Ask A Scientist Chemistry Archive

status       educator
age          30s

Question -   Please explain the anatomy of a candle flame.  How does
it burn?  Why it is not a reversible reaction?  What conditions will
affect the burning of the candle and in what regions?
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Combustion chemistry is very complicated!  Many, many reactions are
occurring  sequentially and simultaneously.  Scientists and engineers do not
fully understand the chemistry and mechanics of a candle flame (or other
types of flames), but quite a bit IS known.

   * In general, the fate of the wax molecules is this: the heat of the
candle flame first melts the wax, and it rises up the candle wick by
capillary action.  Farther up the wick, the greater heat vaporizes the wax
molecules, which move from the wick into the surrounding space.  The heat of
the flame and reactive molecules (free radicals) in the flame break apart
the wax molecules, in particular stripping hydrogen atoms from the
carbon-chain backbone.  Some of the carbon chains fragment into gaseous
carbon (C2) and into small (typically two-carbon atom containing) molecules
and molecular fragments.  The hydrogen atoms stripped from the wax molecules
eventually combine with oxygen atoms from the air to form water molecules.
The carbon atoms eventually combine with oxygen to form carbon monoxide and
carbon dioxide, but first many of them combine to form very large (as far as
molecules are concerned) clumps of carbon-rich solid material, called soot.
Some of this soot burns to make carbon dioxide in the candle flame, and
sometimes some of it escapes the flame.

  *  Several zones of a candle flame can be seen with the eye.  At the bottom
is a region that gives off blue light.  This light is actually molecular
emission from gaseous carbon, C2.  Further up the flame is a region that is
substantially opaque and which gives off yellow light.  This is known as the
"incandescent region", and is where hot soot particles glow, giving off
light like the filament of a light bulb. The inside part of the flame, near
the wick, is oxygen-deficient, and most of the reactions that occur are
heat-induced fragmentations and rearrangements.  In the outer regions, where
oxygen can enter from the surrounding air, the fragments combine with
oxygen, eventually forming water and carbon dioxide.

   *  Many factors that affect the burning of a candle.  Most of them are of
the type that are difficult to vary, such as the air pressure, concentration
of oxygen, thermal conductivity of air, and the buoyancy of the hot reaction
products.  One factor that is easy to vary, however, is wind.  A good
breeze, or even a person's breath, can blow the hot flame gases away from
the source (the candle and wick), interrupting the process and extinguishing
the flame.

Richard E. Barrans Jr., Ph.D.
Director of Academic Programs
PG Research Foundation, Darien, Illinois
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