Candle combustion science

Let me wax poetic about waxes - after I tell you about how candles work.   
  
  
We’ll talk more about what a wax is later, but for now just know that it’s made up of long chains of carbon and hydrogen - so it’s a hydrocarbon. And when you light a candle you use the candle’s wax as fuel for a chemical reaction called combustion which involves reacting hydrocarbons with oxygen, breaking them down into carbon dioxide and water. This reaction is highly exothermic (energy-releasing) because you “get back” the energy required to hold all those carbons and hydrogens together. And that energy is given off as heat and light.   
  
  
But combustion in the candle is “all at once” and although the process is pretty efficient in the sense that it almost all gets burnt, the products are things that, if produced in our bodies, would make a process inefficient for energy-generating purposes - a ton of heat and light is given off - Some of the heat & light are related - when molecules are heated up they start moving around & give off energy in the form of “electromagnetic (EM) radiation.” Basically heat is being transferred from the molecules through the air in the form of waves, and when these waves have a certain energy content we can see it, so we call it “visible light.” This THERMAL RADIATION is given off whenever an object is hotter than its surroundings BUT usually the radiation given off doesn’t have enough energy for us to see (it’s in the infrared range). BUT if we get it hot enough (VERY hot) we reach the visible range & the molecules start to “glow.” We call this INCANDESCENCE, and incandescence of soot (intermediate chunks of carbon produced during the burning process) is responsible for the yellow light we see in flames.   
  
  
When you burn a candle, you’re not actually burning the solid wax - this is why the whole candle doesn’t just burst into flame - instead you’re lighting the evaporated wax. When you light a candle, the wax near the flame starts melting, turning from a solid into a liquid, and then ultimately vaporizing into a gas. These are just physical changes - just like you are the same person whether you’re sleeping, casually strolling, or running, molecules are the same molecules whether they’re in solid, liquid, or gas form, they just have different amounts of energy.   
  
In a solid, molecules only have enough energy to vibrate in place, but give them some more and they get enough to slide back and forth past one another, but they keep getting attracted by neighboring molecules and don’t have enough energy to really break free - this is the form we call a liquid. Add some more energy and molecules have so much energy that they can “fly away” - they’ve gotten enough energy that if another molecule tries to attract them, they can resist and they can break free if they get temporarily sidetracked - congrats, you’ve got a gas.   
  
And once our wax is a gas, it has a chance to run into oxygen, which likes to hang out in the air in pairs (in diatomic form) as O₂. That oxygen-oxygen bond isn’t super strong, so it’s willing to swap if a better option comes along! This oxygen can react with the wax molecules to give you water vapor (H₂O in gas form) and carbon dioxide (CO₂). So now you *do* have chemical changes - bonds are broken and formed and new molecules are made.   

finished in comments

[thebumblingbiochemist]

  
But even then, another thing about the inner-bottom-y part of the flame is that there isn’t much oxygen. So you can’t make water of carbon dioxide here, but you’re still giving the molecules a lot of energy. All this energy breaks up the really long wax molecules into smaller hydrocarbon molecules - in our bodies we use enzymes to help break things up, but turns out if you just give the molecules enough energy (heat’s just energy) the molecules start vibrating so fast they can’t hold onto each other, so they break up - this happens in a process called pyrolysis (lysis for split, pyro for fire). Unlike when we break things with enzymes, and make sure to make fairly “safe” products or at least really well control things, this pyrolysis can generate intermediates that have lone electrons (we call these radicals) and they’re really reactive, so they can also react with other carbon-y chunks to give you all sorts or weird carbon rings and stuff, collectively referred to as “soot.”  
  
They can also be kinda smelly, unlike the combusted products (water vapor and CO₂), which are invisible and don’t smell. So a candle doesn’t usually smell and/or blacken things until you burn it out and there’s not enough energy for combustion, but you still have uncombusted break-down products.   
  
As we talked about before, gas molecules are free to move around - so they do - the hotter things are, the more energy they have, so the more they can move - so you end up with a net movement of molecules from hot areas to cold areas. And this leads to the generation of a convection current with the heated gases rising and fresh air oxygen coming in. And it gives the flame its characteristic teardrop-y shape.   
  
Before they leave the flame, the hydrocarbon molecules reach more oxygen-rich parts. And in those parts of the flame you have combustion occur, generating water vapor, carbon dioxide, light, and heat - about 1/4 of the energy produced escapes as heat. Some of this heat just wanders off, but some of the heat gets absorbed by the wax, so you get more wax melting to replace the wax you melted and then vaporized and then combusted.   
  
And the wick helps the newly-melted wax reach the air and vaporize (it’s a lot easier to vaporize once you’re already at a liquid-air interface because you have an easier escape route). The wick is usually made of absorbent twine - absorbent it means that liquid doesn’t just stick to it (adsorb), it actually gets sucked inside it. And capillary action can then help it climb up (basically because it likes sticking to the twine, but it also likes sticking to its neighboring liquid molecules, it will climb up the twine pulling the liquid with it.   
  
So the candle can keep burning until you run out of fuel (the wax) or the oxygen - if you run out of one of them, you can get flickering which allows some soot to escape. Soot’s just unburned carbon particles - and it can form when there’s not enough oxygen for all of the carbons to get some. The hydrogens still get theirs to form water vapor, but there’s not as much for total CO₂ making, so pure carbon & mostly-carbon products are made instead.   
  
  
Now, as promised - what’s a wax?  
  
A lot of the molecules in our bodies like to hang out with water & will happily put on a water-coat (dissolve in water). We call such water-loving molecules HYDROPHILIC. What makes something hydrophilic? Charge or partial charge. You see, water, H₂O, might “look” neutral - you don’t see any + or - signs indicating it’s an ion (charged molecule) but its charge is unevenly distributed.  
  
Molecules are made up of atoms and atoms are made up of charged parts (positive protons and negative electrons) and neutral parts (neutrons). The electrons like to hang out in certain areas more than others so those parts become partly negative (δ⁻) and the other parts, where the electrons spend less time, become partly positive (δ⁺).  
  
Oxygen is more electron-hogging than hydrogen, so the O in H₂O is partly negative and the H’s are partly positive. This creates a charge imbalance called a dipole - and we call molecules with dipoles POLAR. Because opposite charges attract, the O will be attracted to positive things - either fully-charged anions (fully negatively-charged molecules) or molecules with dipoles (including other water molecules which gives you things like the surface tension that makes water “sticky”)

[MrSpamTrapper]

From the reaction, water should also be produced. I'm assuming it's because of the high temperature that it's just produced as gas and thus we don't see any candle dripping water ? :)