ELI5: If fire likes wind and oxygen to keep burning, why is blowing on a candle enough to extinguish it?

r/

I assume it has something to do with the size of the fire, but in what way?

Comments

  1. preparingtodie Avatar

    When you blow on a candle, you’re cooling it down, essentially blowing the heat away. When the wax is cooled enough, the flame goes out. If the fire is bigger, it takes a lot more effort to cool it down.

  2. capricioustrilium Avatar

    You know how one piece of fudge is good, but eating a box of fudge makes you sick? It’s kind of like that.

  3. graveybrains Avatar

    A fire needs fuel, air, and heat to burn. By blowing on it you take away the heat.

  4. jamcdonald120 Avatar

    for fire you need 3 things. Fuel, Oxygen and HEAT.

    When you blown on a tiny flame you are forcing in A LOT of cold (relative to the fire’s needed temperature) air and displacing the hot flame (the part you can see is just gas/plasma hot enough to see).

    If the fire is small enough, this can extinguish it.

    Its tempting to say it also removes Oxygen, but exhaled breath has a lot of oxygen in it still (this is how Rescue Breathing works at all), and if you are careful you can use your breath to kindle a fire if you blow lightly on a spark.

  5. f50c13t1 Avatar

    This is because blowing off a candle does more than providing oxygen.

    Blowing on a candle disrupts the temperature it needs for the combusion reaction to keep running, that is, you push away the hot gases it needs for its thermal equilibrium.
    You also separate the flame from its fuel (the vaporized wax), so the flame temporarily runs out of fuel if you will.

    Finally, there’s also a cooling effect happening since your breath is cooler than the flame’s temperature. By cooling the burning wax vapor below its ignition requirements, you disrupt/prevent the combusion reaction.

  6. Sylivin Avatar

    In fire safety they like to point out what’s called the fire triangle. For a fire to continue it needs 3 things – heat, oxygen, and fuel. Take away any one of these and the fire goes out. A candle wick is very small and a sharp, intense blast of air will reduce the heat enough to prevent combustion.

  7. Derangedberger Avatar

    Blowing on things cools them down. This is because heat is the jiggling of molecules. When there’s a hot object, it has heated the air around it, cooling slightly in the process. Though, that means the air molecules around it are moving faster, so it’s harder for them to accept more heat, limiting heat loss.

    When you blow, you are removing all of those fast air molecules from around the hot object and replacing them with new, slow ones. This allows more heat to be sucked from the hot object into the air. The temperature drops rapidly, and a tiny candle wick doesn’t produce enough heat to offset the loss and keep the flame lit.

  8. Vorthod Avatar

    The oxygen needs to heat up and bind with the carbon and hydrogen in the fuel.

    Candles ignite at about 500F, your breath is at most 98.6F, so if a ton of air is moving passed the candle faster than it can heat up by 400 degrees, then there’s not really any oxygen being added that can actually participate in the reaction. In fact, the size of the air stream from your mouth covers the entire wick, so you’re also blocking off any normal air that would’ve normally been there to combust. You’re also cooling the wick down the more it tries to keep going

    A bigger fire is less vulnerable to this because it has air coming in from a much larger area, so it’s much more difficult to block it all out. It also has more total heat energy, so it’s harder to cool down with a little wind. and a hypothetical massive air stream that could block everything else would still have to traverse the entire width of the flame without heating all the way up in order to stop the reaction.

  9. NegativeSuspect Avatar

    Fire needs 3 elements to keeping burning – fuel, heat and oxygen. When you blow on a candle, you’re 1) cooling the wick to a point where it isn’t hot enough & 2) removing vaporized wax from around the wick.

    So you’re removing two of the parts required to maintain the fire, which extinguishes the candle.

  10. wdaloz Avatar

    You can blow on coals to help push oxygen in because they have enough heat to keep going.

    Also a lot of people are commenting you remove the heat but you’re also partly removing the fuel- when a candle burns it’s not actually burning the wick or the wax directly, it’s melting and vaporizing the wax, and the vapors are what burn. So the effects are 3 fold:

    1. you’re removing heat to keep the flame hot enough to keep the vapor ignited

    2. you’re blowing away some of the vapors

    3. you’re removing some of the heat that’s vaporizing the wax, so the wax can’t vaporize and then it can’t directly burn

  11. BurnOutBrighter6 Avatar

    The fire triangle – fire needs fuel, oxygen, and heat. All 3 are essential. Any 1 missing = no fire.

    Blowing on a candle removes heat, flame stops.

    Bonus fact: Blowing on or fanning a bigger fire helps by adding oxygen more than it hurts by removing heat. The bigger the fire the more wind needed to cross from making the fire bigger to blowing it out.

    Additional bonus-bonus: you can put out even big fires using explosives! If you set off a bomb like C4, all the nitrogen gas it makes pushes the oxygen away. Fire triangle: no oxygen = no fire. They use this method for oil and gas well fires since no one has to be close if you just blow up the fire..

    Bonus bonus bonus: the USSR put even out oil well fires with nuclear bombs, because of course they did.

  12. whiteb8917 Avatar

    You can snuff a match out with wet fingers, as long as you can remove one of the three things to keep it “alive”, Fuel, Oxygen, and heat.

    On a candle, you induce colder air from your breath to reduce the heat, heat is removed, flame dies.

    In the right circumstances, fire can outrun a vehicle. There has been cases of bush fire in Australia (and most likely in Los Angeles recently) of the fire exceeding 100 Kilometers an hour.

    There was even a case, in the fire of Kings cross station, where the fire generated an effect that had NEVER been seen before, called the “Trench” effect. The fire started in the escalator, and the sides of the escalator acted as an incubator for the fire, but instead of the flames raising straight up (Heat rises right), the flames actually laid down, and hugged the escalator, super heating the air ahead of it, causing it to flash and rush up the stairs, straight in to the ticket hall, consuming EVERYTHING.

    https://youtu.be/PBbJXvJYeq0?t=2268

  13. Absentmindedgenius Avatar

    Fire happens with gasses. It may seem like liquids are burning, but it’s just the gasses coming from the liquid. Weirdly enough, burning wood is also the gasses coming from the wood. If the burning gasses are blown away and replaced with air, the fire stops.

  14. YGoxen Avatar

    You’re drinking water right? No problem. What if some firefighter guy use high pressure water from nozzle? Can you drink it?

  15. LyndinTheAwesome Avatar

    Its the wax thats burning, or more precisely the wax gets vaporized and the vapors burn.

    If you blow hard enough you blow away the vapors and the fire has no fuel for a brief second and extingiushes.

  16. jukkakamala Avatar

    At Iraq war (or was it Iran, i always mix them up) people leaving lit the oil wells on fire.

    Problem is the pressure of oil, you just can not “shut it off” so you must first extinguish the fire.

    So they blew burning oil wells off. With EXPLOSIVES.

    Against what people think no, explosives did not use all the oxygen and that put the fire out.

    They just blew the heat out of the equation.

    Explosives are explosives because they contain all the ingredients to make a fast burning experience.

    The burning stuff, own oxygen and the detonator gives the heat needed to start the show.

    Thats why guns and dynamite works under water and works in space too. They bring their own oxygen.

  17. EarthDwellant Avatar

    DAE think blowing hard on a small flame is nearly instinctual, maybe hard coded into our genes? Or just because of birthdays?

  18. Osato Avatar

    Chemistry-wise, solids and liquids aren’t fuel for a flame. Flammable gases are. You turn a solid such as wax or wood into flammable gases by heating it up a lot.

    Case in point: sufficiently cold gasoline (doesn’t evaporate into gas all that much) won’t catch fire even with a very hot spark, whereas sufficiently hot wax (boils into a lot of gas) will catch fire if you aren’t careful.

    When you blow hard enough on a candle’s flame, you remove all of the flammable gas around the wick and cool it down a little. Without the flammable gas, which is the actual fuel for the flame, the fire cannot continue.

    There are some exceptions to this (coke, magnesium or thermite, which burn without turning into gases), but they burn without a flame and you can’t put them out by blowing on them.

  19. kindanormle Avatar

    If you need water to live, why can you drown in it?

  20. Wadsworth_McStumpy Avatar

    Fire needs oxygen, fuel, and heat. Once it’s started, it usually provides its own heat, but if you blow on a small flame, you can push enough of the heat off it to make it go out.

    Technically you could blow out any size fire if you blow hard enough. They use explosives to put out oil well fires that way. That also takes away oxygen, though, which is sort of cheating.

  21. MXXIV666 Avatar

    The answers about making it cold are kinda misleading, even if technically mostly correct.

    When a candle burns, what burns is evaporated wax. That’s why the wick barely burns, it’s loosing heat to evaporating wax. Just like you do to water if you’re wet.

    The burning wax vapor evaporates more wax. If you blow on it, you blow away the source of heat. Wax will still evaporate for a bit, but will not be hot enough to burn.

    And by the way, if you blow it very slightly, just eenoughto put it out, you can re-lightit with a match without touching it.

  22. Zvenigora Avatar

    A candle flame is homogeneous combustion: it is a mixture of wax vapor and air which burns when ignited. But not just any fuel/air ratio will burn: if the mixture is too rich (too much fuel) or too lean (too much air) it will not burn. Blowing floods the wick area with air and makes the mixture too lean, causing the flame to go out.

    This will not work with burning charcoal because that is heterogeneous combustion (the fuel remains solid until reacting) and there is no flame as such.