This can also be easily demonstrated with a simple experiment. Mixing bicarb soda and vinegar! As we know, the vinegar makes the soda fizz and bubble. This is because the bicarbonate in the soda and the acid in the vinegar react to create carbonic acid which is unstable and causes the bubbles. Sounds complex and of course, you don't have to go into this explanation with your little ones (I surely didn't!).
Eden simply found out that mixing the soda and vinegar was FUN!
I put a fair bit of bicarb soda into a brownie tin, and portioned out vingear into some egg cups. I dyed it with food dye ('cause it just looks prettier that way!), gave her some spoons and a medicine dropper, and stood back.
She began by adding drops of the coloured vinegar to the tray and watching the little bubbles burble and pop, quickly progressing to putting whole eggcups in the tray and adding pinches of soda to the eggcups, watching delightedly as they fizzed over!