Seems like they are not producing any smoke while burning. (At least not noticeable)
A candle burns with enough heat that the wax gets completely combusted into CO2 and other gas-only products. Wax-vapour that hasn’t combusted is still hot enough to spread out by natural convection.
When we blow out a candle, the temperature drops. Wax still on the candle can no longer combust. The smoke we see is not a gas, but rather tiny droplets of solid / liquid wax, much like water-droplets in a cloud.
Once we blow out a candle, any wax-vapour that remains in the air has no option but to stay there – as smoke.
This is also why it’s a good idea to use a candle snuffer to extinguish a burning candle. Proper use can help prevent wax particles from flying around and landing on other surfaces.
Ask yourself: what happens when a candle burns?
A burning candle is hot at the top with the melted wax. At the wick, the wax is “boiled”. This releases flammable gas.
The flame of the candle is even hotter. Here the temperature is high enough to burn the gas. When the gas burns, it releases heat. This heat keeps the candle going.
- It heats the top of the candle, melting the wax and boiling it at the wick.
- It keeps the flame hot enough to burn the gases.
- It keeps the air flowing. Hot air rises. The heat in the burned gas rises upwards, and draws fresh air in. The fresh air contains oxygen and is used in the burning of the gas.
The flame is supporting itself, drawing in fuel, expelling used fuel and air and drawing in new air. Without any movable parts. Neat.
Now you blow it out. The flame is not hot enough to burn the gas. The flame is gone, but the air is still rising above the candle, drawing new air in. And the wick is still hot enough to boil new gas.
The unburned gas that is boiled from the wick does not meet a hot flame, it meets cool air. It is not hot enough to stay a gas, and rains out as small drops. These drops look white to us.
Soot and the visible flame.
Why do we see a flame?
When stuff gets hot, it glows. The flame of a candle is hot enough to make stuff glow. But you need something solid to really get somewhere. Certain flames are (barely) visible, since there is no solid stuff in them.
The answer is soot. When gas burns, it is not as simple as gas + oxygen + heat = CO2.
The molecules in the gas changes gradually, and in a lot of different ways. Some of these molecules may turn into solid particles, usually those with some carbon in them. These particles look black, collide with other similar particles, and grow.
When the soot particles are heated up they glow, and voila, we can see the flame!
If the flame is hot enough (with enough oxygen and mixing) for a long enough time, the soot will burn. There will be released no soot, and all is well.
Watch the tip of a candle. There is a black color there, right? That black tip comes from soot that is not burned off. This soot will not burn and will stay in the room. Burning away all the soot is hard to do.
If you burn pure hydrogen, you do not really get any soot. Soot contains carbon, remember. This is why hydrogen flames are not very bright. (They are still hot, tho).
The wick isn’t actually what burns, it just suspends the flame. The flame vaporizes the wax and burns the vapor, effectively using the wax as a fuel source. When the candle is blown out, the heat still remains and still vaporizes the wax for a little while. The smoke is the vaporized wax. Side note: this is why you can relight a candle by lighting the smoke.
Everything you wanted to know about a candle but where afraid to ask