Do gases of different molecular weights separate like liquids would? Mainly I'm thinking of gases like Cl2 and H2. Would the H2 gas form a "layer" on top that I could release from a container?

 

I ask because I found when electrolyzing salt water it can produce Cl2 and H2, and also that when these two are exposed to sunlight and each other they can "explode to form HCl." I don't want an unsupervised explosion...

In the exampke you use, hydrogen and chlorine would be far easier to separate if they were cooled to liquid. Chlorine boils at a not too chilly -34.6 degrees C, so could be easily separated from a mixture with hydrogen in this way.Hydrogen boils at something like 20K (or -253 C) so would safely remain in gaseous form throughout the separation.

 

Dry ice should do it.

With gases, they mix kind of like two similar liquids mix. You can have two non-polar solvents of different densities, but if you mix them together they will form one mixture which will have the same density. Over a long period of time without any movement, then the two gases may separate but not to any really appreciable extent.

Indeed, as Jdurg states, gases, once mixed, will not separate anymore. It is good that they do not, otherwise we would have a bad time here on earth with the mix of gases in the atmosphere .

 

If you perform electrolysis, then assure that the H2 and Cl2 are collected in different bottles. Never let them bubble into the same container. If you start correctly, then you'll not have the problem of separation at all.