I asked the children how I could blow up a balloon. They offered the standards....blow air into it with my mouth, or fill it with water.
I told them that I could use a chemical reaction to blow up the balloon. We discussed what a chemical reaction is, and they understood that it means 'change'.
They were intrigued as I half filled a narrow bottle with vinegar, then stretched a baking soda filled balloon over the neck of the bottle. Next I gently shook the balloon, sprinkling the baking soda on the vinegar.
The reaction began, inflating the balloon. I had a moment of panic, fearing that the balloon would pop, spraying the children with vinegar. Fortunately it did not.
This was a good jumping off point, to allow each child the opportunity to make their own reactions. They enjoyed combining the ingredients, smelling the vinegar, listening to the popping bubbles, squealing when the mixture overflowed onto their trays and then dipping their hands in the bubbly concoction.