Most people appreciate the beauty of fireworks without knowing the science behind the wonder.
Fireworks are dangerous. It’s something we in the UK get drilled into from a young age as soon as Bonfire Night gets close, and with good reason. They are, put simply, explosives we are allowed to buy and light and cheer about. We actively encourage children to watch with delight as an adult systematically sets off various explosives. But just what makes them so different from the sort of explosives we wouldn’t be allowed to just buy? The answer is obvious: colour. So how are fireworks very colourful?
This all comes down to salts. Obviously not all the basic table salt you have on your chips (sodium chloride, by the way) because otherwise every single firework would just be orange. Instead, fireworks contain a mixture of different metal salt compounds which all burn in different colours. In school you probably, at some point, did an experiment which involved shoving sticks dipped in powder into a lit Bunsen burner before writing down the colour of the flame. Fireworks are literally that packed inside an explosive.
The ‘traditional’ firework was made of saltpetre, which mostly contains potassium nitrate but also has some potassium sulphate in there too. When lit, it burns with a purple flame. That’s why when you light a firework and it whizzes up into the sky, all of the sparks are purple – little glowing balls of flaming salt dissipating into the night.
Nowadays there’s many other compounds in fireworks to provide variation and awe at displays. I had to remember the salt flame colours for chemistry when I was in school many years ago, and I can still remember them now. Perhaps that’s why I’m not invited to firework parties. People don’t necessarily want someone stood next to them going “that one’s probably got barium in it!” My chatting about barium may be a cause of boredom for others. So I’m writing a blog post about it. Lucky you, dear reader.
The general colours seen in fireworks include crimson (lithium and strontium based salts), yellow (calcium based salts), green (barium and copper based salts). The actual definition of colour seen obviously depends upon the exact mixture of compounds and their purity/the quality of the fireworks.
But why do different salts have different colours when put inside a flame? Well, it is all down to the chemical composition of the metal element. The outside of all atoms is made up of electrons. When heated up these get excited and so transfer to a higher energy level. When they get cooled back to their original, standard state (in the air) the electrons fall back down to the previous energy level and a colour is emitted. The different metals all have different coloured wavelengths as part of their transitions. This is why we see them as that colour.
That sounds a bit technical and may not be very easy to imagine. Think of it like this: picture yourself sat on the ground floor of a house, happily doing your own thing. Then, you are informed that something very exciting has occurred upstairs that you just have to experience. It may be that someone has fallen over comically, it may be that there is a funny video playing that somebody wants you to see. Whatever. You go upstairs to see it. When that’s done, you run back down again to get back to what you were doing. As you go back downstairs you’ll make a noise. If you’re careful and measured in your steps it may be a very small noise. If you’re like me, other people in the house will inform you that you galumph downstairs like a herd of elephants. The point is, different people will all have different noise levels and sounds as they head back down the stairs from such an event. This varying scale of noises is a bit like the spectrum of colours which different metals display when they’re recovering from excitement.
Naturally, a firework isn’t just a load of salt shoved inside a missile (though some dangerous homemade ones are). Fireworks companies have lots of extra features and safety precautions designed to give consumers the safest possible experience. That’s why it’s better to buy quality fireworks rather than attempting to botch together your own. No matter how pretty they look lighting up a night sky, fireworks are explosives and can have devastating consequences.
So that’s a very brief summary of the basic chemistry behind the wonderful colours we see in fireworks. Stay safe when watching a display, and if you’re feeling knowledgeable, impress others by trying to name which salts could be in the rockets you see whizzing into the sky.