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Name: Jeff S.
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 5/28/2003


Question:
When you blow at a candle or small flame what makes it go out? Please give me all the details!


Replies:
Jeff,

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 681


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!!!

Vince Calder


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 itself.

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.

Tim Mooney


Jeff,

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.

Thanks for using NEWTON!

Ric Rupnik



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