Exploding Film Canisters

Find out how to make a mini-explosion using a film canister, and find out how your explosion relates to real explosives, such as gunpowder.

What you need

What to Do

What may Happen

You should find that the film canister goes off with a satisfying pop and flies into the air, and even (as we often found) into your neighbour's garden!

What is going on?

The fizzy tablet contains bicarbonate of soda, mixed with a dried acid. This becomes acidic only when it dissolves in water, at which point it reacts with the bicarbonate of soda, releasing carbon dioxide (just like in the Fire Extinguisher experiment).

When this reaction takes place in an open glass of water, the carbon dioxide escapes slowly into the atmosphere, over a period of several minutes as the tablet dissolves, so there is no change in pressure. 

But if the gas is prevented from leaving, by constraining it inside a container like the sealed film canister, the pressure will build up, storing energy. Eventually, the pressure becomes sufficiently high to force open the lid, and all of the gas - and the energy stored in it - is released at once with a "pop", launching the cannister into the air.

What has this got to do with real explosives?

Ordinary "low" explosives, like gunpowder and cordite (the smokeless propellant used in most guns), are just mixtures of two materials - an oxidant and a fuel - that react to produce large volumes of gas, a bit like more energetic versions of the fizzy tablet. If you set fire to them in the open, although they burn very quickly, there is no explosion.

If, on the other hand, you constrain them, they will produce gas until their container bursts, and you get a big bang. This is why, if you contain gunpowder in a card tube and set fire to it, there is a bang (a banger)

This is the sort of behaviour that is desirable for the propellant in a gun: a substance which will produce lots of gas quickly, but not so quickly that it would damage the barrel. The resulting gas pushes the bullet out of the gun, and the bang you hear is the gas escaping, once the bullet leaves.

But what if you want to do lots of damage with an explosive?

This is where high explosives such as TNT or dynamite come in. They react in a different way. Unlike low explosives, where typically the oxidiser and fuel are chemically separate materials, in a high explosive they are very close together, often in the same molecule.

Materials like this are designed to just burn like low explosives if you set fire to them, for safety reasons. However if you hit them hard enough, you can create a wave of compression, which moves through the material, triggering the reaction as it goes. This, in turn, produces large volumes of gas, which then makes the compression shockwave even stronger. This moves through the explosive at supersonic speeds, increasing in power as it goes. Consequently, when it leaves, it is immensely destructive.

So high explosives can go bang without being confined.

Written by Dave Ansell