Chemistry with a Bang: The Science of Fireworks

A colourful firework display! Source: http://londontheinside.com/2015/10/16/east-village-fireworks/

Loud bangs and pretty colours; that’s what fireworks are known for. I imagine we have all been to a huge and impressive firework display at some point, and during a significantly smaller one that my family had at the end of Halloween, I realised I didn’t actually know how these miniature rockets worked. How do they achieve the patterns and colours they are known for?

The basic structure of a firework. Source: http://www.explainthatstuff.com/howfireworkswork.html

Well it turns out they are an excellent example of the everyday application of the physical sciences, with some very interesting Chemistry dictating both the bangs and the colours. But before we get into that, lets take a look at the structure of a firework and see how everything is bound together.

  1. This is the Stick, or “tail” if you prefer, which consists of wood or plastic. This long stick ensures that the firework shoots in a straight line, and doesn’t just fire off in any direction. This not only helps prevent a firework to the face, but also allows for display organisers to position the firework effects with precision, allowing for a well coordinated display.
  2. This is where the fuse is located, which consists of a small bit of paper or fabric that can be lit a flame or by electrical charge. This starts the fuel of the firework burning and can set off other, smaller fuses that can make the effects explode later than the main firework.
  3. This is the Charge, which is a fairly crude explosive, designed to shoot the firework upward. These charges can sometimes reach heights of several hundred meters (roughly 1000 ft) and can achieve speeds of several hundred km or miles per hour. This is also where the fuel is located, and is often made up of tightly packed gunpowder, which we will discuss the composition of later.
  4. This is the fun part where the Effects are contained. The compounds stored in here are what actually produce the display once the firework is in the air. There can be one or multiple effects, usually packed into separate compartments, that can fire off in a predetermined sequence or all at once.
  5. This component is not particularly special. It is often referred to as the “Head” of the firework, and can be aerodynamically designed to improve the flight on more expensive models, but is often just a flat cap on cheaper ones. This part is designed to contain the effects.

Now we get to the hardcore science! All fireworks contain the same basic chemical ingredients: an oxidant, fuel, a colour producer, and a binder. The binder is the simplest of the ingredients, used to hold the everything together in a paste-like mixture. The most common type of binder is known as dextrin, which is a type of starch. The other components are a bit more complex.

The oxidant is required in order for the mixture to burn. Common examples are Potassium Nitrate (K2NO3) or Potassium Perchlorate (KClO4), both of which decompose when heated, and yield Oxygen. This Oxygen then allows for the effective combustion of the fuel.

The fuel can consist of many chemicals, such as Carbon or Sulfur containing compounds, as well as organic based material like poly(vinyl chloride) (PVC). They can also contain powdered Aluminium (Al) or Magnesium (Mg) to help the mixture reach the high temperatures necessary to cause rapid combustion. The most common fuel used today is gunpowder, sometimes called black powder, which consists of a mixture of charcoal, Sulfur, and K2NO3. This allows it to act as both a fuel and an oxidant.

The special effects, such as the bright colours, are provided by additives to the mixture. These additives are often metal compounds known as metal salts, using elements from Group 2 of the periodic table. An example of this is the Barium (Ba) salt Barium Carbonate (BaCO3), which is added to produce green flames when the firework goes off. The species actually responsible for the colour in this case is the gaseous BaCl+, which is produced when Barium ions (Ba2+) combine with Chloride ions (Cl). The Barium ions are produced when the BaCO3 salt decomposes, and the Chloride ions can come from the decomposition of KClO4 oxidant or PVC fuel, depending on which is used.

When the firework explodes, the newly formed BaClgas is extremely hot, and contains a great deal of kinetic energy. This means that the many atoms in the explosion will frequently collide with each other, and the kinetic energy will be transferred from one atom to another. The energy from both these collisions and the heat of the explosion can then be absorbed by the electrons surrounding the atom, and they become “excited” into a higher energy state. Now these excited states are unstable, so the electrons will naturally return the lowest energy state available, known as the “ground state”. The energy that was absorbed is then released in the form of light, with the colour depending on the amount of energy released. In this case, you would see a bright green flame emitted by the BaClgas. The specific range of colours emitted in this process is called the “emission spectrum”, and each element / molecule will have a unique spectrum as the structure will determine which energy states are available for the electrons to occupy.

There are many other metal salts that can be used, each yielding a different colour. Strontium salts can give of red light, whereas Copper compounds produce a nice blue. But pure colours require pure ingredients, and even trace amounts of other metal compounds are sufficient to alter or completely overpower other colours. The skill of the manufacturer, as well as the age of the firework, will therefore greatly influence the final display.

So next time you’re at a public firework display, you can shamelessly point at the sky and scream “SCIENCE!”, or proudly describe the Chemistry to the person next to you. Although I highly doubt either option will make you very popular.

Sources:

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