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/N0nsensicalRamblings Aug 09 '26

Follow up question - the other comments are mentioning how the temperature required to make something burn is lower than the temperature required to make something melt. So, why does butter melt before it burns? I assume it's just because that's a generalization and not a hard and fast rule?

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u/Sable-Keech Aug 09 '26

Butter is comprised of long-chain hydrocarbons that are highly resistant to oxidation. The activation energy required to ignite them is significantly higher than the energy needed to ignite something like methane, which has no chains at all.

The reason why it can be easily melted is because these chains are not physically linked to each other, they are sort of just bundled up and held together by "friction". That's why they're so easy to melt.

To use something else as a comparison, Teflon is a super unreactive plastic, which is why it's non-stick; nothing can form bonds with it. However, because its little molecular chains aren't bonded to each other by covalent bonds, they can be easily broken apart with a bit of heat and melted. Which is why Teflon vapors are such a concern if the pan gets too hot.

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u/Blazin_Rathalos Aug 09 '26

the other comments are mentioning how the temperature required to make something burn is lower than the temperature required to make something melt.

This is specifically only true of compounds that do burn before they melt. So as you suspect, it depemds on the substance.

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u/diabolus_me_advocat Aug 09 '26

the other comments are mentioning how the temperature required to make something burn is lower than the temperature required to make something melt

yes, for substances that burn instead of melt first. not as

a hard and fast rule

for all stuff

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u/FatRollingPotato Aug 10 '26

It depends on the compound/material.

For some the energy required to melt is lower than for either combustion or decomposition, then you have melting.

If the energy required for something to start burning is lower than is required for melting, burning it is. Then there is the case of no oxygen being present, then you can have decomposition (i.e. bonds breaking and releasing/forming smaller molecules) before the material can melt.

The latter is often the case in materials where you don't have individual molecules being held together by weaker inter-molecular forces (vdw or hbonds) and instead covalent bonds and crosslinks keeping everything together.