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The chemistry of fireworks

Happy Diwali and Bandi Chhor Divas! This year I’m busy writing up my PhD thesis and preparing for my viva in London so I had to celebrate the festival of light in the lab. Luckily, I had a load of the chemicals on hand that fireworks use to produce colours, so we set about bench-top festivities.

As you can see, Lithium Carbonate gave us a nice red flame.

Sodium carbonate gave its characteristic orange flame (which is also why streetlamps are that colour)

Potassium chloride’s beautiful lilac flame came out almost invisible on my phone camera but trust me, it’s ace

Copper (II) sulphate glowed a nice green colour

Barium burns a beautiful apple green. Unfortunately the only salt of it we had was the perchlorate which are a group of elements used extensively in the pyrotechnics industry for their explosive nature. We chickened out.

How it works:

Atoms can be thought of as a nucleus that contains the bulk of the atom’s mass with electrons orbiting it in rings. Electrons are only allowed to exist in rings of certain diameters, which corresponds to orbits of certain energies.  When the atoms of the different metals are heated (either via the explosion of the firework, or in the flame of the Bunsen burner) the electrons in the metal get excited and jump from orbits near the nucleus to orbits further out.

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The electron (orange) jumps to an outer orbit when it’s heated

Once they get to this outer orbit, they realise that they don’t really like it: the atom much prefers it when the inner orbits are full. However, the electron has too much energy to be in the inner ring and so it moves to an outer ring associated with higher energy.  The electron in the orbits closest to the nucleus have the lowest energy because they are attracted more to the nucleus, which is positively charged.

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The electron doesn’t really like being up in this outer orbit

As such, the electron drops back down to the vacant inner orbit. To do this, it gives off the energy it absorbed from the heat by emitting it as light.

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Matt’s sole contribution to this post has been this series of rubbish Microsoft Paint diagrams

The difference in orbit diameter is different for each metal. As the colour of the light given off by the electron depends on the distance it has to jump to get back to the inner orbit, this is why different metals give off different colours when you heat them. By using different metals in different components of the rocket, firework designers can ensure the explosions consist of loads of different colours.

If you are not celebrating Diwali, the reason many people light fireworks and candles on this night is because it is the festival of light.  A brief background to this festival is that it signifies good overcoming evil, knowledge winning over ignorance and light being better than darkness.  Many different religions celebrate the festival of light including Hindus, Sikhs and Jains, and all have different stories to tell.

Wherever you are in the world and whatever you believe, have a happy Diwali and Bandi Chhor Divas – it’s a great excuse to set things alight, be with family and friends, and eat a lot of food.


PJ author photoAbout PJ

PJ has *nearly* completed her Thesis in Chemistry at Imperial College London, and will be starting a postdoc there in November. Her interests include heterodinuclear catalysts for the ring opening coploymerisation of cyclohexene oxide and CO2, salsa dancing, NCIS and pulling out Matt’s eyebrow hairs.

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