The products of baking soda and vinegar are water (H2O) + carbon dioxide (CO2). The CO2 gas is what fills the balloon.
Surprising: Notice how the balloon bounces at the end. The balloon filled with CO2 from the chemical reaction is heavier than the reaction products of breathing air or helium (the two ways we would normally blow up balloons). It also sounds different as it bounces. Tie it off and experiment! See if your child can tell the differences between a balloon blown up with air from our lungs versus one blown up from the baking soda + vinegar experiment.
Here's what we did for the experiment:
*Fill the balloon with baking powder (we didn't measure - but it was enough to fill up the tip of the balloon).
*Fill the bottle 1/4 full of vinegar
*Attach the balloon to the top of the bottle without letting the baking soda fall into the bottle yet
*Tip the balloon so the baking soda falls into the vinegar
*Watch the reaction
*Comment on the reaction
Vary the amount of vinegar/baking soda/size of balloon and try again!
Edited 1/26/14 to Note: This post has become very popular with Pinterest. The pin links to this blog post, but the picture is not mine (after some research, I found it on Mom to 2 Posh Lil Divas and want to give them credit). For the record, I did not start that particular Pinterest pin with someone else's picture nor did I link it as an alternative to helium for party balloons. I apologize to disappoint. I want to ensure you that CO2 is HEAVIER/more dense than air, which means it's heavier/more dense than helium (as stated above in my original post). It will not float. The main point of this experiment is the chemical reaction between baking soda and vinegar. The CO2 gas fills up the balloon and the now water stays in the bottle. It is a fun and easy experiment for anyone who loves science. Enjoy!!