Chemistry behind Fireworks

Fireworks come in all sorts of shapes, colours and sounds, and are used to celebrate different occasions all over the world. Have you ever thought about how they work? Well today, this article will explain the science behind how they work.

Historians believe that fireworks originated in ancient China in the second century B.C. It is thought that these natural firecrackers were bamboo stalks that, when thrown in a fire, would explode with a bang because the hollow pockets in the bamboo were overheated. This became a tradition as it was said to ward off evil spirits and is still done today at Chinese New Year.

(https://www.historyhit.com/the-history-of-fireworksfrom-ancient-china-to-the-present-day/ )

More and more people have experimented with fireworks and they have evolved to use all types of ingredients and all sorts of styles. Infact, there are 19 different types, all with their own colours. However, with evolution comes danger, and there are now very large and dangerous fireworks; there are four categories of fireworks which are ranked in order of ‘hazard types’: HT1, HT2, HT3 and HT4, with HT1 being the most dangerous. HT1 and HT2 are not sold in common stores.

Concerning the make of a basic firework, gunpowder is needed. There are three reagents which make up gunpowder potassium nitrate, carbon and sulphur. The most important part of the gunpowder is the potassium nitrate as this is what propels the firework into the sky. A fuse is used to light the gunpowder, igniting the firework and thrusting it into the sky.

Other components are required for the firework to shoot into the sky: fuel, an oxidiser, and a binder (not to mention the plentiful supply of oxygen in the atmosphere). The fuel stores energy as it’s a source of electrons and essentially burns up during the explosion ( charcoal is typically used as fuel). Next a chemical reaction (usually combustion) takes place between the fuel and the oxidiser (an oxidising agent oxidises another substance by gaining electrons from the other substance and it itself is reduced). Upon this reaction, the electrons are being transferred between the two substances - creating a lot of stored potential energy which is then ready to be released. The binder is a substance that holds these components in place to ensure the explosion does not go off unexpectedly as well as reducing the sensitivity to both shock and impact. The binder is typically used to engineer the timing of the explosion to coordinate with others in a fireworks display.

While in the sky, a combustion reaction takes place between the reactants and a detonation explosion occurs. As they react, the products formed are solid potassium carbonate, solid potassium sulphate, nitrogen gas, and carbon dioxide gas. Finally, the explosion spreads out all of the material, all while being under a superheated state.

Now, if this type of firework were to be sent into the sky, there would be a loud ‘bang’, however there wouldn't be any light or colour. The components responsible for creating a dazzling spectacle of light and colour are metal compounds - particularly metal salts.

Metal salts within the metal body of the firework are often coated with gunpowder to aid with the ignition. As the reaction creates heat, the electrons in the metal become so excited that they travel back and forth between higher energy levels (shells) and their original energy levels (shells). The excess energy emitted when the electrons fall back down to the lower energy level is visible light energy. Different metals will have larger or smaller gaps between their ‘excited’ energy level states and original energy level states, which causes emissions of different colours (this is also what happens during flame tests on different metals).

(https://www.compoundchem.com/2013/12/30/t he-chemistry-of-fireworks/amp/ )

The metal compounds are put into ‘stars’; the pattern shown in the sky depends on how the ‘stars’ are arranged. For example, if the body of the firework is made up into sections, the stars can be put into different compartments. These compartments explode at different times making different patterns. The pattern of stars around the central gunpowder also creates different patterns of fireworks. For example, if the stars are in a circle around the gunpowder, a circle display of colour is shown in the sky. These are placed with a lot of precision!

Although metals are in salt forms, due to the easier dispersion and typically more stable states, there are some colours which are difficult to produce due to the unstable nature of the metal. For example, copper salts at high temperatures tend to be unstable. If such high temperatures are reached, it breaks apart and the blue colour is not exhibited. Purple colours are also tricky to produce as red producing compounds (strontium) and blue producing compounds (copper) are required in combination.

There are other limitations which those preparing fireworks have to be careful of. Firstly, the reactants cannot collect moisture in the air, as otherwise it won't burn properly, so they must be stored dry. Secondly, the fireworks should not expel any toxic substances into the atmosphere. This means all compounds must be benign (safe to use). For example, blue colouring previously used in fireworks many years ago contained arsenic, which is lethal to humans when ingested.

As shown above, there are many complicated (and quite dangerous) events which take place during a firework show. When you go to your next firework display, it’ll be like watching it for the first time, now that you’ll be able to appreciate the work and science behind it! This overview has hopefully presented an interesting insight into the science behind fireworks.

Sources: https://penntoday.upenn.edu/news/chemistrybehind-fireworks