Marsh gas, or hydrocarbons on the heath

Take a walk on a marshy, boggy area of heath or moor land, for example the New Forest in Hampshire. There are shallow pools of stagnant water, with rotting vegetation at the bottom. No chemists worth their salt go for a walk without pH paper, so test the water - it is weakly acidic. Often, you will see bubbles on the surface. Occasionally, you may see the rainbow colours of an oily film on the surface, like petrol spilled on a wet road. This suggests long hydrocarbon chains. 

Stir the bottom with a stick. More bubbles rise to the surface. With care, this gas may be collected by using a plastic bag filled with water. It does not matter too much if a little air is collected as well. Set fire to the gas you have collected. It burns if there is at least 15 per cent of the gas by volume, or it will explode if there is less than 15 per cent but more than 5 per cent.  

You have collected marsh gas, methane, made under strongly reducing, anaerobic, conditions. Because we think of natural gas as a fossil fuel, made millions of years ago, it is worthwhile students seeing that the process still continues today.