Festive Series: Fireworks

What is more magical than the screech, explosion, and colours of a firework? Standing outside on a cold New Year’s Eve counting down to midnight, waiting for the firework display. Let’s discover the science that makes them.

Fireworks have been a part of celebrations since ancient times. They were first used by the Chinese, who are believed to have made explosive rockets in the 6th century. However, the custom of using fireworks for celebrations didn’t become popular in Europe until the 17th century. In the 20th century, American scientists swapped solid fuel in fireworks for a liquid fuel system which lead to modern space rocket technology, aiding with the Moon landing in 1969.

Each type of firework functions slightly differently but generally, aerial fireworks have five sections: the stick, tail fuse, charge, effect and head. Fireworks are usually varied, otherwise watching the same one for half an hour would be a tad boring. Essentially all fireworks combine the power of a missile with the excitement of colourful, burning metal compounds. Rockets produce the high, spectacular displays whilst Catherine wheels work closer to the ground, with several small fireworks attached to the edge of a wooden wheel which spins as they fire off. Another common type is the firecracker, which are designed to produce sound rather than light and are usually incorporated into the rockets.

What about the science? Fireworks can teach us about both chemistry and physics. The explosion of a firework is a lot of chemical reactions happening at once. For this to happen you need to light the firework. Ignition provides enough energy for solid chemical compounds in the firework to overcome the activation energy (i.e. the energy needed for a reaction to happen), combusting and reacting with oxygen. Smoke and gases (such as CO2 and N2) are released during the reaction.

fireworkactivation

The magic chemistry however, is how fireworks produce different colours. The various colours come from different metal compounds packed within the firework; if metals are burnt in a hot flame, bright colours are produced. Sodium produces yellow and orange colours, copper and barium salts give green or blue, and calcium or strontium make red when they burn.

Na Sodium.jpg

Cu Copper.jpg

Ca Calcium.jpg

Images from Wonk! magazine

Firework Colours 2015

Image from Compound Interest

How about physics? Chemical energy inside the compounds of a firework is converted into four other kinds of energy: heat, light, sound, and kinetic energy (i.e. movement). One law of physics determines that the total energy in a firework before ignition must be the same as the total remaining after it has exploded, plus the energy released as light, heat, sound, and movement. This is the first law of thermodynamics: energy is not created or destroyed, simply converted to another form.

Physics can also tell us why fireworks shoot into the air. The charge component of a firework is no more than a missile. When the powder in the charge section burns, hot exhaust gases are produced, firing backwards. The backward force of these exhaust gases is similar to the blast of a space rocket at take-off. These forces create an equal and opposite reaction force, sending the firework shooting into the air. This is also known as Newton’s third law of motion: for every action, there is an equal and opposite reaction.

fireworkenergy

What about the symmetry of explosions? You never see a lopsided firework, the explosion is always perfectly symmetrical, but why? This is another basic law of physics: the conservation of momentum. The momentum of a firework (i.e. the amount of stuff moving in each direction) must be the same before and after an explosion. Therefore, the explosions to the left must be the same as the explosions to the right.

Most people will imagine bonfire night, New Year’s Eve or weddings when they think about fireworks. However, the same technology is also used for more practical situations. Flares can be used by the military or as distress signals at sea. These work in almost the same way, though instead of metal compounds such as sodium, they use even brighter compounds such as magnesium, and are designed to burn for much longer.

This New Year’s Eve, when you spot a firework or attend a display, remember all the science you have to thank for such an entertaining evening! Finally, thank you for all your support in 2019, I can’t wait to share all the science of 2020 with you. Happy New Year!

2019

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