A yellow powder cannot be ignited when placed on a watch glass in quantity, but ignites vigorously when blown into a burner flame, due to increased surface area.  When the powder is placed on a surface of water, a barrier effect is illustrated.  Next, a candle is lit to illustrate the properties of it burning.  Finally, a lead in to a safety first demonstration is presented.

Lycopodium, a yellow powder is introduced to the audience.  Some of this powder is placed on a watch glass, and the lecturer attempts to ignite the powder with a butane lighter and nothing happens.  A portable torch is then lit.  Some of the powder on the watch glass is gathered into a glass rod to which is attached a piece of tygon tubing.  The powder is then blown into the flame, resulting in a vigorous combustion and a large fireball.  The powder is very finely divided – what is observed is a surface area effect – if the powder is clumped, the powder does not ignite – on the outer edges of the watch glass where the powder layer is relatively thin, it is singed.  Next, a scoop full of the powder is placed in a beaker of water – notice how it floats on the surface but does not penetrate.  Next, a bare finger is introduced into the system.  The finger gets coated with the powder and does not wet.  The finger feels the coolness of the water.  Next, a candle is lit and the question posed – would the candle wax burn without the wick in the candle?  The wick actually slows down the burning of the wax which is the fuel for the flame.  The wick does burn, but by itself would be consumed by the flame in a matter of seconds.  There is actually a space between the flame and the wax – when the candle is lit, a pool of melted wax forms which travels up the wick via capillary action, then vaporizes with increasing temperature so that the wax vapor is what is burning.  There is no contact between the flame and the surface of the wax.  Next, a wooden splint is lit, the candle flame is blown out and as the flame from the splint approaches the wick, the residual wax vapor easily reignites.  This is the way an ordinary candle burns.  Next, an empty can is used as a container for candle wax (without the wick).  In the next video, the wax will be melted and eventually its temperature will be such that the wax vapor formed will ignite.  The next demonstration is conducted in a chemical vapor hood for safety.


Surface Area Effects as Related to Flammability