Where does the wax from a burning candle go? Also, why do beeswax candles burn virtually completely, leaving no wax behind at all? — SC, Rhode Island
The wax molecules in the candle react with oxygen in the candle flame and are converted into water molecules and carbon dioxide molecules. That reaction is associated with combustion and it releases energy so that the candle produces light and heat. The molecules formed by this combustion drift off into the air.
Normal candle wax (paraffin wax) consists of relatively large hydrocarbon molecules. Each molecule in paraffin is a chain of between 30 and 50 carbon atoms that are surrounded by hydrogen atoms. Because its molecules are fairly long and they stick together reasonably well, paraffin is a firm, crystalline solid. If the chains were shorter, say 20 to 30 carbon atoms long, the material would be softer—it would be a liquid-like wax known as petroleum jelly. If the chains were much longer, say 2000 to 3000 carbon atoms long, the material would be firmer—it would be a solid known as polyethylene. Still shorter chains are used in machine oil, diesel fuel, unrefined gasoline, and finally petroleum gases such as propane and methane. The shorter the chain, the softer, thinner, and more volatile the hydrocarbon is at any given temperature. All of these hydrocarbon molecules can burn completely, leaving only water molecules and carbon dioxide. In a candle, the heat of the flame vaporizes the wax molecules—they become a gas—and they then burn completely in the flame itself. As long as the wax doesn’t drip away from the flame, the flame will consume it all completely and leave no ash or wax. Although the structure of the molecules in beeswax is slightly different from that in paraffin, beeswax also vaporizes from the heat of the flame and then burns completely.