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Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide. Also, our breath does not cool the flame as it is itself warm. So what is happening here?

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The answers are already spot on. I would only add a comment: our breath is warm relative to air (just like our body temperature is usually higher than that of the surrounding environment), but is much cooler than the flame. Proof: you can burn yourself if you touch the flame and not if you breath on your hand. So that part of your reasoning is not relevant for the question at hand. – Francesco Mar 14 '14 at 13:41
You might be amused by this video, which shows how blowing out a candle scales up to larger fires. – Eric Lippert Mar 14 '14 at 15:08
Our breath is considerably cooler than the flame itself, so yes, it does cool the flame (as well as separating the flame from its fuel source, as described below). Any air or gas that's not as hot as the flame is going to cool it. How significant that effect is in putting out the fire is another matter. – Phil Perry Mar 14 '14 at 16:37
@Raja: the temperature required to cool a candle is ~1400 °C or less... – Mooing Duck Mar 14 '14 at 20:20
@HenningMakholm, water that is dissolved in air is not the same thing as water in the gaseous state. I am pretty sure it still absorbs heat of vaporisation to raise its temperature over 100c, but even if it doesn't, it still has considerable specific heat, so warm, moist air still will absorb more heat from a source than warm dry air. – psusi Mar 15 '14 at 23:01
up vote 56 down vote accepted

You blow away the flame from its fuel source. If you would blow less hard the flame might burn harder because more air is supplied to the flame (similar to a Bunsen burner). Because normally the flame of a candle gets its oxygen through a convectional airflow generated by the heat of the flame.

The reason why the flame is blown away from the candle is because the air you blow towards it moves faster than the speed of the flame front. So the air you blow at it moves the flame away from its fuel source, where the flame burns out due to the lack of fuel.

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This is a newtonian physics way of looking at it, ignoring thermodynamics. Heat is what determines the speed of the flame front. If the reaction were more exothermic, then it would have a faster flame front. Really what you are doing is both diluting the fuel/air mix, and lowering the temperature below the ignition threshold. – psusi Mar 14 '14 at 19:35
What is flame? It is hot air area. – Anixx Mar 15 '14 at 8:08
@Anixx I assume you are asking what the flame front is? The name is a bit misleading since it would imply it is a surface. However it is actually referring to an volume in which lots of chemical reactions take place, since burning fuel with oxygen does not take place in one simple chemical formula but is lots of different sub-reactions. I am not an expert on this, but you could say that a candle has multiple flame fronts (or one very thick one). – fibonatic Mar 15 '14 at 12:32
@psusi Maybe you are right and that it is a combination of both. But also keep in mind that the fuel/air mix also affects flame front speed, this is also why the flame does not get closer to the wick in "steady-state". So blowing out a candle might be more complicated than it seems. – fibonatic Mar 15 '14 at 12:43

Combustion is a gas phase reaction. There are two requirements to generate a stable flame. Firstly the temperature must be high enough to vapourise the combustible material (wax in this case), and secondly the temperature must be high enough to generate the activation energy needed for the reaction. Heat is needed because gas phase molecules of wax and molecule of air won't react unless they collide with enough energy to break the interatomic bonds in the molecules.

So when you look at the flame on a candle you are not seeing the solid wax burn. Instead the heat of the flame vapourises the wax then heats the vapour and the hot wax vapour reacts with oxygen to produce the flame.

When you blow on a candle you are simultaneously diluting the wax vapour and cooling it below the point where the wax and air molecules can react. As fibonatic points out in his comment and excellent video wax vapour continues to be produced after the candle is extinguished, but without a source of enough heat to provide the activation energy the reaction won't go. Applying a source of heat can restart the reaction and reignite the flame.

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This is not true, since the smoke of the candle can easily be relight. The reason is that the air you blow towards the flame of the candle moves faster than the flame front can burn the fuel, so it is moved away from the fuel source after which the frame front burns out due to the lack of fuel. – fibonatic Mar 14 '14 at 10:46
@fibonatic: hmm, yes, I've edited my answer in the light of your comment. That's a great video - I must try that :-) – John Rennie Mar 14 '14 at 11:04
@fibonatic: I watched the video and while I understand that you can relight the candle because the wax vapor is still present in the smoke, I don't get why the flame cleanly falls to the candle wick. – user34801 Mar 19 '14 at 7:26
@user34801: the smoke is a dispersion of solid wax particles. When you apply heat you vaporise the nearby wax particles and the vapour can then burn. The flame then vaporises more nearby wax particles so they burn, and the flame spreads. However since it's a small flame the vaporisation of the nearby wax particles is relatively slow so the flame moves only slowly. – John Rennie Mar 19 '14 at 7:30

If you look closely at the candle flame, you will notice that the flame hovers just over the wick, but does not touch it. This is because the flame is boiling the wax, which becomes a vapor, which then burns. All of these processes are driven by the heat from the flame. As you blow on the flame, you moving it away from the wax and disrupt this process. Eventually, the chemical reaction (the flame) moves too far from the fuel source (which is no longer boiling as quickly) to sustain itself.

Note: while the candle wick itself does burn, the vast majority of the light and heat in the flame comes from the burning of the wax.

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As fibonatic noted, you are blowing the flame activity away from the wick, but that's not the entire story: if the wick were still the same temperature it would immediately reignite. You are super-cooling the system by introducing a large mass which can't be heated enough to sustain the fire in time.

This is, incidentally, one of the primary reasons why water is so good at extinguishing fires -- its not just that water can provide a barrier between free oxygen and the fuel (that's after the system has cooled, as vaporized water would simply evacuate itself), its that it is extremely dense and dissipates a vast number of calories in a very short time, robbing the system of its sustainable base energy. The reason this does not work on vapor fires (oil, gas, liquified plastics, etc) is because those systems are usually not carrying enough calories in a solid body to impact and they are extremely volatile (a single spark...). It is possible to put out a gas fire with cool water vapor in a laboratory setting, just extremely unreliable in the real world.

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It's more accurate to say that water has a very high heat capacity (rather than dense). – chase Mar 16 '14 at 9:33
@chase Aha! This is useful to remember -- and, incidentally, is fascinating topic all its own. – zxq9 Mar 16 '14 at 12:04
I thought the difficulty with putting out liquid fires with water is that the water would generally either submerge under the liquid and either sit there uselessly without absorbing much heat (if the liquid was below 212F) or expand in such a fashion as to rapidly distribute hot droplets of flammable liquid. I would think that even dumping enough liquid water into a bucket of burning oil would extinguish it once it had cooled the liquid enough, but that would only be useful if there wasn't any danger of the spewing grease igniting anything else (in which case... – supercat Mar 16 '14 at 20:34
...simply letting the grease fire burn itself out would generally be more practical). – supercat Mar 16 '14 at 20:35

This is because the cold air replaces hot one. Once the temperature on the surface of the fuel drops below critical level, the flame disappears.

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protected by Qmechanic Mar 16 '14 at 18:57

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