Ask A Scientist Chemistry Archive

From:       observe
Text:       In order for a flame to continue burning, it must be provided with 
a source of oxygen.  An air stream is one source of oxygen.  Why then, should 
a rapid air stream every cause a flame to go out, why does "blowing out a 
candle" work?

Response #:  1 of 4
Author:      Robert Topper
Text:        When you "blow out" a candle flame, you are breathing a gaseous 
mixture which is rich in carbon dioxide and poor in oxygen.  So when you blow 
on the flame, you decrease the amount of available oxygen.  If you were to use 
a rapid oxygen stream to hit your flame, you would find that the flame would 
burn more brightly.  Perhaps I am not understanding your question correctly?  
Let me know if I have missed the mark or am in error in some way, please. Even 
scientists have their bad days...

Response #:  2 of 4
Author:      poach
Text:        Actually, a fast oxygen stream would probably blow out the 
candle.  For one thing, the oxygen might dilute the combustible vapors below 
the ratio that would support combustion.  More likely, the fast oxygen stream 
would overcome the flame propagation velocity.  If the flame cannot propagate 
upstream fast enough, it gets carried away from the fuel source: it gets blown 
out.

Response #:  3 of 4
Author:      Robert Topper
Text:        But how fast is fast?  I guess fast must be as you have defined 
it; fast relative to the flame propagation velocity.  This explanation makes 
sense to me...hope it makes sense to "observe."  Thanks to poach.

Response #:  4 of 4
Author:      sysop 
Text:        Just to add my 2 cents....... you must understand that the flame 
needs three things to burn:  1) Fuel, 2)Oxygen or Oxidizing Agent, and  3) 
Heat which form a Chain Reaction.  Interrupting the chain reaction in any why 
will put out the fire.  This is how fire extinguishers work.  Fuel - If you 
remove the fuel (say the candle wax which under heat produces a flammable 
vapor) the fire will go out - a hard blow will do this, Oxygen - Cover the 
candle with a jar.  As the oxygen level goes down (the flame uses it up) the 
flame will go out - a hard blow of CO2 will help the above step, Heat - As the 
flame goes out because of the above two conditions, it does not relight 
because the heat drops without the flame.  How do those relighting candles 
work - A guess might be that the wax produces a chemical in which the re-
ignition temp might be lower than the melted wax temp - just a guess.