physics

[ikemen]

the burning point of a piece of paper is 450 degrees cel. (from what i read, but i could be wrong)

imagine if you heat some water vapor to the same temperature and fan it towards the paper, would it light up or just get wet?

 

[Douglas]

Originally Posted by ikemen

the burning point of a piece of paper is 450 degrees cel. (from what i read)

what was that fahrenheit 451 book on about then?

 

[Thomas]

the paper would burn

 

[MrG]

The piece of paper would turn into a magnet.

 

[Thomas]

Originally Posted by MrG

The piece of paper would turn into a magnet.

which, also would burn - and because of the third law of thermodynamics (heat induction), so would the oven in which you placed the paper magnet.

 

[Biggskip]

Outside of a very tightly controlled environment (say inside of a steam engine), wouldn't it be hard to heat water vapor above the point where it becomes steam?

 

[Thomas]

Originally Posted by Biggskip

Outside of a very tightly controlled environment (say inside of a steam engine), wouldn't it be hard to heat water vapor above the point where it becomes steam?

Actually, the water vapor becomes magnetic as well.

(real answer, steam can be heated above 100C (so can water, actually, depending on pressure and/or superheating) - but the big reason that water tends to quell fires is 1: temperature and b: smothering effect, so when the water is at the same temp at which paper burns, then the temperature effect is mitigated, and because it's a vapor the smothering effect is also lost)

 

[dcg]

Originally Posted by Douglas

what was that fahrenheit 451 book on about then?

Bradbury thought "Fahrenheit 451" sounded better than "Celsius 451".

 

[Mr Herbert]

Originally Posted by ikemen

the burning point of a piece of paper is 450 degrees cel. (from what i read, but i could be wrong)

imagine if you heat some water vapor to the same temperature and fan it towards the paper, would it light up or just get wet?

Paper would not burn.

Superheated steam would cool before it reached the paper unless surrounding air temperature was also 450F. If this was the case the paper would have burnt already anyway.

If you were relying on the steam to heat the paper and the air to the auto-ignition point then the paper would be smothered by condensing steam.

I assume this hypothetical situation is occuring in a pressure vessel at 420psi?

If you are asking if paper will burn in an atmosphere with steam vapour in it, the answer is, it depends how much steam vapour (or more to the point, how much oxygen compared to steam).

 

[Huntsman]

Originally Posted by ikemen

the burning point of a piece of paper is 450 degrees cel. (from what i read, but i could be wrong) imagine if you heat some water vapor to the same temperature and fan it towards the paper, would it light up or just get wet?
Your hypo violates the physics you want an answer to. But the idea can be answered this way -- water has no magic fire-quelling abilities (though water can, admittedly, absorb incredible quanities of heat energy), it is just a substance that works well to put out fires that occur in our atmosphere. The main problem with the hypo is that even if you had a substance that ignited at a temperature at which steam can exist at regular pressures, you'd pretty much heat the substance to that temp in the environment before you could get the steam to it. If it was cooler than the steam, the water would condense upon it, cooling it and not igniting it. So it's more a question that it's just to impractical for a variety of reasons than that water cannot ignite a substance. Now if you put a piece of paper in a vessel with some steam, close it, and start heating it -- now that is a more interesting problem that will probably come down to whether you have enough vapor in the vessel to lower the oxygen content below that required to sustain combustion of the paper. If you do, it won't ignite. If you don't, it will. ~ H

 

[brown eyes]

it'll probably just get wet.

 

[tagutcow]

The temperature at which something burns is not necessarily the temperature at which something must burn.

Heat an oven to 450° and hold your hand inside-- it doesn't burn. Touch the metal interior of the oven and your hand does burn, even though the temperature of the metal and the temperature of the air are the same. Temperature isn't the same thing as heat, brah.

 

[Smartalox]

The 450 degree figure is the ignition temperature for paper in air.

To burn anything, there must be the appropriate balance of three things:

- oxygen
- (thermal) energy to initiate the reaction
- fuel

paper will ignite at temperatures lower than 450 degrees, if the oxygen content is elevated. Air has an oxygen content of 21%. The rest is nitrogen 78% (doesn't cause things to burn), CO2 (also doesn't cause things to burn), and trace gasses.

Liquid Oxygen is extremely cold (-180C) but if you pour it on a book (or meat, or metal, or hydrocarbon grease, for that matter) the paper will ignite and burn, because when the liquid O2 touches the paper, it's a 100% Oxygen environment in the immediate area of the contact.

In a vessel with saturated water vapour, there is not enough free oxygen to support combustion, so the paper won't burn.

When water is sprayed on a fire, the enormous energy required to the liquid water droplets to steam deprives the fire of the thermal energy needed to sustain combustion, and displaces some of the oxygen too. So the fire goes out. Ever tried to light a wet sponge on fire?