Solving the Mystery of "Heavier" Gas Mass

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In summary, the conversation discusses the concept of why hot air rises over cold air in the atmosphere. The initial explanation is that the hot air is less dense and therefore lighter, but the conversation delves into the idea that this explanation may not be entirely accurate for an ideal gas. It is suggested that the ideal gas approximation breaks down in the presence of sharp temperature gradients and that there is actually attraction between the molecules, which can affect the movement of air masses. The concept of mass and density is also discussed, with Archimedes' principle being brought up as a possible explanation for why less dense portions of air can rise. The conversation concludes with a clarification on the use of the term "mass" and a thank you to those involved in the
  • #1
rach27
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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.
 
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  • #2
rach27 said:
However, if we consider air as an ideal gas...

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
So in this case, then, there is attraction between the molecules, and that is why we may speak of masses of air?
 
  • #4
Are you sure you're using term "mass" correctly ?

If so, we could not really say that a mass of air is 'heavier' than another one,
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
jim hardy said:
Are you sure you're using term "mass" correctly ?

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


jim hardy said:
Look up Archimedes principle.

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
Even liquids have been known to have a density.
 

1. What is "heavier" gas mass?

"Heavier" gas mass refers to the concept that some gases are denser or have a greater mass per unit volume than others. This can affect their behavior and interactions with other substances.

2. How is the mass of a gas determined?

The mass of a gas is determined by its density, which is calculated by dividing its mass by its volume. This can be measured using various techniques such as gravimetric analysis or through the ideal gas law equation.

3. What factors contribute to the heaviness of a gas?

The heaviness of a gas can be influenced by several factors, including its molecular weight, temperature, and pressure. Heavier gases tend to have higher molecular weights and can also become denser at lower temperatures and higher pressures.

4. How does the heaviness of a gas affect its behavior?

The heaviness of a gas can impact its behavior in various ways. For example, heavier gases tend to sink in air because they are denser, while lighter gases rise. This can also affect their ability to dissolve in liquids or react with other substances.

5. Why is it important to study the mystery of "heavier" gas mass?

Studying the mystery of "heavier" gas mass is important because it can help us better understand the behavior and properties of different gases. This knowledge can have various applications, from predicting the movement of pollutants in the atmosphere to developing new technologies for gas separation and purification.

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