Here’s an example of one of those silly little throw away toys that you sometimes got as a free gift with packets of breakfast cereal. The Baking Powder Submarine was a thing of awe the first time you saw it in action, though chances are you never bothered to play with it again afterwards…
At first glance it was little more than a plastic submarine around 6cm in length, but open it up and fill the little compartment inside with baking powder, then pop it in a sink full of water, and the little submarine would astound you by repeatedly sinking to the bottom, then rising back to the surface, until it’s fuel load was depleted.
Whilst it seemed magical, it was of course a fairly simple chemical reaction causing this behaviour. At first the sub is too heavy to be supported by the water, so it sinks to the bottom. The baking powder starts to react with the water and lets off a gas (carbon dioxide I believe), forming bubbles on the underside of the sub. These bubbles want to rise to the surface, so they carry the submarine back up, popping when they reach the surface and so the sub starts to drop again.
These toys were initially produced in the 1950’s (funnily enough as breakfast cereal give aways) but I distinctly remember getting one out of a packet of some cereal or other when I was a kid during the Eighties. I believe there was also another variation on the theme given away, in the form of a little plastic SCUBA diver.