Blowing Out Candles
Name: Jeff S.
When you blow at a candle or small flame what makes it go out? Please give me all the
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.
Any fire, a candle is a specific example, requires three components to burn: 1. A fuel (candle wax). 2.
Oxygen and 3. Sufficient heat for the fuel to reach its ignition temperature (every substance has such
an ignition temperature) and sufficient heat to keep the temperature of the fuel above its ignition
temperature. It is a bit more complicated than these three. For example, you can easily light a pile
of saw dust with a match, but would have a hard time getting a log of the same wood to begin to burn.
And in the case of a candle there needs to be sufficient heat from the match (technically called the
"ignition source") to melt a bit of the wax. But for
the purpose of your inquiry let's just ignore those "details". When you blow at a candle you remove
heat away from the flame. This
reduces the temperature of the wick below the ignition temperature, and to a lesser extent causes the
wax to solidify so that it can't move up the wick to the combustion zone, that is just above the wick.
That is also the reason you can extinguish a candle with your thumb and finger. You "squish" the fuel
going up the wick, interrupting the flow of fuel and cool the ignition zone since your fingers absorb
sufficient heat to lower the combustion zone below the ignition temperature. You do not get burned
because your fingers are pretty massive compared to the flame, and so can absorb a lot of heat without
burning you. You can also make a candle go out by placing a glass over the flame. It burns the
oxygen in the glass but then it no longer has sufficient oxygen to burn so the flame goes out. Do
not use a Polystyrene foam cup to test this!!!! Finally, the burning flame temperature of a candle is not
much hotter than the ignition temperature, so that helps. So for example, you would not want to try
to extinguish a propane torch with your fingers!!!
When you blow out a candle, what you are actually doing is moving the flame away from the candle, so
that it cannot make more burnable fuel.
Although it may appear that the candle, or the wick, is burning, what actually burns is a gas composed
of the smaller molecules that make up wax. These molecules are released from heated wax just as steam
is released from heated water, while heavier molecules -- which might also burn if they were heated to
a higher temperature -- remain liquid and drip down the candle. As gas
is released from the wax, it combines with oxygen and the chemical reaction gives off heat and light.
The heat from this burning gas causes more gas to be driven from the wax, so the fire perpetuates
But if you move the heat from the vicinity of the wax, no more gas will be driven off. When you blow
out a candle, the stream of air from your mouth pushes the burning gas far enough away from the candle
that its heat is not able to raise the wax's temperature enough to drive off any gas.
Another thing happens at the same time: the stream of air brings oxygen to the flame more quickly that
it had been arriving. This oxygen speeds up the combustion, and depletes the supply of burnable gas in
the vicinity of the flame.
The requirements for fire are fuel, oxygen, heat, and source of ignition. Once the flame is burning,
take away the fuel, oxygen, or heat and the flame is extinguished. Blowing at the flame removes the
heat and successfully extinguishes it. (Note that if a large fire is burning, blowing on the flames
to put out the fire is ineffective because the amount of heat present overwhelms the minuscule breath
attempting to put out the flame. In this case, supplying water in sufficient quantity will remove
both heat and oxygen and effectively put out the fire.
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Update: June 2012