I’m in the midst of a huge argument with my family over this, so anyone who could step in would help me greatly.
My sister says, when candles burn, the wax is just being melted away and that it has no additive effect to the candle’s burning.
I say, well yeah, but if you embed a wick in clay or water, for example, and let it burn down, it burns for a few seconds and goes out. Furthermore, the wax disappears over the life of a candle–an old candle has very little wax. Therefore, the wax is fuelling the candle in some way.
My sister replies by arguing that wax merely slows the burning reaction and does nothing additive–the wax is just an inhibitor. After all, wax isn’t flammable.
I respond with, “Well, if you burn a piece of string, you get the same size flame, but it burns much more quickly and leaves more waste. How can a candle burn so efficiently and yet yield the same flame unless the wax of the candle is feeding it somehow? Also, cigarettes are chemically engineered to burn slowly, but they don’t burst into flame and they burn a hell of a lot faster than candles do.”
I must embarassingly confess that I have no idea how something so simplistic a candle works. My own theory (wax fuels the reaction) seems so counterintuitive that it strikes me as wrong. But given my observations, I keep drawing this conclusion. Someone please explain to me what is going on in the most technical way possible. I get the feeling that I am very wrong…
The wax is the fuel. Molten wax is drawn up in to the wick where it is burned, producing, carbon, carbon dioxide, carbon monoxide and water, along with a few other chemicals. Wax is a type of fat, in fact, and we all know what the fuel in oil lamps is. In fact, candles used to be made from beef tallow–a type of solid fat.
Here’s more. The heat of the flame melts the wax and then evaporates it. There’s wax gas - yes, the mix of hydrocarbons you call wax is turning into a gas - and it’s diffusing and rising away from the wick, and also mixing with oxygen and combusting. When you blow the candle out, you are speeding up the mixing and carrying away of this gas faster than the fire can keep igniting it. When you have blown it out and you see smoke coming off the wick, that is actually wax gas recondensing into droplets or balls of wax (I don’t happen to know what the temperatures are so I don’t know which). It is very easy to ignite this wax aerosol because of its high surface area and fairly high temperature, and you often see candles reignite themselves, especially if you interfere with the rising of that aerosol or if the candles are formulated to increase the effect.
Exactly right Napier. The truth is that nothing that burns, actually burns, the vapor from the heated material is what burns. When heat is applied to the wick through direct flame contact, the composition of the wax changes from solid to liquid, when that occurs, the heated liquid gives off a flammable vapor, which burns at a small height above the wax itself.
The same kind of thing happens on wood, paper, cotton etc. it doesn’t all turn to liquid, but it does change to provide vapor which is what burns, and thus causes a chemical reaction, leading to the byproducts of incomplete combustion, such as smoke, soot and ash.
I do not believe this is correct. I believe Carbon burns as a solid, coal burns as a solid and wood , once reduced to carbon embers, burns as a solid. Countless things oxidize at ambient temperature too, including iron.
That’s an easy one to prove. The sublimation point of carbon is somewhere above 3500 [sup]o[/sup]C. The flame temperature of carbon in air is only around 1000 [sup]o[/sup]C. So there’s just no way in hell that the carbon is vaporising before combustion.
Whether carbon burns as a solid or gas depends, I think, on what sort of carbon you’re talking about.
I think charcoal briquettes in a barbecue are probably burning as solids, glowing red as the reaction takes place at their surface. There are also faint flames around them, which I’m guessing are due to various things (including hydrocarbon compounds) outgassing from the charcoal, which is the quintessential sorber after all.
Coal may be burning more as a gas than as a solid. A hundred years ago, there were coal gas plants in most sizeable towns, wherein coal was heated to vaporize the coal gas and send it out to homes etc. I’m a little hazy just what is going on here, but am pretty certain it’s not just sublimation of elemental carbon.
In a candle, you’re oxidizing carbon and hydrogen and maybe traces of other things like sulfur. The carbon and hydrogen come from hydrocarbons that get pyrolized. Do we count this carbon as “burning carbon”?
I think you are more than just a little hazy. “Coal Gas” is not vaporized carbon but a mixture of hidrocarbons and other compounds which are gases at ambient temperature. There is no way to have carbon vapor at ambient temperature.
There are different types of coal gas, according to this page.
Heating coal drives off a mixture of hydrocarbons, which is what provides the flame in an ordinary coal fire, and leaves ‘coke’, fairly pure carbon that burns as a solid. The coal gas used for lighting was made either by blasting the hot coke with steam to make a mixture of carbon monoxide, hydrogen, and mixed hydrocarbons, or by mixing oil into the hot gas coming from the coking oven.
I knew about the steam process, but just found out lots more!
The link to Uncle Cecil’s column, from Return of the Straight Dope, just in case Cecil drops by to see if anyone remembered to include a link to his column, which I did. and it is: Where does the candle wax go?