# Heavier mass of gas

1. Jan 1, 2013

### rach27

Ok, so the other day I was chatting with a friend of mine, and this doubt came up...
It is my understanding that hot air rises over cold air in the atmosphere because, being hotter, it is less dense and, hence, lighter.
However, if we consider air as an ideal gas, doesn't that mean that the attraction between the molecules is negligible? If so, we could not really say that a mass of air is 'heavier' than another one, since gravity would pull on each of the molecules individually - as opposed to a solid object, where the molecules pull on each other and we can abstract their weight into a 'center of mass'.... Here each molecule is on its own, so how would it matter if a certain portion of air is less dense than another one?

One possibility I imagined, discarding the 'heavier' or 'lighter' explanation, is that hotter air molecules have more kinetic energy so it takes a greater force to pull them down, and that because of this they tend to rise... But it is a guess mostly, and I have heard the 'heavier' gas explanation a lot of times, is there a way that it comes to terms with there being no attraction between the molecules?

Please do say if I have not been clear and I will attempt to rephrase it better.
I hope it is the right section to ask the question, bare with me please since I am new to the forum. Oh, and thanks in advance to anyone stopping by on this one.

2. Jan 1, 2013

### Staff: Mentor

It's not.

The ideal gas approximation breaks down if there are sharp temperature gradients within the gas, so if you want to see ideal gas behavior, you have to allow the temperature (and kinetic energy - consider just how non-ideal the boundary between a jet engine exhaust and the air around it is) to reach equilibrium first.

3. Jan 1, 2013

### rach27

So in this case, then, there is attraction between the molecules, and that is why we may speak of masses of air?

4. Jan 1, 2013

### jim hardy

Are you sure you're using term "mass" correctly ?

Mass isn't the only measure .
if one mass of air occupies more volume than the other (equal)masses around it, it will be bouyed upward. Look up Archimedes principle.

5. Jan 1, 2013

### rach27

Sorry, here I was using 'mass' as in a given set of air molecules.

I think that cleared it up. Somehow it was strange to me that the density of something that is not a solid would have meaning (being that we could just define an arbitrary boundary of volume of which to calculate the density)... But it makes sense now that simply the less dense portions of air get a greater upward force than their weight.

Thank you both!

6. Jan 2, 2013

### SteamKing

Staff Emeritus
Even liquids have been known to have a density.