Eli5: Why do some objects burn while others melt?
u/Xelopheris did a good ELI5 imo, so my comment adds on his one.
– Many materials are chemically speaking already burned (even if they weren’t made by holding something in a flame): water, CO2, baking soda, chalk, glass,… So they can’t burn any furthrr.
– Things can melt before they burn. If the molecules they are made out of bind weakly to each other the melting point is low. This is why many polymers melt before they burn.
Burning is a chemical reaction. Thing plus oxygen at sufficient temperature react together, and that reaction releases more heat.
For example, some hydrocarbons (combinations of hydrogen and carbon) will react with oxygen and create CO, CO2, and H2O vapor.
Melting is just a phase change. When ice melts into liquid water, it’s still H2O before and after, just in a different chemical phase.
As for why some burn and some melt, it’s basically whether or not the specific chemical combination will react, and if so, what temperature is required. You could melt wood in theory in a vacuum chamber by removing the oxygen (although because of the structure you would need an insanely high heat).