The colours in fireworks stem from a wide variety of metal compounds – particularly metal salts. ‘Salt’ as a word conjures up images of the normal table salt you probably use every day; whilst this is one type of salt (sodium chloride), in chemistry ‘salt’ refers to any compound that contains metal and non-metal atoms ionically bonded together. So, how do these compounds give a huge range of colours, and what else is needed to produce fireworks?
Silver tarnishes in air to form a black coating of silver sulphide, which has the formula Ag2S. The chemical equation for the formation of silver tarnish following reaction with hydrogen sulphide (H2S) is as follows:
2 Ag(s) + H2S(g) → Ag2S(s) + H2(g)
There’s speculation that, in the present day, tarnishing of silver actually occurs more quickly due to the increased amounts of hydrogen sulphide being released into the atmosphere by the combustion of fossil fuels and the like. This silver sulphide tarnish can be quite easily removed using polishes that contain abrasives to rub off the silver sulphide, but these have the disadvantage of also removing small amounts of silver. Handily, chemistry provides a method which sidesteps this problem.
