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Metal chlorides add colour to fireworks - January 2006
Chemistry of a celebrationHow does chemistry produce beautiful bursts of stars and cascading streams of colour in fireworks? Pyrotechnics - the science and art of making fireworks – uses techniques that have been fundamentally unchanged for many years. Black powder (a mixture of potassium nitrate, charcoal and sulphur) was used for fireworks before it was adapted as gunpowder. Eventually, the chlorine-based compound, potassium chlorate (KClO3), took the place of potassium nitrate as the oxidiser. Today, this has been replaced by a more stable oxidiser – also based on chlorine - potassium perchlorate (KClO4). Not only does chlorine chemistry provide the chief oxidiser used in modern fireworks, but metal chlorides – of barium, strontium and copper - are responsible for the dazzling colours that flash against the night sky. Chlorides offer greens, reds or bluesBefore the 1800s, fireworks consisted of loud explosions and a few sparkles, made by bits of metals such as iron, copper or zinc. Colours were mostly limited to shades of yellow and orange. Fireworks became spectacularly colourful when certain new laboratory-made colour-producing compounds were added to the chemical mixture. Today, most colours are produced by metal chlorides. The table below is a "menu" of chloride colour possibilities:
The chloride compounds shown in the table have one disadvantage for firework manufacturers and users: as hygroscopic materials, they absorb moisture easily, becoming less effective as colour-producers. A recent advance in firework science solves this problem. Instead of producing colour from the glow of very hot solid particles of metal chlorides, the new technique brings metal and chlorine together in a vapour during the burning process, providing energy to excite the molecules’ electrons. This produces colourful results. Excitement can be colourful
Reference: Wilson, E. (2 July 2001). "What's that stuff? Fireworks." Science and Technology, Vol. 79, No. 27 Click for details
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Celebrating important events with fireworks is a very old tradition believed to have started in China more than a thousand years ago. Gunpowder was probably first used in Europe in the early 1200s. Today, fireworks are popular on a variety of dates, from New Year’s Eve in virtually all European countries, to national celebrations such as Guy Fawkes’ night on 5th November in England. 
Electrons within atoms – the tiny, negatively charged particles that move around an atom’s nucleus – become excited when they absorb energy. And when they release that energy, they may emit flashes of colourful light. The first step is for an electron to move between various energy levels in the atom - from the lowest, most stable level, to a higher, excited, but unstable level – when it absorbs energy (from the chemical reactions in fireworks, for example). The electron soon falls back from this unstable level to the lower energy level, releasing energy as a burst of colored light.