Exploding Film Canisters
|film canister||A fizzy tablet or two|
|a little water|
First, if your tablets are large, snap one in half (so as not to waste too much).
Now, half-fill your canister with tap water.
Next, find an open space where it won't matter if things get wet.
Then as quickly as you can:
Drop the half-tablet into the water.
Seal the lid on tightly.
Place the cannister upside down on the floor.
Stand back! (At least 2-3m away is advisable).
Now wait (but note that it can take up to 2 minutes to go off).
BUT - DO NOT GO BACK TO THE FILM CANISTER WHEN IT IS CLOSED because the liquid can fly into your eyes, which is unpleasant.
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!
|The film canister should launch with a satisfying mini explosion|
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).
|The tablet reacts when it gets wet to form bubbles of carbon-dioxide.||If you put the lid on, the reaction continues, with nowhere for the gas to go.|
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.
|As the gas builds up, so does the pressure.||Eventually something has to fail, and the lid is pushed off, releasing all the gas at once in a small pop, or explosion.|
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)
|The loose gunpowder burns fast but doesn't explode. But if it is confined, it goes off with a bang!|