Why do some things melt and some things burn?

Question

Why, when heated, do some things burn and some things melt?

Answer 1

All flammable substances MUST melt and/or vaporise before they will burn. (Only gaseous substances will react with Oxygen in a combustion reaction).

If the Oxygen is excluded, they will simply melt and/or change to other solid, semi solid, liquid or gaseous substances by decomposition when at the correct temperatures.

Answer 2

chcolate melt cuz of the heat temperature. it get burn cuz of the same thing excpt when it doesnt contain that much water in it for it to melt. i mean water evaprotes when touchs heat the coco powder doesnt evaporte that much therefore stays on the hot spot and turn black. basic carbon science

Answer 3

Everything has a melting point, but unfortunately for char and ash its much hotter.

Answer 4

Remove Oxygen (or whatever the reactive chemical may be) from the burning equation and objects that burn also have a melting point.

For argument's sake we'll take a complex object like wood which we all know burns quite well.  Yet if it's heated up in a corked test tube it begins to blacken and tar becomes extracted.  Carbon is an essential building block of the wood pulp yet even carbon has a melting point of ... check this out: 3823 K!  Woah!

Answer 5

Mack got it right, before I got to it.

Yes, one is a physical change and the other is a chemical change.

What I think you want to know is why doesn't everything first melt and then burn? That's because not everything supports combustion. They have different compositions, different states of matter, they are not all elements like in a periodic table, there are mixtures and compounds.....depends what you specifically need to know about.

Good question, opens up a pandora's box, doesn't it.

Answer 6

Interesting question.  Melting and burning are actually quite different processes; melting is an example of a physical change while burning is an example of a chemical change.

In a physical change, no chemical reaction occurs but, instead, the molecules that make up the substance are rearranged.  When a solid object is heated (assuming it doesn't burn),   the added energy makes the molecules move around more, thus overcoming some of the intermolecular forces preserving the object's structure.  Consequently, the molecules are more free to move around than in its previous state.  If enough energy is added, a molecule will be able to move around completely independently of its neighbors (=a gas).

In a chemical change, the molecules of the object interact with others to form new molecules.  An example of this is burning or, more specifically, a combustion reaction.  Here is the equation for the combustion of methane, a simple hydrocarbon:

CH4 + 2O2 → 2H2O + CO2

As you can see, after the reaction takes place, there is no more methane.