water vs fire

[COMMUNIST FLISK]

why does water put out fire?
is it that it starves the flames of oxygen?

also why does something damp/wet not catch fire.

and on the same sort of subject, bedsheets/clothes etc can be made flame resistant... how?

 

[Spase]

why does water put out fire?
is it that it starves the flames of oxygen?

also why does something damp/wet not catch fire.

and on the same sort of subject, bedsheets/clothes etc can be made flame resistant... how?

Fire is a chemical reaction where you have some carbon compound absorb energy until it's chemical bonds break and then it forms CO2 and H2O. CO2 and water are extremely stable and therefor release energy when they form as heat. This heat they release is enough to 'burn' more things creating a chain reaction.

One thing to note that a lot of people misunderstand. Breaking bonds absorbs energy, making them releases energy. A reaction releases energy when the reactants go from less stable to more stable.

The reason water puts out fire is it absorbs energy breaking what would otherwise be a chain reaction where the heat output by one reaction fuels the next resulting in a spreading fire. Water happens to be a *very* good means of absorbing energy because of its particular structure. I won't go into detail unless you want me to but take my word for it.

The phase change water undergoes from liquid to gas takes a lot of energy and so when you put water on something burning the energy is absorbed by the water to make steam before it can be used to keep something burning. The reason the water takes precedence is because water boils at a lower temperature than most things light on fire. So... you have to boil off the water before you can keep burning.

I don't really know what they use to make things flame retardant...

 

[COMMUNIST FLISK]

i guess also that waters very high specific heat capacity has something to do with it?

 

[Spase]

Yep, that's the technical wording for, "It absorbs a lot of energy."

well.. I guess what I wrote was it's very good at absorbing energy... but either way. Same thing. Yes, water's specific heat makes it take much less of it (water) to bleed much more heat off a reaction than many other compounds.

 

[ebbixx]

Water can retard fire, it doesn't necessarily stop it. Water retains it's strong binding of oxygen, though, even when it vaporizes. So in most ordinary fires it works by depriving the fire of a source of free oxygen needed to sustain the cascading reaction Spase spells out in his response.

If something is damp, before you can set it burning (by most ordinary means, excluding chemical reactions that may in some cases break the chemical bonds of water molecules) you first would have to drive/ vaporize most of the water, to give the material access to free oxygen in the atmosphere.

Heat a flammable but wet material long enough and you will set it on fire. As long as the water doesn't smother your source, assuming it's an open flame of some kind.

 

[ebbixx]

I don't really know what they use to make things flame retardant...

"It's complicated." And I don't necessarily endorse this source but it seems a good place to start reading.

 

[ebbixx]

For a fire you don't want to put water on see:

 

[mknorman]

One thing to note that a lot of people misunderstand. Breaking bonds absorbs energy, making them releases energy. A reaction releases energy when the reactants go from less stable to more stable.

I'm not sure that this is correct. There are compounds that break down exothermically, as well as some that are formed endothermically. Take, for example, monopropellant rocket fuels.

It is true, however, that reactions release energy as they move to a more stable state. The only question is whether this end state *always* consists of products with fewer bonds than the reactants.

It's been a long time since I've done any chemical equilibrium calculations, so if anyone knows for sure, please chime in!

Edit:

I lose.

Chemical energy is part of all chemical reactions. Energy is needed to break chemical bonds in the starting substances. As new bonds form in the final substances, energy is released. By comparing the chemical energy of the original substances with the chemical energy of the final substances, you can decide if energy is released or absorbed in the overall reaction.

From http://en.wikipedia.org/wiki/Chemical_reaction

 

[Spase]

It is true, however, that reactions release energy as they move to a more stable state. The only question is whether this end state *always* consists of products with fewer bonds than the reactants.

To be clear... You're right that it doesn't always mean fewer bonds. It does always mean a net increase in stability though. Some bonds are much more stable than others. As a general rule the more ionic or strongly polarized a bond is the more energetically stable it is.