Wednesday, August 31, 2011

31/8/11 - Fireworks

Midway through our trip to France was a French bank holiday, this was celebrated by one of the best firework displays I have ever seen (and set to The Lion King music of all things...)

I was hoping to find a video of it but I can't see any lurking on t'interweb so you'll just have to take my word for it, it was awesome!

Anyway, while everyone else was "oooh"ing and "aaah"ing (this seems to be universal between English, French, Dutch and whoever else was on the beach too) I couldn't help but let my mind wander into the chemistry of fireworks. It struck me how little I really know about how they actually work so I decided some research was in order once I was back home and online!

I already knew that the different were colours were down to which metal salts where used (although I wouldn't have been able to tell you which does which!). Didn't need the internet for that bit as we've covered it numerous times before thanks to flame tests:
Red: Strontium and Lithium
Orange: Calcium
Yellow: Sodium
Green: Barium
Blue: Copper
Purple: A combination of strontium and copper)
Silver: Magnesium / Aluminium / Titanium

This is useful to know, but what I was really interested in was finding out the mechanics of it - so how they get the different patterns, what causes the delayed explosions in some of them, etc?

The answers turned out to be a lot more simple that I was expecting - the short answer is that some very clever pyrotechnic experts design the composition of a firework so that it can do whatever they want!

The first thing that must happen is universal amongst fireworks and is relatively simple - a form of gunpowder is ignited by a fuse, the highly exothermic reaction that follows provides the power to send the firework into the air, this is achieved by trapping the gas produced and allowing sufficient pressure to build up.

A second fuse is lit at the same time as the first, however this one burns much more slowly and will only ignite the packed chemicals (giving the pretty colours and patterns) some time after the firework is in the air. This has to be precisely timed and will occur when the firework is at its peak height, as well as causing a colourful explosion other chemicals will be present to give the big bang.

The explosions are as a result of a variety of oxidation and reduction reactions, nitrates being the most commonly used oxidising agents, as well as perchlorates.

I am now going to have to resist the urge to see how easy getting hold of some of these chemicals might be...home made fireworks anyone?

Saw this on youtube, not real but thought it showed the likely outcome if I started playing!

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