The premise of this demonstration is the knowledge that a chemical reaction of an acid and a base produces carbon dioxide and a salt, so it's a good one to conduct after you've been studying acids and bases for a bit.
You know, make the volcano, mix citric acid and baking soda and then add water, play with litmus paper, make red cabbage pH indicator solution, and THEN light things on fire!
NOTE: I bought some long-handled matches just for the kids, because yep, I let them play with matches.
First, have the kiddos stick a match into an empty test tube, just to prove that putting a match into the test tube won't necessarily cause it to go out.
Next, put a teensy scoop of baking soda in the bottom of a test tube. Punch a hole in a piece of paper, then syringe up a teensy bit of vinegar into an eyedropper and poke the eyedropper through the hole in the paper.
Set the paper on top of the test tube, so that the gasses formed by the chemical reaction won't be able to completely escape, and drop a couple of drops of vinegar into the test tube. You should see the chemical reaction when the acid and base mix.
Light another match, then remove the paper from the test tube but tip it, so that the gasses still can't escape. Just as you did before, stick a match into the empty test tube:
This demonstrates two facts: fire can't exist in all gasses, and the result of the vinegar and baking soda reaction is one of those gasses in which fire can't exist. The textbooks tell us that this particular gas is carbon dioxide--on another day, we can do more playing to try to prove or disprove this claim.
After the kiddos finished writing up and illustrating their demonstration and its results, Will enjoyed trapping the smoke from lit matches in an overturned test tube and observing it--
--and Syd and I later went out onto the driveway so that she could light some small fires while I put branches through the wood chipper:
Frankly, most of our science classes end with starting fires.
And that's okay by me!