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Name: Adetunji
Status: educator
Grade: n/a
Country: Nigeria
Date: Fall 2009


Question:
Why do some substances like wood burn on heating rather than melting like most other substances, or charring like some plastics? What determines burn vs. melt vs. char?


Replies:
Adetunji,

Burning is a chemical reaction which involves the substance being burned plus an oxidant (usually oxygen). In colloquial use, burning is also accompanied by a high amount of heat being released and flames.

Melting is a physical change (no chemical reaction takes place). The substance is changing from solid to liquid.

So the controlling factor then is whether the substance easily forms compounds with some oxidant. This term "easily" can be taken to mean that the energy required to start the reaction (called the energy of activation) is low enough so that the reaction starts (as in striking a match), or the compounds produced in the burning are low enough in energy that there is a tendency to form it. Of these two, energy of activation is usually the controlling factor in deciding if a reaction will take place.

Thus, while iron can "burn" in the presence of oxygen to form iron oxides (in fact, iron filings produce when tossed into a flame produce a big - and dangerous - fireball), a large block of iron tends to react slowly with oxygen and form rust - not generally considered a "burning". As such, the controlling factor here is that the energy of activation to form the oxides are easily supplied (the reaction proceeds under mild conditions), but the oxidation of the next layer of iron does not happen because it is not in easy contact with oxygen and so, rusting is a slow process. Thus, heating iron will tend to melt it, rather than burn it.

Charring is a complicated process that involves many things: the amount of oxidant available, the energy of activation of the process, the amount of volatile substance that are released during the heating process, etc. Because it is a function of so many different things, deciding if a substance will char instead of burn is best treated on a case-by-case basis.

Greg (Roberto Gregorius)
Canisius College



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