It’s probably something you’ve never really thought about. But there’s an interesting reason why a candle doesn’t emit any smoke while it’s burning; only after it’s blown out. The following answers from the internet should give you a clear understanding of it.
When the flame is lit…that smoke is being burned. The smoke is vaporized wax. When you blow it out, the wick is still hot enough to vaporize wax, but not ignite it.
If you cool the wick like lick your finger or put in water, the wick is no longer hot enough to vaporize wax.
Candles are surprisingly complicated from a chemistry perspective. Candle wax isn’t just any mix of hydrocarbons, they have to all be saturated, which makes them very nonreactive compared to other organic compounds. (I believe their name came from a Greek or Latin phrase, something like “para affinis”, meaning “next to no activity”.)
Saturated hydrocarbons burn cleaner in a candle, because their single bonds can be broken by lower temperatures, meaning that they’re less likely to escape a candle flame as carbon particles (soot).* They can be pyrolyzed with a carbon catalyst though, to make a flammable mix of gases. Cellulose string, when burnt, forms a porous mass of carbon at the end that can catalyze this decomposition. It does this when you light a candle, with the flame growing as it liquefies more and more wax. But eventually the catalytic tip is overwhelmed by molten wax and slows down, creating equilibrium. If you blow out a candle, you’ll often see a glowing bit on the tip that’s releasing smoke. This is the catalytic carbon part of the tip, oxidizing the candle wax into the mix of smaller molecules that are found in smoke.
So, why are some natural oils saturated and some unsaturated, anyway? It all comes down to what temperatures they experience, of all things. Birds and mammals have high enough body temperatures to keep saturated fat from solidifying. Fish, on the other hand, do not. So their fat molecules have kinks in them, in the form of carbon-carbon double bonds, to keep them liquid at cold temperatures. This goes for plants too. Most plants grown in temperate climates have kinks in their fat molecules, to keep them from solidifying. But plants that grow only in tropical climates, like coconuts and other palms, have saturated fat molecules that solidify when they get cold.
You might be wondering why soy candles are able to be solid at room temperature, even though soy grows in temperate climates. Well, that’s because soy candles are actually made from “trans fats”, or “hydrogenated fats” like margarine and shortening are made from. These have had the kinks chemically removed from their fat molecules, so they’re solid at room temperature. This is supposed to be slightly bad for you if you spend your life eating it, but there’s certainly no harm in burning it in a candle.
the smoke is flammable, because it’s just unburned, vaporized candle wax. When the candle is lit, that same wax vapor is what sustains the flame in the first place.
In 1848, Michael Faraday literally explained the science of candles to five-year-olds in a series of lectures. In his own words:
There is another condition which you must learn as regards the candle, without which you would not be able fully to understand the science of it, and that is the vaporous condition of the fuel. In order that you may understand that, let me show you a very pretty experiment. If you blow a candle out carefully, you will see the vapor rise from it. You have, I know, often smelled the vapor of a blown-out candle—and a very bad smell it is; but if you blow it out lightly; you will be able to see pretty well the vapor into which this solid matter is transformed. When I hold a lighted match two or three inches from the wick, you can observe a train of fire going through the air till it reaches the candle.
Engineer guy recorded himself giving the lectures with all of the practical demonstrations, and put them on YouTube. Here’s the part where he reads the paragraph above and does the experiment.
In theory, a perfectly efficient flame will have no smoke, because the fuel combusts with the surrounding oxygen, and in a perfect world, you will have C02 and H20 byproducts, both of which are a gas and invisible. But in the real world, it is difficult to get a perfect rate of combustion. Instead, we often get incomplete combustion, when there is too much fuel for the air to mix with.
In the case of a candle, while it’s burning, you will often see a little whiff of smoke every now and then; since we cannot control the rate of which the wax burns (the wick and candle design can get it close, but not perfect). When you put out the flame, the fuel continues to vaporize, but is unable to burn and thus you have smoke.
This is the same for all sources of combustion. If your campfire is really smoky, stir up the wood and get some air flowing through it, bringing more flame and less smoke.
Old cars usually smoke more than newer more efficient cars, and in winter, a cold start generally has a lot of smoke because the engine is fed more fuel to help it run until it’s at operating temperature.