Energy and entropy > hydrogen

‘Rockets’ are made by filling plastic fizzy drink bottles with a mixture of hydrogen gas and air. The rocket is launched by igniting the mixture with an electric spark, and will fly several metres. The rocket can be set up to fly tethered to a string or launched from a short section of drainpipe.

A large fizzy drink bottle, from which the base has been removed, is filled with hydrogen. The hydrogen is allowed to burn at a small jet in the stopper of the bottle. As the hydrogen-air mixture changes in composition, an explosive mixture is reached, which then explodes with a load bang.

Test-tubes containing three different proportions of hydrogen and air are prepared and the gas mixtures ignited. The loudness of explosions (‘pops’) is noted and related, at least in part, to the amount of energy produced when the hydrogen is burnt. The mixture producing the loudest ‘pop’ is used to estimate the ratio in which hydrogen and oxygen combine.