It is time to watch basic chemistry in action again with the sights and sounds of Diwali. The Chinese first developed pyrotechnics, the science — and art — of fireworks, more than a thousand years ago when they invented gunpowder. But it was not until the Middle Ages — when it reached Europe — that black powder (as gunpowder is also known) became popular for creating spectacular displays to mark important occasions.
The basic formula for making black powder — blending potassium nitrate, charcoal and sulphur in the ratio of 75:15:10 — has remained unchanged through centuries. When you ignite this mixture, exothermic (energy-releasing) reactions discharge energy in three forms: heat, light and sound. The charcoal and sulphur act as ‘fuels’, burning by drawing oxygen from potassium nitrate.
Heat splits the chemical bonds in a pyrotechnic mixture and allows ‘fuel’ atoms to combine with oxygen atoms of the oxidiser. The resulting compounds are more stable, and need less energy to maintain their chemical structure. This excess energy is released as heat, which we see as light, and hear as a loud boom (as the expanding gases collide with the surrounding air.)
Additives in the basic black powder mixture, packed in papier-mâché shells, determine the colour of a firework. An aluminum mixture containing barium, for instance, generates brilliant white sparks, while compounds with strontium emit fluorescent red dazzlers, just as lithium compounds produce purple fireworks.
Some shells have several compartments, each with its own bursting charge and ‘stars’ (flash-and-sound powder) separated by nothing more exotic than cardboard. Each exploding compartment ignites a time-delay fuse that leads to the next segment. The result: a shell that produces multiple bursts. Some combinations do not produce much heat and light, just lots of gas. When pressed into a narrow tube, they create whistling sounds as the shells rise into the sky. These days, computer programmes synchronise the firing of thousands of fireworks, each fitted with a metal ‘match head’ that electrically lights the fuse to send it skywards.
Nevertheless, the appeal of fireworks for most of us is probably more instinctive than intellectual. So, during Diwali, no one really says, ‘What a fine exothermic oxidation-reduction reaction!’ Or, ‘Was that strontium hydroxide or strontium chloride?’ More likely, they will be reaching out for the next box of patakas.