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Friday, January 2, 2009

Why does water not burn?

(My Original Blog Post:
Mike from Salem writes:

If water is composed of both Hydrogen and Oxygen, both explosive elements, why is water not only not explosive but fire retardant as well?

Hi Mike, thanks for the question.

That is a very good question.  That type of molecular chemistry is a bit out of my expertise, but I'll answer it as to how I understand it.

Hydrogen exists normally in what's called a diatomic state.  That is, it likes to pair up with another hydrogen atom. In this state, it's quite inert.

However, when energy is added, such as a spark or a flame, the following happens:

H2 --->(spark)----> 2H

H, or H* (* represents the excited state) is now an energetic and can react with a number of things.

Then you add oxygen to this excited hydrogen, and the following happens:

H* + O2 ---> HO* + O*; O* + H2 ---> HO* + H*; HO* + H2 ---> H2O + H*

As you can see the end result of these equations is more H* atoms. The H* atom will find more oxygen and repeat the cycle until all the immediate available hydrogen is used up.  This is a chain reaction and why hydrogen and oxygen burn.

That reaction also produces water.  In the absence of a spark, H2 and O2 can produce water without any burning reactions like so:

2 H2 + 02 = 2 H2O

Water does not burn because the hydrogen and oxygen are bound together and must be separated first before the first reactions I showed you above can happen. In order to break those bonds, a lot of energy is required, much more than a spark or open flame is able to deliver.

When you throw water on a flame then, it denies the flame oxygen and that is why it stops burning.

I hope this answers your question.  If you would like further clarification or have another question please do not hesitate to contact me at any time

0 keen observations: