Eli5 Given a high temperature, why do some things melts while other burn to ashes?
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When you heat something up, the molecules start to jiggle faster and this extra energy can cause a lot of things to happen.
Melting is the simplest option; the molecules get enough energy that they’re not so attached to their neighbors and can mingle around the area freely (but not shoot off on their own as a gas). Generally things will melt at a high enough temperature, but sometimes that temperature is VERY high (graphite doesn’t melt until 3600C) so the question is: **does something else happen first?**
In some things (e.g. wood) as the molecules heat up and jiggle around, they begin to smash into other things (e.g. oxygen) and combine into more stable molecules, releasing extra heat in the process. If somethings starts burning, the extra heat will make it burn FASTER, and destroy the thing before you can get it hot enough to melt. But if you remove oxygen (and other “oxidizers”), it won’t have anything to burn with and you may be able to get it hot enough to melt. Even metal burns (“oxidizes”) but it’s usually much harder for oxygen to reach the tight-packed molecules, especially as the resulting metal-oxide forms a protective layer. Thermite gets around this by using powdered aluminum mixed with a powdered oxidizer and it burns VERRRY hot.
Plastic has fairly low melting & burning temps so it’s easy to see this in action. Heat plastic up slowly and you’ll notice it’ll calmly melt. But if you get it too hot, it’ll get enough energy to start burning. Now it’ll still be melting, but it may not spend much time in this melted phase before catching fire that it’ll just feel like part of the burning process. And the carbon-heavy soot left over has a MUCH higher melting point and won’t melt so easy.
EDIT: Burning plastic can produce very toxic chemicals. Only try this in well-ventilated areas, and not much as it’s a nasty polluter.
Melting is a state transition from solid to liquid. All solids will undergo it at a certain temperature and pressure.
Burning is a chemical reaction. In air, this is a chemical reaction with oxygen, although there are other oxidisers. This ignition therefore requires both the presence of an oxidiser and sufficient surface area.
Not everything is reactive with an oxidiser. For example, water, ice, steam, *H2O*, doesn’t matter how hot you get it, it’s *already burned*. Specifically it’s the product of hydrogen combustion. Dihydrogen Monoxide.
So some things melt at lower temperatures than they burn, some don’t burn, some scenarios don’t have any oxidiser, and sometimes the surface are is insufficient for ignition.
Melting is a transition where molecules that are structured within each other have that structure broken due to the heat.
Burning is a reaction that occurs within individual molecules, where the extra energy added causes the molecules to split and reform, typically consuming atmospheric oxygen as well.
Think of melting like taking apart legos and burning being like running legos through a blender.
*Edit* realized I’ve never answered the question…
If a thing has a chemical structure capable of burning, then whether or not it burns or melts depends on which one takes less energy to accomplish. However, you can bypass burning if you heat something up in an area deprived of oxygen.
It depends on the material. Melting is when a solid transitions to a liquid state while burning is when a material breaks down into a different state instead.
The reason something might burn rather than melt is they have a lower combustion point compared to a melting point. Ice for example has a low melting point and so melts Instead of burns.
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