Why don't wet things burn?

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Why don't wet things burn?
« on: 18/04/2007 05:23:34 »
Sounds simple, but if i soak a towel in water and then hold it over a flame it does not burn! Water is Hydrogen and oxygen, so why does it not burn?


Offline Karen W.

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Why don't wet things burn?
« Reply #1 on: 18/04/2007 08:27:52 »
CAUSE IT'S WET??? LOL LOL...crimany does hydrogen burn, You mean same as in hydrogen bomb..?? Isn't hydrogen just water,and water has oxygen..? I don't get it! I know oxygen can be explosive, maybe the combination kind of cancels each other out.. I really never thought about that before.. now I am curious!
« Last Edit: 18/04/2007 08:35:46 by Karen W. »

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Why don't wet things burn?
« Reply #2 on: 18/04/2007 10:48:31 »
OK, firstly, hydrogen does burn, but water is the combustion product of hydrogen in air - so water is already burnt hydrogen.  That explains why water does not burn in air, but does not explain the answer to the question, which is why water will help prevent something else from burning.

There are two reasons why water will help suppress the burning of other materials - both are to do with steam.

Firstly, as the water boils off, it creates steam, so that steam displaces some of the oxygen around, so less oxygen available for burning, will partially suffocate the flame.

But the more significant effect is one of direct colling, so reducing the local temperature to below the ignition temperature of the combustible material (assuming the combustible material has an ignition temperature in air of above 100 centigrade, this being the temperature at which water boils - and most things do have an ignition temperature above this).

If you remember the discussion about compressing water, one of the aspects discussed was the latent heat as water solidified to ice - so you could keep drawing energy out of the water but the temperature would not fall until all the water had turned to ice.

The same is true in the other direction - if you heat ice, you can keep pumping energy into the ice, but its temperature will not rise above 0 centigrade until all the ice had melted.  You have the same things again when you go from water to steam - you can keep pumping ever more energy into the water, but its temperature will not rise above the boiling temperature of water until all of the water has been converted to steam.  This is why when you are boiling something on the stove, you can never actually raise the cooking temperature of the food you are boiling above 100 centigrade (at least, no until you allow it to boil dry) - the exception being when boiling in a pressure cooker, since water boils at a higher temperature as you increase the pressure, so the temperature you can reach in a pressure cooker can be much higher.


Offline DrDick

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Why don't wet things burn?
« Reply #3 on: 18/04/2007 16:42:13 »
A cool demo to demonstrate this ability is to take about a 2:1 mixture of rubbing alcohol and water, soak a paper towel in it and light the paper towel.  You'll notice that the paper towel won't burn.  The alcohol in the mixture will burn, but the water will keep the temperature low enough that the paper won't ignite (at least until the water evaporates sufficiently).

As with any experiment of this sort, remember to take appropriate safety precautions.  In other words, don't freak out and drop the paper towel while it's burning.  Kids, make sure there's a responsible adult nearby.  A fire extinguisher wouldn't hurt, either, just in case.


P.S., This would be a good kitchen science segment, if it hasn't been done already.