Why is water not flammable?

[noagname]

why is water not flammable, because H20 is hydrogen and oxygen. I have learned that hydrogen is very flammable and oxygen speeds up a fire. so wouldn't that mean that it is very flammable

[mgb_phys]

It is already burnt, the hydrogen has burnt in the oxygen to make water.
The molecule H2O means that the hydrogen and oxygen have combined - it's very different from a mixture of oxygen and hydrogen.

[mrnecho]

mgb_phys is right

You can think of it in terms of energy: hydrogen and oxygen gases are at a higher potential energy level than water. THAT change in energy level is what is released (in the form of explosion/fire etc.) when going from (2)H2 + O2 -> (2)H20 (burning hydrogen in presence of oxygen to form water.

Since water is at a lower energy level there isn't any more energy that can be released by "burning" it.

[russ_watters]

Water is like the ash that is left in the fireplace (and soot lining the chimney) after wood is burned.

[Borek]

Water is like the ash that is left in the fireplace

Agreed.

and soot lining the chimney

Disagreed. Soot burns pretty well.

[russ_watters]

Disagreed. Soot burns pretty well.
Is soot unburned particles carried away? I guess I figured soot was like airborne ash.

[mgb_phys]

I said that but edited it - I thought somebody would say that soot is unburnt fuel.

[Borek]

Is soot unburned particles carried away? I guess I figured soot was like airborne ash.

Soot is mostly product of wood distillation - ie when you heat up wood it gets degassed. These gases are in general highly flammable. Some of them have low molar mass (like CO and methanol), some are much heavier. Depending on the situation they will be either burn in the flame - or not. If they were not burnt, they form tar, which covers inside of the chimney. It dries there (and it doesn't mean just loosing water, but loosing many low molecular mass, volatile substances). What is left is soot.

If you burn coal situation is very similar, although there are not so many low molecular mass products, so there is much less soot produced, and it is much drier (less tar-like).

As I wrote before, soot is higly flammable. This is one of the reasons why you need to clean the chimneys. If soot starts to burn temperature in the chimney can go very high, much higher then during normal exploatation. That's dangerous. It is very difficult to put down burning soot, as you usually have no access to chimney, and just closing the air access at the chimney bottom is not enough; strange things can happen. I have seen several soot fires. The most spectacular one ended with chimney erupting with clouds of sparks and pieces of burning soot every 30 seconds for about 10 minutes. Evidently it was burning without air access, periodically sucking air through the chimney top.

[Simon_Moore]

why is water not flammable, because H20 is hydrogen and oxygen. I have learned that hydrogen is very flammable and oxygen speeds up a fire. so wouldn't that mean that it is very flammable

To put it simply: hydrogen is very flammable when it is in the form of hydrogen molecules, and oxygen speeds up a fire when it is in the form of oxygen molecules. But when the two are combined in the chemical compound water H20 then the molecules are neither of hydrogen, nor of oxygen, but of a different thing altogether (i.e. water), so they take on the properties of water rather than those of molecular hydrogen or molecular oxygen.

Elements combined as compounds always perform differently from the consituent elements. For example, calcium compounds are found in certain foods (e.g. milk) and are good for teeth. The element calcium is a metal - probably not ideal for eating !