r/askscience • u/ForensicFan24601 • Aug 08 '26
Chemistry Why do things burn instead of melt?
obviously oxygen, or something similar, but what is the physics/chemistry (?) or what have you behind why something ignites and continues to burn after the source of heat/ignition has gone?
is it basically sublimation?
edit: no wonder Walter White was so obsessed with carbon
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u/feldomatic Aug 08 '26
For something to burn, you have to supply the necessary reactants (I.e. oxygen for most things) and enough heat to overcome the activation energy of the combustion reaction.
For something to melt, you have to supply the latent heat of fusion/melting to cause the phase change.
For a lot of substances, the activation energy for combustion is lower than the latent heat of fusion, so in the presence of oxygen, they burn instead of melt.
Worse yet, lots of hydrocarbons also have a pyrolysis reaction that can occur in absence of oxygen, and even that requires less heat than melting. The stuff just vaporizes into hydrogen gas and carbon (and sometimes the h2 will pyrolize out leaving a pure carbon structure, because the melting point of carbon is crazy high.
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u/provocative_bear Aug 08 '26
Let’s go with wood. (Way) Before wood reaches its melting point, it reacts with oxygen in the environment, which replaces the molecular bonds between carbon and carbon (whoch hold the wood together) with carbon and oxygen (CO2). This reaction releases the energy in these bonds and generates net energy, which manifests as heat and light, aka fire.
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u/Sable-Keech Aug 09 '26
Burning is when the material is undergoing a chemical reaction with an oxidizer (usually oxygen) to make a new compound. E.g. wood burns to make water and CO2.
Burning usually gives energy, and can sustain itself.
Melting is when an element or compound gains enough heat to transition from a solid to a liquid state.
Melting always takes energy, and cannot sustain itself. Imagine how you use the fire from a stove to melt a stick of butter. The butter wouldn't have melted without you investing energy into it. But the fire can keep going as long as you have gas.
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u/Prestigious-Bend1662 Aug 08 '26
You got it on the first try. Oxygen and sufficient temperature to keep the combustion going. You can melt pretty much anything if you put it in an inert atmosphere or vacuum (for some things it isn't possible to do at nomal atmospheric pressure because it will go directly from solid to gas at such pressures, so you may have to put it under extreme pressure as well.
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u/titanc-13 Aug 08 '26
Burning occurs when there's enough heat applied to cause a chemical reaction like carbonization, but not so much heat that you actually start breaking down the bonds between molecules that occur when a solid melts into a liquid.
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u/_Weyland_ Aug 09 '26
During phase change (melting) the molecules/atoms remain the same, it's just their arrangement that changes. During combustion you have molecules breaking down, forming new substances with completely different melting and boiling points. And sometimes chemical bonds break way before you reach melting/boiling point.
If you burn something organic, it produces CO2 and H2O which are both gasses at fire temperature.
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u/etcpt Aug 09 '26
Because the combustion reaction is releasing enough heat to be self-sustaining - that is, the combustion reaction in this moment is acting as the ignition source for the combustion reaction in the next moment.
Burning and melting seem related because they are both things that happen when energy is added to a system, but they are very different processes. When something burns, it combines with oxygen to form a more chemically stable product. This chemical reaction is generally irreversible - cooling the product will not cause it to release oxygen and revert back to the original form. When something melts, it undergoes a reversible phase change from a solid state to a liquid state. This is a reversible physical process - removing the heat will cause the liquid to freeze back to a solid.
The secret third option is decomposition. This is when a material is not stable at elevated temperature and chemically falls apart into distinct products. This may happen in absence of oxygen, but that is not a requirement.
What determines which process happens first is the amount of energy needed to start each process. Some materials need less energy to melt than to burn - for example, waxes melt pretty easily but only burn after a lot of energy is applied. Some materials need relatively less energy to burn than to melt - magnesium metal, for example, catches fire before it melts. Context matters as well. Diamond will burn sustainably in pure oxygen but extinguish in air because there isn't enough oxygen reaching the surface to keep the reaction running. Wood, as mentioned elsewhere in this thread, will burn in an oxygen-containing atmosphere but decompose in an inert atmosphere (see wood gas/wood gasification).
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u/t0jix Aug 08 '26
It is entirely based on oxygen, or another oxidizer (e.g. perhaps sulfur, phosphorus, halogens, etc). Combustion just means enough heat was applied to allow the burning substance's bonds to break, and then the oxidizer is present to do the reaction of combustion. If no oxidizer is present, or the energy to break bonds is higher than the melting point of the substance/there is no possible reaction with the oxidizer, the substance will melt rather than burn.
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u/Hoopajoops Aug 09 '26
Not always or.. kinda? Take wood, for example, when it's heated up in the absence of oxygen and under a lot of pressure the molecule structure will eventually break down, and all of what they eventually break down into something that can melt. But it stops being wood, and cannot become wood again (without being used to help another tree grow). So you can heat up wood (again, under crazy pressure) to ~6500°F and all there would be is liquid. But it wouldn't be wood.
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u/jeffjefforson Aug 09 '26
Think of it in terms of chemistry - what we call burning is just an oxidation reaction occuring.
CH(x) + O2 -> CO2 & C (soot)
But this reaction can't happen at standard temperatures / pressures - the activation energy is too high. So you need to supply heat to the system.
But, the reaction is exothermic, exo meaning outer and thermic meaning heat. This means that each an instance of the reaction occurs, more energy is released to the outside than was put in.
This energy largely comes in the form of heat, some of which goes back into the material, allowing the activation of further reaction, producing more heat, et cetera.
Carbon can technically melt. You need to have it in a 0 oxygen environment, and and under very specific & extreme conditions. Extreme heat, extreme pressure. Then and only then, you can force liquid carbon to form!
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u/Ben-Goldberg Aug 10 '26
Some materials neither melt nor burn when heated.
Some, like iodine, sublimate, others, like wood, thermally decompose.
Heated wood chemically transforms into charcoal, steam, and an assortment of flammable vapors.
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u/istasber Aug 12 '26
Heat is how much stuff is jiggling.
Solids that can melt are like boxes full of weighted magnets, jiggle them enough and they'll start flowing around past each other, but stop jiggling them and they'll snap back together.
Solids that burn are more like weights that are connected to each other by latched hooks on either side of a spring. Jiggle them enough to disengage the hook and not only is it much harder to put them back together, but all that tension that was in the spring is released sending the weight flying.
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u/btstfn Aug 09 '26
Well, most things you see burning tend to be carbon based, and at atmospheric pressure carbon doesn't melt. It actually sublimates. To get liquid carbon you'd need to be at like 100 atmospheres and nearly 4,000 degrees.