Re: Why does combining two gases produce a liquid?

Good question! To begin to answer that question, we should talk about what 
the difference between a gas and a liquid (and a solid, for that matter) 
really is. In fact, many substances (such as water) can exist as a solid, 
a liquid, OR a gas. For instance, when it's REALLY cold outside, the pond 
that you may swim in during the summer turns as hard as a rock! Does this 
mean that the water went away? NO. All that happened is the little tiny 
bits of water (called molecules) slowed down a LOT (because they got cold) 
and eventually stuck together! The opposite of this happens when someone 
puts some (liquid) water on the stove to boil for tea. When the water gets 
hot enough, you can see steam coming out, and the water really starts to 
bubble. That is because the water molecules have gotten so hot that they 
are literally bouncing off of each other, and escaping in the form of a 
gas! So you see, ice, "water", and steam are ALL water, just really hot, 
really cold, or just right! Also, oxygen and hydrogen are both gases at 
temperatures that YOU would find comfortable (usually called room 
temperature). However, if you got EITHER of them cold enough, they would 
eventually CONDENSE to form a liquid! So you see, a lot of things can 
exist as either a liquid or a gas. Water can be a gas if you heat it up to 
a boil, and oxygen can be a liquid when you REALLY cool it down (to more 
than 300 degrees Farenheit below freezing)! It just depends on how you 
look at it!

Good Luck,
Dr. Scott Miller
Air Force Research Laboratory