Ask A Scientist Chemistry Archive

name         Will F.
status       educator
age          50s

Question -   One of my students asked me under what conditions hydrogen gas and oxygen gas can 
come together to form water.  In particular, he wanted to know the exact process by which 
these two gases combine to form a totally different substance in the liquid state.  Can you 
help me out here?
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That is the whole thing about rearranging atoms - the new substances ALWAYS have new properties 
compared to the original substances. In this case with new molecules there is a ton of hydrogen
 bonding that pulls the molecules into a liquid state at moderate temps. No such bonding in the 
 original gases since there is no unequal sharing of electrons. In the water molecule,
oxygen hogs the electrons which creates + and - charges across the molecule.  And this creates 
H-bonding between water molecules.

Lou Harnisch
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Dear Will,

If you fill a balloon with H2 and pop it in the atmosphere with a cold needle, nothing much 
happens (except the balloon pops). However, if you pop it with a hot needle, you get a loud 
"bang" as the highly exothermic reaction

H2 + O2 ---> H2O

occurs. This reaction is so slow at room temperature that it does not happen measurably when 
you just mix the gases together. However, add a little extra energy and.... you have the 
Hindenberg (which was filled with hydrogen gas).

It is hard to go into much more detail without knowing whether your students are K-5, 6-8 or 
9-12 students. But the short
answer is, it happens when the H2 and O2 are combined in the presence of a little applied 
heat.

Dr. Topper
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Hydrogen burns in the presence of oxygen.   The combustion product is water vapor.

To get liquid water you must cool it below the dew point.

Greg Bradburn
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