Question

What is the exact difference between heating and burning?

Answer 1

When heating something a small amount, as not to cause it to burst into flames, you are actually speeding up the kinetic energy of the molecules; making them vibrate faster. Since solids are usually in covalently/ionically bonded lattices, these strong chemical bonds are not broken.

However, once you heat the solid to a point at which it burns, you are oxidizing the bonds. Oxygen in the air reacts with hydrocarbons by breaking C-C bonds and creating a carbonyl (aldehydes/ketones/carboxylic acids/etc. Each additional carbon broken off the end of the chain is oxidized into CO2 which is extremely stable. Since the free energy of all the CO2 formed is much lower than the parent hydrocarbon, it is a very exothermic reaction. The additional heat causes the oxidation to speed up into a chain reaction. Most flammable stuff is made of hydrocarbons.

In the case of non-carbon materials burning, you have the oxidation of metals into metal oxides. However, metal oxides are generally not as stable as CO2 and thus do not create as much heat during the process. Usually you need to heat up metals to pretty extreme temperatures to get this reaction to go.