Why do candles only smoke when they’re put out?



Why do candles only smoke when they’re put out?

In: Chemistry

Smoke is the result of incomplete combustion. Because there is not enough heat to cause the wax to burn cleanly, you wind up with smoke

The smoke is little bits of candle that started to burn but didn’t burn completely. When the candle is lit it’s hot enough to completely burn the wax. When you put it out the temperature goes down but not enough to completely stop it from burning so it continues to poorly burn the wax and give off smoke until it is cool enough that no burning at all happens.

In addition what others have said, you can demonstrate that that smoke is leftover flammable material. If you’re fast enough, you can relight the candle by touching a flame to that smoke trail. https://m.youtube.com/watch?v=0Zrw_4v1xes

The flame vaporizes the melting wax and the mixture of vaporized hydrocarbons and oxygen from the air is what causes the flame. When you put out the flame, the residual heat continues to vaporize the wax for a short while and these particles are carried up into the air by the heat. It is basically unburned fuel. You can even reignite it above the wick and if that smoke column is cohesive and dense enough, the flame will burn down to the wick and reignite the wick and restart the melting and vaporizing of the wax fuel.

(Solid) candle wax doesn’t burn. It has to be torn up into little molecules – then, if it’s hot enough, it will react with oxygen in the air to burn.

A candle flame is a tiny cloud of vaporized candle wax that’s super hot (that’s why it glows). When the hot molecules reach the edge of the cloud they hit oxygen and burn. This reaction heats up the cloud more and keeps the flame burning.

When a candle goes out, that little cloud gets too cold and no longer burns when it touches oxygen. You can see the vaporized wax drifting away as ‘smoke’.

This is why you need a wick, to start burning and melt the wax which is then drawn up and vaporized. The wick is protected from burning because the wax cloud keeps oxygen away from it. In the old days candlewicks would need to be trimmed every now and then, as they’d get super long and start producing too much wax vapor to burn cleanly. Candles would be super smoky after a while unless you cut the wick down. Modern candles have flattened wicks that actually curve to the side as they come free from the wax so that they poke out of the flame and burn down to a consistent length. Pretty cool technology! There are lots more cool candle facts but I’ll stop there.

If you’re using a jar candle, it won’t smoke if you dip the wick into the wax to put it out. There’s an L shaped tool for just that purpose and you can straighten up the wick after it’s out so it’s ready to be lit the next time.

Fire doesn’t burn solid material. The initial spark vaporizes the solid material which then ignites. You blow out the candle, and the smoke is vapor created by leftover heat or that which wasn’t burned.

A fire is a process that feeds itself. The flame produces heat, which heats up the candle wax under it, which then evaporates into a gas and burns to create the flame.
When you stop the flame, the wax is still hot and still producing gases. But the fire is not there to burn it anymore.

Because smoke is flammable.

The the smoke coming from the wick is also catching fire as it’s not fully oxidized, so it’s burning further in the fire coming from the wick.

Smoke occurs when there isn’t enough oxygen to burn the fuel completely. Further away from the wick, more oxygen become available and the smoke continues to burn.