Everybody has seen about a thousand explosions - on TV etc, for example, where
pyrotechnicians use carefully calculated explosions to make us
think that something bad has happened. Also, unfortunately, more real explosions
can be seen, like explosions from bombs or planes crashing.
What exactly is an explosion?
An explosion is a kind of chain reaction that happens when a material very
quickly changes from being in solid, liquid or gas form, into hot, expanding
gas. When the energy from this chemical reaction is released, a quick increase
in pressure happens.
For an explosion to occur, three things have to be present, namely oxygen,
flammable material and temperature (the fire triangle). When something
explodes, however, the actual burning process usually produces oxygene to burn.
(if you mix sugar with a material with lots of oxigene in it (an oxidant),
you can make it explode)
What determines how "hard" an explosion is?
Apart from the obvious factors of what the chemical combination of your
explosion is, the same material can also create various amounts of strength
When studying pyrotechnics, the following experiment is common:
What to do:
First, make a loose pile of 20 grams of gunpowder. Use the electronic fuse,
and set off.
Second, take a film canister, have 20 grams of gunpowder in it, close it,
use the electronic fuse, and set off
Third, take a film canister, have 20 grams of gunpowder in it, Fill it up
with sand, close the canister, tape it, put it in another sealed plastic container,
use the electronic fuse, and set off
In scenario 1, there is no explosion. There is a "whoosh" sound,
a lot of smoke, and that's is. Nice flame, but nothing dramatic
In scenario 2, the film canister gets blown to bits, there is a loud bang,
and a lot of flames flying everywhere
In scenario 3, the film canister and outer container is blasted to smithereens,
there is an extremely loud bang, and there are little or no flames.
Why did this happen?
The experiment shows how important containment is to an explosion. There are
only very few solid state materials that will explode in free air (dispersing
the material is a different matter altogether, see below). When contained, however,
the explosion is "delayed" as pressure builds up enough to blow through
whatever contains it.
The flames in scenario 2 happen because the 20 grams of gunpowder is very
much overkill to blow up the poor photo canister. There is first an explosion,
but whatever of the gunpowder that was not needed to push the lid off the canister
doesn't explode - it burns.
In scenario 3, our loudest explosion, all the gunpowder has to burn to generate
enough pressure to blow through the cannisters.
What can explode?
To keep it short; Everything that can burn can (in theory) explode. Flour
can explode. Wood can explode (if you grind it very finely and disperse it).
Gasoline can explode (when you use gasoline in your car, it explodes (or combusts)
when you start your engine - hence internal combustion engine)
What happens when things get dispersed?
Gunpowder, when thrown into the air, will burn and look pretty (if you like
flames, that is). Other materials (like flour) might explode. This is because
flour can burn, but it can't burn if it doesn't get enough oxygen. If you
try to set fire to a pack of flour, it will not burn very well. if you would
throw the pack across the room, so there is flour dust flying in the air, you
will get a fairly big explosion.
How come planes can make big explosions?
Plane fuel is rather nasty stuff. Not because it burns extremely quickly, but
because there is a horrible lot of it.
- A huge tanker trailer (the type that fills your local gas station)
can hold roughly 10,000 gallons (38 000 Liters) of fuel.
- A Boeing 737 can hold roughly 5,000 gallons (19 000 Liters) of fuel
- A Boeing 747 can hold roughly 50,000 gallons (190 000 liters) of fuel
Needless to say, if a 747 should crash with full tanks (or even with half tanks,
for that matter), it can make quite a bang.
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