Explosions

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 in explosions.

When studying pyrotechnics, the following experiment is common:

Ingredients:

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

Results:

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.

For comparison:

  • 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.


Back to the node on pyrotechnics

Ex*plo"sion (?), n. [L. explosio a driving off by clapping: cf. F. explosion explosion. See Explode.]

1.

The act of exploding; detonation; a chemical action which causes the sudden formation of a great volume of expanded gas; as, the explosion of gunpowder, of fire damp,etc.

2.

A bursting with violence and loud noise, because of internal pressure; as, the explosion of a gun, a bomb, a steam boiler, etc.

3.

A violent outburst of feeling, manifested by excited language, action, etc.; as, an explosion of wrath.

A formidable explosion of high-church fanaticism. Macaulay.

 

© Webster 1913.

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