The Sinking Submarine

We have a little plastic submarine in our classroom. We tried filling it with baking powder and then submerging it in a basin of water. We hoped the baking powder would react with the water and make bubbles of carbon dioxide that would float the submarine. This didn't happen. We could see the bubbles of air popping slowly out the top of the submarine but the submarine stayed stubbornly on the bottom of the basin. We tried again, this time with less baking powder, but that didn't work either. We'll go on trying until we discover the right combination!