Density of Gases

Hey Alex,

One of the questions asked: which of the following is true concerning a gas? The answer was Gas molecules generally move so fast and for such short distances that the change in their velocities due to gravity has a negligible effect on their behavior.

For this question, think about what gravity would do to each individual gas molecule -- it's trying to pull them down towards the ground.  How much of an effect will this have?  Well, the amount of time between collisions in the gas particles (and with walls of the container) is really really small (probably on the order of thousandths of seconds... at most).  Over than incredibly short time, how much change in velocity will these particles really experience?  Honestly, very little.  This should also just make intuitive sense to you -- air doesn't tend to "settle" in a room; granted, there is turbulence and all sorts of other factors, but the idea still remains.  Ultimately, the effect gravity has on these particles has basically as much impact as things like small eddys and currents that may occur from heat differences throughout the gas, etc.  Not much of an effect overall.

But then a question later asked: which of the following phases experiences the greatest change in density due to gravity: The answer was gas.  I'm not sure I understand this, I know gas is the most compressible, but isn't gravity supposed to not affect it that much? 

Understandably, this is somewhat hard to reconcile with the previous question.  But the question doesn't say which one has a significant change in density, just which has the most of the various phases.  Since gases are the most compressible, they'll have the largest change in density (but the absolute change here will be on the order of like 10^-20 -- I made up that number, but hopefully you get the idea).

Also..I think I confuse how to understand density because I know density of an object is an intrinsic property... then why is it (another answer to a question) that if you have two rooms connected by a closed door and you open the door - the mass and energy change but not density and pressure? I thought that you would have to use rho = mass/vol. And the answer says that density is an intrinsic property of gas... but that implies that density won't ever change won't it? I guess my question is.. when do we think of density as an intrinsic property and when can we think of it as a changing variable in a problem?

The definition of the word "intrinsic" in physics means something that does not depend on the amount of material you have.  In other words, 1 L of a gas with a density of 1 gram per cubic meter has the same density as 100 L of that same gas.  Other intrinsic properties include molarity, molality, specific heat, etc.  An extrinsic property is one that depends on amount, such as mass, volume, or heat capacity.  Thus, density is an intrinsic property but you can always change it.  Basically, "intrinsic" does not mean the same thing in physics as it does in common English usage.