Why do some things burn and other things melt?


Why do some things burn and other things melt?

In: Chemistry

Burning is a chemical reaction between the burning object and the air (usually oxygen). Melting is a state change from solid to liquid. If a substance gets hot enough that the reaction can be sustained it will burn, if not it may melt when the melting point is reached. In some cases it will both burn and melt (waxes, plastics, soft metals).

Not a chemist by any means, so if I’m using slightly incorrect vocabulary, I’m sorry.

Combustion is a chemical reaction, while melting is just the matter changing state from solid to liquid. For instance, in a combustion, the matter reacts with oxygen and creates a different molecule (e.g. carbon + oxygen -> carbon dioxide), while some ice that melts into water is still the same H2O before and after. The only thing they have in common is that they need a certain temperature to trigger the reaction.

Most things will do both, but one is easier than the other.

Melting is a state change from a solid to a liquid. This requires enough energy (heat) to raise the temperature high enough for the current atmospheric pressure. Some materials simply require too much energy or too low of a pressure for this to happen easily in normal life.

Burning is a chemical reaction between fuel and oxygen. It also takes energy to start (heat), but usually releases even more energy which makes the reaction continue. For some materials, this initial energy is too high to happen easily in normal life.

It gets more complicated when your substance isn’t a single material. For example, wood. While we generally think of wood as “burning”, there are a lot of other materials in a piece of wood that will melt or evaporate while the cellulose (woody part of wood) burns.

So essentially, it comes down to which of the two needs less energy for the current conditions. This is known as the combustion temperature (burning) vs the melting point (melt).

A somewhat familiar example is magnesium: this is a solid metal at room normal temperatures and pressures that is commonly used in firestarters because it is so flammable and easy to burn. However, if you put it in a non-oxygen environment at the same pressures, it melts rather easily.

Most of these comments imply that things only melt or burn depending on which thing happens first. That’s not the whole story though. Some materials simply don’t melt. In atmosphere they may burn, but if you take them into a vacuum with no oxygen available they still won’t melt.

IIRC all elemental materials have an associated phase diagram and will liquify under certain conditions. Molecular materials may or may not. In some cases they simply decompose into smaller molecules and/or sublimate away, if there is no oxidizer available to cause it to burn.

Most things melt (a few skip melting and go straight to gas), it’s just that many things also burn.

So if the melting point is higher than the flash point, and there is an oxidiser of some sort around, it will ignite before it melts.

Fire is a chemical reaction. Carbon-based fuels, plus Oxygen, plus heat equals CO2 and more heat. Things that don’t make great fuels will melt rather than ignite because it’s missing vital parts of the triangle.

If you were to apply heat to a non-oxidized fuel source in a vacuum it would “melt”, using the term generously: