Chemistry Lecture Demonstration Facility - Demos

Surface Area Effects as Related to Flammability Part II

A simulation of a kitchen grease fire is demonstrated, explained again by surface area effects.

An empty can is used as a container for candle wax (without the wick).  In this video, the wax will be melted and eventually its temperature will be such that the wax vapor formed will ignite.  This demonstration is conducted in a chemical vapor hood for safety.  As observed, when the wax completely melts, its temperature keeps increasing with time, the wax starts to smoke, and eventually, the wax vapor ignites.  Next, the question is posed as to how to extinguish the flame – most would use water, but as shown when a stream of water is introduced into the flame from a squirt bottle, the result is a vigorous fireball.  This is known as the grease fire demonstration – DO NOT DO THIS AT HOME IN A FRYING PAN, FOR INSTANCE!!  Let’s discuss this – why does water extinguish a normal wood fire for example?  Three ingredients for a fire are fuel, oxygen and enough heat – when the water is introduced, the heat is taken away and the fire goes out.  But something special happens in this demo, imagine the wax which is a hydrocarbon takes the place of grease.  The wax is non polar while the water is polar.  They will not mix.  Next, the water is denser than liquid wax, which means it goes to the bottom.  Finally, the temperature of the resultant melted wax is much higher than the normal boiling point of water.  When the water hits the wax, it is instantly vaporized and occupies much more space.  It spreads out and carries with it little droplets of wax – this is a surface area effect similar to the lycopodium powder demo in the previous video.  The wax droplets burn very fast resulting in a huge fire.  How can you put out this fire – obviously not with water?  Smothering the flame with baking soda or salt would do it, you could put a lid on it.  With enough water, the wax would get used up and eventually the flame would extinguish. 

 

Surface Area Effects as Related to Flammability Part II

Contact

Bob Porcja
porcja@chem.rutgers.edu
848-445-5157

Location

Beck Hall Auditorium
99 Avenue E
Piscataway, NJ 08854

The CLD Facility is located in the prep room adjacent to the Beck Hall Auditorium (map).

CLD Facility