[Jesse Owens]

Hello!
I've been struggling for a while now with producing the B gas which is pretty soluble in water.
By thermal decomposion of A, h2o(g) and B(g) are produced. In our words: A(s)---> B(g) + h2o(g)
(I do not want to use specific names since this is a general topic)

My goal is to eliminate water from our equation. I've used the following sheme:
http://shrani.si/f/42/mt/4stzWk8I/wwwqwq.jpg
The problem is the backflow. Water is being sucked into pipe and therefore the reaction is not possible, because i do not want to heat water. I would be very thankful if a more skillful chemist advised me how to prevent the backflow. I'm also interested in any other method that dries the gas and not contamins it.

[Arkcon]

You can just collect the gas in an open tube -- open end up if the gas is denser than air, open end down if the gas is lighter than air.  In either case, the gas displaces the air, and you have a tube full for further analysis -- sniff test, moist litmus paper, or what have you.  If you need to quantitate the gas, you can use a gas syringe to hold and measure it.  But if you need pure, clean, dry gas for another reaction, you might as well just buy a tank.

[Jesse Owens]

Thanks for replying. Gas is denser than air.
Well let me think at loud:  if I attach gas syringe on a tube and heat the reactant A(s), the product B(g) and h2o(g) is made. Water will condens and fall back on the bottom of the tube and the process repeats its self. I conclude that this does not dries the gas.

[Arkcon]

That won't really dry it, no.  You will need a drying tube: https://secure.drierite.com/catalog3/page7.cfm?

(For some reason the Drierite corp. insists on using secure http for this page.  But it has a better picture than the Wikipedia page, so I went with it)

[Jesse Owens]

Thank you! I didn't even know such stuff exists.
Now I just have to look it up in my country.

[curiouscat]

Thank you! I didn't even know such stuff exists.
Now I just have to look it up in my country.

Even if you don't find a pre-packaged product you can make one yourself by packing a tube with anhydrous calcium sulfate. Should be cheap and easily accessible. You might also be able to use silica gel or Calcium Chloride or some such.

[ajkoer]

Here is a chemical approach. Consider Chlorine and water. The reaction is:

 Cl2 (g) + H2O <--> HOCl + H(+) + Cl(-)

So, if you are collecting the Cl2 over an acidic solution, the equilibrium is moved to the left. In fact, the solubility of Chlorine in water is reported as poor in neutral to acidic conditions. I would then use CaCl2 to dry the gas.