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The Science of Fireworks

 

In today’s health-and-safety conscious society, it’s hard to believe we are actually allowed to fire explosives in the air! As well as dazzling us with their colourful and magical aerial displays, fireworks are really interesting from a scientific point of view – both physics and chemistry!

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What is a firework?

A firework is a missile containing combustible chemicals, designed to explode in a controlled manner with loud bangs and bursts of brightly coloured light. The word actually evolved from the Greek ‘pyrotechnics’ which means ‘fire art’ or ‘fire skill’. They were first used to ward off evil spirits and then later for celebrations.

 

Anatomy of a firework

There are lots of different types of firework which all work in different ways. So, let’s look at the simplest kind – a rocket. A rocket has five main parts to it, working from the bottom to the top:

  1. Tail – a long wooden or plastic stick protrudes from the bottom of the firework to ensure it shoots in a straight line. This is important for two reasons: to help prevent injuries or damage occurring from fireworks flying in random directions, and also to help organise the actual display with some level of accuracy
  2. Fuse – this is the part that starts the charge burning and ignites other smaller fuses which in turn, light up other parts which explode later. Although you can light this with a match, for more complex displays, they are lit by electrical contacts when a button is pressed. This can be done at a distance and is much safer!
  3. Charge – this is the main explosive usually made of gunpowder – traditionally 75% potassium nitrate + 15% charcoal + 10% sulphur – modern fireworks sometimes use different mixtures of chemicals instead. When it is ignited, the nitrate oxidises the sulphur and charcoal which results in hot gases. This charge can blast a firework into the sky sometimes for a distance of several hundred metres at a speed of up to several hundred kilometres per hour – as fast as a jet-fighter! Interestingly, the charge only shoots the firework into the sky, it doesn’t actually make the explosions that you see!
  4. Effect – now this is the part that makes the spectacular display you watch once the firework is safely high in the sky. A firework can have multiple effects packed into separate compartments which fire off in sequence as they are slowly ignited by a time-delay fuse. Although these are explosives, each one is more loosely packed and made into separate ‘stars’ which make up the little colourful explosions that are all part of the main one. How each effect is packed will influence how the firework explodes – they can range from the size of a pea to a tennis ball!
  5. Head – this is the top part of the firework that contains the payload of effects. Sometimes it might have a pointed end to make it go faster.

 

The chemistry of a firework

An exploding firework is simply a number of chemical reactions happening at the same time or in a rapid sequence. Heat provides enough activation energy to make the solid chemical compounds burn with oxygen in the air and release smoke and exhaust gases.

The different colours from fireworks come from the burning of different metal compounds causing chemical reactions on the sky: yellow gold/orange colours come from sodium compounds, green/blue from copper/barium and red from calcium!

 

The physics of a firework

Energy locked inside the firework is converted into heat, light, sound and kinetic energy.  As it burns, a firework gives off hot exhaust gases that fire backwards like the blast out of a rocket engine which creates an equal and opposite reaction that shoots the firework up, like a rocket! Interestingly, fireworks always make symmetrical explosions due to the conservation of momentum.

Fireworks may be entertaining but the same technology has more practical uses for military forces and ships which still carry flares as a backup method of signalling that they need help.

 

Remember – fireworks are explosive missiles and therefore extremely dangerous – let a responsible adult handle them and stay safe!