Good ol' H 2 O - a pathetic science question!

I know how this will sound to a lot of people, but since water is made up of good ol' Hydrogen and oxygen, why on earth does it not combust? Other than the fact that "water is wet, stupid!", I'm just interested in learning the real scientific reason behind water.

Is this a homework assignment? Reason why I ask is I received the exact same one in grade 11 Chem way back.

[edit on 8-12-2006 by sardion2000]

Originally posted by Catfish

I know how this will sound to a lot of people, but since water is made up of good ol' Hydrogen and oxygen, why on earth does it not combust? Other than the fact that "water is wet, stupid!

", I'm just interested in learning the real scientific reason behind water.

Because water is results from burning hydrogen with oxygen. Water in essence is the ash.

By definition, any time a chemical substance reacts with oxygen it is said to be burning. Therefore, in a rocket engine using hydrogen and oxygen as fuel, the hydrogen is what is burning, since it is reacting with the oxygen. In this reaction, the only product is water vapor. So the water doesn't burn, but the hydrogen does.

www.madsci.org...

No it's not homework, I'm genuinely curious about it. Hydrogen is extremely combustable, and since it requires oxygen to ignite, I can't seem to understand why I can't blow up cups of water

Just curious is all. Coincidence.

Anyways, for something to burn, you first need a fuel and an oxidizer.

In many cases, the oxidizer is O2 but not always.

The other requirement for something to burn is that when the fuel and oxidizer combine, they have to form a product, or products, which are more energetically stable than the starting compound(s). Graphite will burn because CO2 is more energetically stable than Graphite and Oxygen are on their own. Helium won't burn because any compound it could possibly form would be far less stable than any mixture of Helium and an oxidizer. Water doesn't 'burn' in the classic sense because water and free oxygen is far more stable than any compound those two could create together. However, that doesn't mean that water can't burn.

Add some fluorine gas to water and you will see water burn. This is because the product formed when fluorine and water react is far more energetically stable than pure water and pure fluorine alone. So in order for things to 'burn', you need three things;

1) An oxidizer.
2) A fuel.
3) Product(s) which is/are more energetically stable than the reactant(s).

[edit on 8-12-2006 by sardion2000]

Thanks for the heads up, good reading

Originally posted by Catfish
No it's not homework, I'm genuinely curious about it. Hydrogen is extremely combustable, and since it requires oxygen to ignite, I can't seem to understand why I can't blow up cups of water

When you burn Hydrogen gas, you generate water. Water is the product of burned hydrogen, not the precursor.

An analogous question is why can't you burn the ash your fireplace.