The science of fireworks

Published on the 21st December 2018 by ANSTO Staff

Whether you’ll be watching fireworks along Sydney Harbour or somewhere else on New Years Eve, you won’t be the only one relaxing this holiday.

That’s because fireworks get their colour from a process in which metal salts are heated, then relax. And some of these salts are more particular than others, explained Associate Professor Michael Pollastri, Chair of chemistry at Northeastern University in the US.

Can you explain the chemical reaction that makes fireworks explode?

An explosion is more or less a very fast and intense burning event. In order to accomplish that, metal salts are mixed with chemicals (oxidizing agents) that cause a very rapid oxidation reaction to occur. This reaction is very fast and exothermic, which means it gives off energy as heat—and anytime you have a very fast and hot reaction, you get an explosion. This launches the fireworks into the sky and the heat from this explosion is what provides the energy to create the colours.

Why don’t they just explode on the ground where they’re lit? What propels them into the air?

Fireworks are like little rockets. They are indeed exploding, but in a controlled way that directs the explosion in a specific direction—down—so the firework shells go up.

What determines the colour of the firework?

The colour is determined by the metal salts that are present in it. The heat that metal salts experience excites the metal atoms to a higher energy state, and when the atoms relax back to their more stable “ground” state, they emit colours. The wavelength (or colour) of light that’s emitted when these atoms relax are characteristic of specific atoms: strontium glows red, sodium burns orange, copper burns green, etc. Other colours can be made by mixing these metal salts in the fireworks, which is called “painting” in the fireworks trade.

Are certain colours easier or harder to create?

Blue fireworks are particularly difficult to create, because the copper salt needs a very precise temperature to be excited to the energy state that emits blue light. If it burns too hot or too cool, the colour gets washed out to a lighter blue hue.

Thanks to Molly Callahan and Northeastern University for sharing the content.

New colours for 2019 Sydney Harbour fireworks

In early December, the City of Sydney announced that two new summer colours, lime and peach, would join the fireworks palette. Foti Fireworks will be using a new technology that delivers pastel colours.