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Name: Jennifer
Status: educator
Grade: 6-8
Location: AR
Country: N/A
Date: December 2006

Question:
How is hydrogen gas and oxygen gas different from water vapor?



Replies:
Hydrogen gas in nature is present as the diatomic (2 atoms) molecule, H2. Likewise oxygen gas in nature is present as the diatomic molecule O2. On the other hand, water consists of two hydrogen atoms chemically bonded to an oxygen atom, that is: H2O. These are very different chemical substances.

Vince Calder


In water vapor, the hydrogen and oxygen are still bound together as water even though it is invisible and fills the air. That is one molecule of oxygen is still bound to two of hydrogen. This water vapor becomes visible as fog or the liquid that condenses on the outside of a cold glass of iced tea or your windshield in the mornings.

Gaseous hydrogen and oxygen are pure. That is, each molecule is composed of only hydrogen or oxygen atoms. These two gasses can be combined to form water. See a chemistry lab manual for an experiment you can do to demonstrate this to your students.

Another difference is that water vapor is not explosive where the pure gasses can put on a real show when exposed to flame.

Bob Avakian
OSU - Okmulgee


First it "kaboomb"s, then it is water vapor. One "kaboomb" is the difference. H2 gas + O2 gas has more potential energy than H2O gas. Think of electrolyzing water (H2O->H2+O2) as prying apart two strong magnets. They make quite a clashing event when they get back together again.

H2 and O2 can be distilled apart. Liquid nitrogen temperature, 77K, would do it. O2 at one atmosphere would liquify in a cold bowl having LN2 under it, but the H2 would stay a gas and could be piped away into a separate container. You can't distill part H2O. It is all one "substance", made of only one kind of molecule. So at a given temperature, it will all take the same state, solid, liquid, or gas.

Dry gasses like H2 and O2 have a very different flavor to a scientist, than does a commonly interactive substance like water (H2O).

Water is stickier than air and other light gasses. Sticks to itself, sticks to other things. H2 and O2 are dry gasses that must be cooled far below room temperature to be liquid, and not much of them bothers to dissolve in any room-temperature liquid either. You can pressurize a balloon with 100% H2, or 100% O2, and it inflates, because being hundreds of degrees above boiling, they stay gasses.

Water, on the other hand:

- is liquid till 80 degrees C hotter than room temp.

- dissolves to >2% in many solids, and >50% in many liquids.

- has noticeable adsorbed layers on solid bodies, tens or hundreds of molecules thick, even when the object seems dry.

- only does a few % in air at room temp. If you put a little more vapor into a balloon, it goes and adds itself to the liquid water on the walls, and the balloon will never inflate.

Water vapor, by going to and from the liquid state, often transports heat in the "heat-pipe" fashion: absorbing heat and evaporating, drifting across as gas, condensing and releasing heat, and wicking as liquid right back to the starting point.

So your jacket won't keep you warm if it's wet, which it will be if has been permeated with dense-enough water vapor. H2 and/or O2 gasses do not have this effect. They are just like nice dry air in that regard.

Water is a polar molecule, but H2 and O2 are non-polar. Polar means it has a (+) end and a (-) end, and can be spun around by an electric field. So water vapor (steam) in a microwave oven absorbs some (2.4GHz) microwaves and gets hot. H2 and O2 absorb microwaves too, but at different frequencies and generally more weakly. Really difficult to heat up dry air in a microwave oven. Moist air is not too difficult.

The balanced reaction is H2 + H2 + O2 --> H2O + H2O + heat. You can see there are 3 molecules on the left, and 2 molecules on the right. So once the heat is taken away, and if all the H2O stays vapor, it's possible that some balloon would be 1/3 smaller at a given pressure and less buoyant in a given surrounding atmosphere.

An X2 molecule (eg H2, O2, N2, F2, I2) is mechanically simpler than H2O. An X2 molecule cannot spin around it's long (atom-to-atom) axis, if it did, all that would be revolving is an electron cloud having almost no mass.

Just about all the mass is in the nuclei, which are infinitesimal points right on that axis of spin. So H2 and O2 molecules can't use that axis to have kinetic energy. So they pick up less kinetic energy in a sea of mechanical chaos (heat), So they take less energy to be rattled up to a given temperature. It is called specific heat, and it is lower for an H2 or O2 molecule than for an H2O molecule. The V-shape of the H2O molecule can also wiggle, called angular modes of vibration. That adds to specific heat too. X2 molecules have only the stretch-mode of vibration.

Jim Swenson


Jennifer,

Water vapor is still made up of water molecules which are composed of 2 hydrogen atoms *and* 1 oxygen atom bonded together per molecule. Hydrogen gas is composed of the hydrogen molecule which has 2 hydrogen atoms bonded together per molecule (no oxygen atoms), and oxygen gas is composed of the oxygen molecule which has 2 oxygen atoms bonded together per molecule (no hydrogen atoms). As such these gases all act differently. For example, water vapor when cooled to below 100C at 1 atm of pressure will revert to liquid water. Oxygen gas, on the other hand would need to be brought down to -118C in order to liquify it.

Greg (Roberto Gregorius)



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