Floating Gas Bubbles

Several years ago, when I also taught physics, we used a demonstration to compare densities of liquids and solids called the 'knickerbocker glory' of densities. This was demonstrated in a tall gas jar and involved paraffin floating on water. Different solids were then added: ice floated on the water but below the paraffin (between the paraffin/water boundary), polystyrene floated on the very top (on the paraffin), whilst metal objects sank to the very bottom.

Then, many years later I was watching the popular US TV series MythBusters where they managed to float a paper boat on carbon disulfide gas. This gave me the idea of floating air bubbles on carbon dioxide.

This demonstration is enjoyed by all ages, but is particularly useful for Year 8 students when studying the properties of gases. It helps to reinforce the idea of density, which can be related to methods of collecting gases (upwards displacement of air etc), and to enforce the idea that gases, despite many being colourless, still have substance.