Ask A Scientist Chemistry Archive

name         LaShondra C.
status       student
age          18

Question -   What causes a candle to blow out? Example: You blow out
the candles on a birthday cake.
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Combustion requires three contributing factors that must all be present
for the combustion to occur.
1. Fuel (in this case candle wax).
2. Oxygen (in this case air).
3. Minimum ignition temperature. These are the three principles that fire
fighters use to extinguish fires. Remove any one and the fire goes out.
     The minimum ignition temperature is the least obvious, but an extreme
example is to place a candle on the birthday cake, and do not "light" it. It
will sit there indefinitely without burning. So if you can remove a
sufficient amount of heat from the flame zone at a sufficient rate, the
flame will extinguish. That, and the removal of fuel are primarily what is
happening when you blow out the candle. You increase the air speed and
remove both heat and vaporized wax from the flame zone depriving the flame
of two of its requirements. However, if  you do not blow hard enough the
candle will re-ignite when you run out of breath.
     In fighting forest fire a spectacularly deadly process occurs where the
rising hot air "sucks" in colder air along the ground and actually "feeds"
the fire with more oxygen without removing the fuel (the forest and brush)
and without being able to cool the fire to reduce the rate of burning. The
result is a "fire storm" in which the fire can move at very high speeds,
pushed by the onrush of incoming air.

Vince Calder
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LaShondra,

In order for the candle to burn it needs an air (oxygen) supply, fuel (the
wax) and enough heat to keep it going. When you blow across the candles on a
birthday cake, you "blow" away the heat faster than it can be generated by
the flame. So. even though you still have a fuel supply and lots of air, the
candle dies because your breath cooled the fuel below its combustion
temperature.

By the way, stay away from candles that re-light after you blow them out. 
If not
handled carefully, they can re-light after you throw them into the trash.
That could cause a disaster

Regards,
ProfHoff 441
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LaShondra,

In order for the flame to continue burning, you need
heat, air (oxygen, specifically) and fuel.  The wax is
the fuel.  Oxygen is a component of the surrounding
air.  The heat is introduced by a match or whatever is
used to "light" the candle; as the candle burns, the
heat produced keeps the candle burning.

If you take away any component the candle will go out.
  When you run out of fuel (wax) the candle has nothing
else to burn. If the oxygen in the surrounding air is
used up, the candle will go out.

The way we normally put out a candle is by blowing on
it. By doing this, we remove the heat necessary to
keep the candle burning and it goes out.

Thanks for using NEWTON!

Ric Rupnik
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