Gases at different pressures reaching uniform pressure First of all, sorry if my question is dumb, I have very little understanding of physics. Also, English is not my native language.
If two gases, initially with different pressures, are mixed in a closed vessel, do they (the mixture being treated as a single gas) generally reach uniform pressure?
Now, atmospheric pressure reduces with altitude. However, how can the air that is above exert pressure on the air below without them reaching the same density? How can it be that the less dense air pushes the denser air and the system is stable?
 A: The total pressure of the gas in a closed mixture is the sum of the so called partial pressures of the gasses: the pressure of each gas in that container if it were alone.
In a container that you could have in a lab, the differences in gravity in all the points are neglegible, so you can assume the pressure is constant. But, if the vessel is big enough and the masses different, the heavier gasses will preferentially accumulate on the lower parts. How big is "big enough" depends on the mass difference between gases and the temperature.
If you want to see it experimentally, mix vinegar and baking soda (Na2CO3) in a big glass to generate CO2, and pour the gas (not the liquid) over a candle flame from above. The CO2 is denser than air, so it will fall over the candle replacing the oxygen and extinguishing the fire.
If you go to the top of a mountain, the gravity starts to be notably different. The molecules at sea level are being pulled downwards by gravity more strongly than the ones over the mountain, and thus, they get closer to each other, so the pressure is higher. 
