Asked by: Jack Hemmings, by email
Smoke is unburned particles of carbon released when the hydrocarbon chain of candle wax breaks down. When the candle is alight, most of the carbon gets burned to carbon dioxide, but some escapes. If you hold a plate above a candle flame, you’ll see the carbon accumulate as a sooty smear.
When the flame goes out, the glowing wick has enough heat left to break up the wax molecules for a while, but not enough to burn the carbon, so you get a trail of smoke until it cools.