Burn, Char, Melt
Date: Fall 2009
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?
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)
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Update: June 2012