Ask A Scientist© Chemistry Archive

 
name       Brian
status     student
age        13

Question - I know this sounds dumb, but if Hydrogen is flammable and
fire uses oxygen to burn then why isn't H2O flammable?

P.S. I have asked many of my friends this question and their best reply
was " Because it's a liquid..." but that's not it, isn't Gasoline
flammable, and gasoline is a liquid
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Because water has hydrogen that has already been "burned".

The energy in the bonds of 2 water molecules (H20) are lower than the bonds
of 2 molecules of hydrogen (H2) and one molecule of oxygen (O2). So when
you bring H2 and O2 together with a tiny bit of energy (a flame or spark
just to start it), it will burn, releasing energy, and making water.

You can make hydrogen and oxygen from water by electrolysis, but that uses
energy, sort of like the reverse of burning.

The burning of gasoline is very similar, the bonds start off with more
energy, which is released on burning with oxygen to make CO2 (carbon
dioxide) and water. However, making gasoline from CO2 and water is
extremely difficult although with enough energy it's theoretically possible.

Don

Donald Yee Ph.D.               San Francisco Estuary Institute
180 Richmond Field Station,     1325 South 46th St. Richmond, CA 94804
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Dear Brian,
I'm not sure how to give you an easy answer,
except to say that knowing what elements are in
a substance (in this case, knowing that hydrogen and
oxygen are both present in water) doesn't tell you
everything you need to know. Every water molecule
is made up of 2 hydrogen atoms, each bonded to
an oxygen atom. This means that water molecules
have very different properties than oxygen or
hydrogen molecules. Chemists say that the hydrogen and
oxygen in h2o are in "different oxidation states"
then they are in hydrogen gas (h2) or oxygen gas (o2)
and are mostly unavailable for combustion.

But you are right; it has nothing
to do with the liquid part, because water can be
a gas (there is plenty of water vapor in the room you're
in right now, unless you live in Nevada or something).

Even knowing that something is made only of
a single element, like carbon,
doesn't tell you much about the properties
of the thing in question.
Carbon can be in the form of diamond, in which case
it is hard and crystalline, or in the form of
graphite, in which case it is soft, black and
powdery, or even in other forms.

I hope this helps.
best regards,
prof. topper
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The problem here is one of definitions.

Literally, oxydation (sic) [oxidation] means the "reaction with oxygen" to 
form an oxide. However, there are a whole bunch of reactions also referred 
to as "oxidation reactions" that don't involve oxygen at all, but rather 
involve an increase in the valence of one of the reactants. One example is 
the explosive reaction of Na with Cl2 producing NaCl. The topic of valence 
is too large to be discussed here, but you can find it discussed in any 
general chemistry text. Look in the index for: Oxidation-Reduction 
reactions, or the acronym [redox] reactions.

Flammable refers to the formation of a flame, which means [usually] a fast 
reaction that gives off visible light and heat (infrared radiation). Often 
these are oxidation reactions, but there are many reactions that produce a 
flame that do not involve oxygen. The reaction of Na mentioned above is an 
example of something being highly flammable that does not involve a 
reaction with oxygen.

There are also many oxidation reactions that occur without the formation 
of a flame -- rusting for example -- the reaction of oxygen and iron 
forming iron oxides.

Neither of these terms have any relation or restriction involving the 
state (gas, liquid, or solid) of the reactants or products.

Vince Calder
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Hydrogen gas is made up of molecules each containing two atoms of hydrogen,
H2. When hydrogen burns, it combines with oxygen to make water, H2O,
according to the overall reaction
2 H2 + O2 -> 2 H2O.
When this happens, a great deal of energy is released as heat. The energy
released is equal to the change in energy between the starting material (2
H2 + O2) and the final product (2 H2O). The H2O isn't flammable because it
is already done burning. Water is the low-energy final product.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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You are very close to answering your own question! When hydrogen burns (in
oxygen), the product of this chemical reaction is, quite simply, water. In
simplistic terms, because this reaction is exothermic (releases heat), the
product (water) requires much heat to reverse the reaction to recover
reactants (hydrogen and oxygen). Since water does not easily release
oxygen, it can be used to smother (deny oxygen to ) a fire. Under
circumstances where water comes in contact with certain active metals, water
can release flammable hydrogen, but it is usually very stable.
Tim Spry
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