The Science and Art of Fireworks

Fireworks are an essential part of many celebration such as New Year's, Diwali, Fourth of July and Bonfire Night. These colourful bombs produce beautiful colours and patterns in the sky, a spectacle that can be seen from miles away. However, what most people look past is the carefully engineered combination of chemicals put into each firework, responsible for its colours, sounds and effects. Today we'll be looking at what actually makes a firework!

Photo by Matthew Landers on Unsplash

- The Base - 

Since fireworks explode in a combustion reaction, oxygen is crucial to this reaction. An oxidiser such as a nitrate ((KNO₃), chlorate (KClO₃), or a perchlorate (KClO₄) reacts with it. This oxidiser will gain electrons from the fuel which releases some energy which will start the combustion of the fuel-oxidiser compound made in the process. Once this starts, a massive amount of energy is released as well a gaseous by products such as greenhouse gases. The rapid expansion of the hot gases increases the pressure inside the firework shell leading to an explosion!

Common fuels that can be used in fireworks are charcoal, sulfur and, surprisingly, sugar! The fuel combines with the oxidiser to produce heat and gas which propels the firework into the air. If you calculate the amount of energy needed for a 1kg firework to reach 200m above the ground, only small amounts ~1g of fuel, such as black powder, is needed. However, when you account for drag forces resisting the propulsion of the firework, this amount can increase to around 50-100g, increasing for heavier fireworks.

- Colours - 

So, we've covered the fuel source and the propulsion of the firework, but what produces its bright colours? A range of metallic salts are responsible for this phenomenon.

Learning about transition metals in A-level Chemistry, I thought transition metal compounds may be used in fireworks, but I was surprised to find that a range of group 2 salts are used instead!

As Group 2 metal salts are heated, electrons gain energy and are excited to a higher energy level. When these electrons return to their lower energy state they release energy in the form of light. This will be a specific frequency of light according to amount of energy absorbed and corresponds to a colour. This produces bright, distinct colours.

Some Group 2 metal salts used and colours produced are: strontium salts (red), barium salts (green), sodium salts (yellow) and calcium salts (orange).

The reason why transition metal salts aren't used is because they have various oxidation states so produce unpredictable colours. These colours are often less bright than that required for fireworks and some transition metal salts can be toxic or hazardous. However, copper salts are used in fireworks to produce a blue colour but it needs specific conditions. Titanium and iron powder is used to make glitter and spark effects.

- Effects -

One of the most distinct features of a firework is its signature whistling sound. Even this effect is created by chemical compounds!

Narrow tubes are filled with organic chemicals such as potassium benzoate or salicylate. The rapid release of gases during combustion causes these compounds to vibrate creating a whistling sound which is amplified by the tube.

Glitter effects are created by small amount of metal powders e.g. antimony or bismuth which combust and leave trails that are glittering in appearance.

You can even produce shape patterns e.g. stars by arranging small pellets of combustible material (made from fuel, oxidisers, metal salts etc) inside the firework shell. Careful timing and ignition, as well as precise arrangement can make simple and complex shapes. A second explosive in the centre of the firework ignites and propels the pellets out. They must also ignite at the same time burn at the same rate to retain their shape.

Fireworks are a truly amazing example of how engineering, physics are chemistry intertwine in the production of common items. I hope this outline gave you a good insight into the science behind fireworks, but I urge you to do some of your own research into the vast chemical processes behind each component of a firework. Have a good day, and God bless you!