Isn't water vapor a gas?
Anyway, a more serious answer is that it's likely because water molecules are rather polar while most of what they're probably talking about when using the word "gases" are non-polar..
That tends to have a strong influence on how molecules interact.
Last edited by HenrikOlsen; 2012-Jul-16 at 10:12 PM.
Reason: waPor, not valor. Spell check for the lose.
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