The chemistry behind fireworks started over 2000 years ago when, according to legend, the firecracker was invented when a Chinese cook mixed up charcoal, sulphur and saltpeter. He discovered that it would explode if packed into a bamboo tube and set alight. In the 9th century the Chinese invented gunpowder and produced fireworks for important events such as the Moon Festival and New Year using a combination of potassium nitrate (also called saltpeter), sulphur and charcoal.
Each of these chemicals burns in a different way. Charcoal burns slowly, potassium nitrate quickly and sulfur crackles and pops as it burns. Using different proportions of these chemicals produced various kinds of displays. They also invented rockets by placing gunpowder in a roll of paper and igniting it at one end.
Chemistry of fireworks
To start off the reaction energy must be supplied by lighting the fuse. Potassium nitrate acts as an oxidiser by providing oxygen for the charcoal or fuel to burn, sulphur helps to keep the reaction stable. Without the oxidiser the reaction would be too slow, the oxygen provided by the potassium nitrate speeds up the reaction. The three ingredients produce potassium sulphide, carbon dioxide and nitrogen which expand with the heat and provide the propelling force. In addition the reactions are exothermic, that is they produce heat, which contributes to the rate at which the gases expand and increases the explosive power of the reaction.
Fireworks were originally only able to produce yellow or white light which was emitted by heating up the gunpowder mixture. The effect can be varied to produce more glitter by increasing the amount of sulphur or a quick flash by adding more potassium nitrate. When white or yellow light is emitted in this way it is called incandescence. As a substance is heated it glows first with a red light (~480C) through bright red (~730C) to bright orange (~930C) and yellow (~1100C) then white at over 1400C. Until the late 18th century these were all the colours that could be produced in fireworks. Chlorates were produced industrially in the 19th century and allowed reds and greens to be produced in firework displays. It was only in the 20th century that purples and blues could be produced.
How does the oxidiser work?
When heated potassium nitrate releases oxygen and nitrogen but not all of the oxygen is released. Some remains bound to potassium ions.
When chlorates were manufactured industrially they began to be used in fireworks as they are better oxidisers than the nitrates. They release all their oxygen on heating so they are better oxidisers and can produce higher temperatures in the firework which allows more intense colours to be seen and a faster explosion.
However chlorates are fairly unstable so need very special care and today perchlorates are used as they are more stable but, as they also release all their oxygen, are also good oxidisers.
The chemistry of fireworks colors
Today we have fireworks that emit red, blue, green, yellow and lavender light so how is this possible? The answer lies in the way metals emit light as they burn.
Some metals and the colour of light they emit
Sodium yellow
Barium green
Strontium red
Copper blue
Potassium lavender
Caesium violet
Magnesium brilliant white
You may have done flame tests at school to discover the identity of a metal by placing it in a flame and noting the characteristic colour that is emitted.
How do metals emit coloured light?
To find out what happens when we burn a metal we need to know something about the atoms of the metal. All atoms have a nucleus containing protons and neutrons (except hydrogen which is the lightest atom and doesn't have any neutrons). Electrons are in orbitals at various distances from the nucleus. The electrons will always occupy the lowest energy level possible, which are the ones closest to the nucleus.
When the electrons absorb energy, e.g. if they are heated, the energy allows them to jump to a higher energy level further away from the nucleus. Once in the higher energy levels they are unstable and will fall back to a lower energy level. When they do so they emit radiation in the form of light. The wavelength of the radiation emitted depends on the energy difference between the energy levels and is different for different metal atoms and for different energy levels within the same metal, so the colour of the light you see emitted will vary with the metal.
Some metals that burn brightly such as magnesium and titanium are used for both the bright light they emit and to increase the temperature of the burning compounds.
So next time you watch a firework display not only will you marvel at the wonderful colours and sounds but you will know more about how they are produced and the fascinating chemistry of fireworks!
