I Do things burn because of the flame or the heat?

Interesting question. Probably my answer will not be the best you can get but what I believe will happen is that yes the cactus will catch fire in a room of 500C. The only difference is that in the room of 500C the cactus will start burning everywhere, while if you use a flame the cactus will only catch fire locally at the point the flame touches it.

 

I'll agree with delta. I just tried it on a heated stove with paper. I think it's heat that burns things. In a 500 degree room the cactus will probably burst into flames in a matter of seconds (all the molecules in the cactus are excited). While a flame will only burn a specific area of a cactus (the molecules closest to the flames are excited).

 

I can't disagree with anything here as I know nothing about this. I just ponder your question.

What is a flame here? Could it be a jet of air heated electrically to 500^oC? In that case wouldn't a whole room full of air at the same temperature have more heat to raise the temperature of the cactus to 500^oC and be more likely to ignite it?

Do you think that the flame having some active burning actually has something more than temperature that it can pass on to the cactus?
It might depend on what part of the flame touches the cactus. I'd guess a living flame has significant variation in temperature. The unburnt fuel & air mixture would be cool, then getting warmed by the flame to combustion temperature, it starts to burn and gets rapidly hotter until it has all reacted. Then it will start to cool as heat is lost to the surroundings and the incoming unburnt gas.
I'd have thought the trouble with the hottest part of the flame might be that there was no oxygen left to burn the cactus. So you could use it to heat the cactus, then you'd have to move it away to let surrounding air get to the cactus. Then the cactus would have to heat up that air and cool down itself a bit. I wonder if the cactus burns when it is at 500 ^oC and the air is still 20^oC, or whether you need both air and cactus to be over 500^oC? But the jet of electrically heated air would have the advantage that it still contained lots of oxygen.

I don't know much about cacti (catuses?) but they seem a funny thing to burn. I thought they were full of water. So would you have to wait until the water had all boiled away before the cactus could warm above 500^oC and catch fire. Some things, when you light them, they burn for a bit and then go out. Damp wood for example. So I wonder if water is the problem? The wood gets heated by a flame, say, then catches fire and burns a bit. That presumably makes more heat to heat up more of the wood, so that it catches fire too, in a sort of chain reaction. But if there is a lot of water in the wood, the heat generated is used up boiling off the water and there is not enough left to get the next bit of wood hot enough to catch fire. So it goes out once you get away from the part of the wood that was heated and dried by the flame. Do cactuses generate so much heat when they burn, that they can boil the water and still heat the next bit of cactus to 500^oC? In the hot room, maybe the mass of hot air would be drying all of the cactus ready to be lit. (I suppose there must be something keeping the air in the room hot.)

But coming back to your original point, maybe a flame is hotter than air even at the same temperature? A flame glows (emits light?) but the air in a room doesn't. (Though I've never seen a room at 500^oC. Perhaps that glows like a flame as well? Would air that is red or blue hot qualify as a flame, even though it isn't burning?) Air molecules however hot aren't reacting like molecules in a flame. Perhaps reacting molecules are more effective at starting fire than non-reacting molecules at the same temperature. They must be more energetic at the moment of reaction, as they then collide with surrounding molecules and spread the energy out. Perhaps if you put the reacting part of the flame to the cactus, it is a lot hotter than the flame as a whole.

 

Auto-ignition, fire point, flash point.
A reading of some base information.
https://en.wikipedia.org/wiki/Autoignition_temperature
https://en.wikipedia.org/wiki/Fire_point
https://en.wikipedia.org/wiki/Flash_point

If a solid combustable material is heated, it gives off gases. It is these gases that will do the burning, or make a flame.

 

If you think a little, the answer is in your question. What do you think that statement above (bold) even mean? It has nothing in it about flame or any other way to reach these 500 degrees. If the cactus reaches 500 C the violent reaction with oxygen (which we call burning) will start, right?
This is what "auto-ignition temperature" usually means. The temperature at what the material starts to burn without the presence of a flame or spark.

However, if you mean something else by that first statement, then the answer may be different, for sure. If you use a flame hot enough to ignite the material, then the initial temperature of either material or the medium are not relevant even though they may influence how long it will take to ignite.

 

Nasu, do you imply that in the presence of a flame or spark a material can be ignited even if the temperature of the flame is much lower than the auto-ignition temperature?

 

No, I did not say such a thing.
I specifically said that the material can be ignited if the flame is hot enough (hotter than ignition temperature) and that the initial (before the flame is brought near the material) temperature of the medium or of the material could be below the ignition temperature.

 

The heat. There's a classic demonstration that proves it: take some copper tubing, tightly wrap some paper around one end, and then place a lighter flame under the paper. The paper wont ignite for quite a while because the copper conducts the heat away too quickly.

 

Of course, the paper has to reach Fahrenheit 451 in order to ignite.
And you can burn paper or cloth with the light from the Sun and a lens, no flame, no spark.

 

The only time that a 'flame' would cause combustion at a lower temperature than 'just' heating, would be if a product of combustion in the applied flame happened to act as a catalyst. But that's not a level playing field.

 

There are times when the soot in the flame will assist the conduction of heat, creating a local hot spot that will begin fuming (and therefore flaming) sooner than the rest of the material.
For example, attempting to burn sugar with hot air or focused sunlight is slower than with a slightly sooty flame.

But I don't suppose that cactus is white.