The same volume of oxygen gas weighs 16 times more than hydrogen gas. When the hydrogen is burnt to produce water vapour, it does so at a weight ratio of 1:8. 2 volumes of hydrogen are reacted with 1 volume of oxygen to produce 2 volumes of water vapour. That's 2 volumes of hydrogen with the atomic weight of 2 being added to one volume of oxygen with an atomic weight of 16. That should give us a total atomic weight of 18. But how is this possible if the weight ratio is now 1:8 - which means the total is actually 9?
I suspect that the hydrogen gas atom shrinks to half its size when it becomes joined to oxygen in water vapour. Therefore the 2 volumes of water vapour are made up with 1 volume of hydrogen and 1 volume of oxygen. The volumes are of equal measure in water vapour.
Normally, as two seperate gases, oxygen gas weighs 16 times more than hydrogen gas. In water vapour it is one volume of hydrogen joined to one volume of oxygen. In water vapour one volume of oxygen now weighs only eight times more than the hydrogen. Or, to put it another way, one volume of hydrogen as part of a water molecule, now weighs twice as much as one volume of simple hydrogen gas.
This is the thing with density though. It's almost illusory. I could say that the oxygen atom now weighs only eight times more than hydrogen, therefore making hydrogen more dense. Or I could say that one hydrogen atom now weighs only half that of an oxygen atom, therefore giving hydrogen half its previous density. It reminds me of that drawing where you see either an old woman, or a glamorous lady, but never both at the same time. It just depends on how you look at it.
If we add the weight ratio of 1:8 together it gives us a total of 9. I think this might make the atomic weight of water vapour 9. I know I'm going over old ground. I'm repeating myself. But writing this over and over helps me get to grips with it. I'm probably brainwashing myself. Is it the case of the Emperor's new clothes? Am I the boy in the crowd? Or the guy in the buff?
Does the hydrogen atom in a water molecule get heavier, or lighter? An atom of hydrogen gas has a much greater volume than a water molecule. Water vapour occupies a space some 1700 times greater than liquid water. If we were to break down the steam into its component gases of oxygen and hydrogen, the hydrogen gas would still occupy a volume 1700 times greater than liquid water. The oxygen gas which we have seperated will occupy a volume that is half that. The oxygen would have a volume 850 times more than liquid water.
A hydrogen gas balloon is known to rise upwards. Simple hydrogen gas is more buoyant than air. The water out at sea seems to pretty much remain where it is, unless it's ice which floats on the surface, or water vapour which rises completely up into the air. Steam from my kettle goes up. Where does it go after that?
My thanks for the picture: