a huge surprise, since cabbage gets rather stinky, and most parents don't care to mess with it (although it's so cheap, easy, and fun that they should).
Okay, now we've seen a couple of ways to tell rather anonymous-looking white powders apart -- looking for chemical changes that produce gas, and looking for chemical changes that cause color change. So we set about sorting out what would happen when we tested baking soda, baking powder, cream of tartar, corn starch, and detergent with drops of various liquids -- vinegar, water, iodine (turns from red to black when it combines with starch -- this was another activity practically straight out of the old Junior Girl Scout badge book from back when GSUSA cared about getting girls interested in STEM, and also the reason I had iodine around the house), and red cabbage indicator. The kids worked in groups of 4. They also had an unknown (baking powder) to identify.
It was really tough to keep this organized and on track -- you practically need an adult with each group of kids, helping them figure out how to organize their work. But it was really fascinating stuff. I'd say that over the course of prepping for this activity I learned more about the history of cream of tartar and of baking powder than I ever thought I would.
After the extravaganza of testing all of those powders we were running a bit short on time, but we still tried to cram in the last to demonstrations I'd chosen (I'm starting to leave stuff out of the curriculum). First I poured some vinegar in a graduated cylinder and had the kids note the temperature of it with a partial immersion thermometer (both the cylinders and the thermometers were purchased from Home Science Tools). Then I added baking soda, let it bubble up all over the place (always a crowd pleaser), then looked again at the temperature. It had dropped! An endothermic reaction.
The curriculum suggests using calcium chloride for the "other" reaction, but I decided to go with the easier-to-source hydrogen peroxide and yeast reaction. This time we put the yeast in the graduated cylinder first, measured the temperature of the peroxide while it was still in the bottle, then dumped it into the cylinder. It bubbled up, of course, and the temperature shot up. Exothermic!
So we managed to make it through 3 possible outcomes of chemical changes -- a gas formed, a change in color, and a change in temperature. It was an action-packed hour.