Celebrating Chanukah calls for chatting about cool candle combustion chemistry! And cleaning last year’s wax off of the menorahs – What’s wax? None of your beeswax – Just kidding! 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.
Combustion takes place in our bodies too – we usually call it oxidation in that case, and it’s helped along by proteins called enzymes that hold things together and help them react. Through lots of super efficient, controlled steps, our bodies can break down all sorts of hydrocarbon-based molecules – from lipids to sugars to proteins. Our bodies do this super efficiently and in a controlled fashion, capturing almost all the little bits of energy released along the way and storing it (such as in ATP) or immediately putting it to use.
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 happened in our bodies, would make a process inefficient for energy-generating purposes – a ton of heat and light is given off – 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 it’s responsible for the light we see.
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.
Chemical changes usually take place before the wax (or “was once a wax”) meets the oxygen. In the part of the flame near the wick, things actually aren’t *that* hot – “only” around 400-600°C (compared to ~1400°C in the real hot spots). The reason for this is that, unlike combustion, melting and vaporization are endothermic (they require energy) – this is why candles don’t just spontaneously combust – you have to add activation energy by lighting the wick. So the energy by the wick & wax is being used for those physical changes.
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 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.
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 it leaves 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 help it climb up.
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 and the other parts, where the electrons spend less time, become partly positive.
Oxygen is more electron-hogging than hydrogen, so the O in H2O is partly negative and the H’s partly positive. And 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 or molecules with dipoles (even other water molecules which gives you things like the surface tension that makes water “sticky”)
Hydrophobic molecules are ones that avoid water. Don’t let the name scare you off – water doesn’t even “scare” off these molecules despite the “phobia” in the name. The molecules aren’t really “scared” of water – they’d just rather hang out with other things. And the water doesn’t want to hang out with them either. The reason for this is that hydrophobic molecules (or at least the hydrophobic parts of molecules (it’s not all or nothing) don’t have charges (full or partial) so no one stands to gain. There’s no charge attractions possible and water doesn’t want to give up the attractions it can find elsewhere to hang out with something that can’t make it happy. And the hydrophobic molecules don’t want to hang out around charge, so they team up to reduce their contact with water through so-called hydrophobic interactions.
“Lipid” is a kinda a catch-all term we give to such hydrophobic molecules made up of carbon and hydrogen (hydrocarbons) – those elements share electrons pretty fairly, so they’re largely non-polar and hydrophobic. Lipids include fats and oils, with the difference being that fats are solid at room temp whereas oils are liquid at room temp as well as steroids (which have hydrocarbon rinds and include things like cholesterol) and – what I’m going to focus on today, waxes.
Pure hydrocarbons are pretty “boring” as is – they lack so-called “functional groups” that give them “special powers” like enabling them to react and/or combine with other molecules. So, instead of just plain chains, the “starter kit” for a lipid is typically a fatty acid. It’s a hydrocarbon chain with a carboxylic acid (C=O)-OH group stuck onto the end. That carboxylic acid *is* reactive, so now you can make different things from these fatty acids. (you can think of the carboxylic acid kinda like putting the bump on a LEGO)
If you use that carboxylic acid to join up to a long-chain long-chain alcohol (ends with an OH) you can get what we call a wax. This linkage leaves them with an ester group (a -C-O-(O=O)-C- group) in between hydrocarbon chains. Some examples of this type of wax are: CARNAUBA WAX , which coats leaves of Brazilian palm trees to keep them from drying out (it acts as a barrier so new water can’t come in, but water that’s already there can’t leave)’ LANOLIN, which coats lambs’ wool to give them a permanent raincoat; BEESWAX, which secreted by bees to make honeycomb homes & seal rooms filled w/pollen & honey for winter storage; and SPERMACETI WAX, which found in head cavities of sperm whales & may help w/echolocation (“seeing” w/sound) &/or buoyancy (floatation).
BUT having an ester isn’t really what defines a wax – instead, classifications between types of lipids are kinda wishy-washy, but waxes are typically…
Based on long chains of hydrogen (H) & carbon (C) atoms – we call these “alkanes” and waxes typically ~12-32 carbons long. They’re soft, moldable solids at room temp but they have a fairly low melting point, so you don’t have to heat them much to get liquid. And they’re water-repellent. This comes from that non-polar nature (even charge distribution) that makes them HYDROPHOBIC (water-avoiding) because water, being highly polar, likes charged things. So if you put water in contact w/wax, water’ll try to minimize contact & it’ll bead up & roll off
Waxes are used to make cosmetics, foods, crayons & much more – including candles! BUT the most frequently used wax for candle-making, PARAFFIN WAX, is ester-less – in fact, it lacks any “functional groups.” PARAFFIN WAX is a mix of alkanes of different lengths w/general formula CnH(2n+2) , where n’s ≥16 (e.g. a 16-C alkane paraffin alkane would have 34 Hs, giving you formula C₁₆H₃₄). It comes from a waxy byproduct of petroleum distillation.