Pressure - intensive or extensive property?

[Mountaineurons]

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This is more of a clarification question. According to ExamKrackers, Pressure is an intensive property (independent of amount). According to PV=nRT though, if you increase moles 👎 you must increase P, suggesting that pressure is only intensive if volume is NOT fixed.

Do you agree?

 

[phltz]

If you increase the number of moles by cramming more into the same volume, the pressure increases. However, it's more natural to think about taking another equivalent "chunk" of gas, and placing it alongside the original.

Compare to another intrinsic property, mass. If you start with a 1g mass of iron, and add a second gram of iron, do you squeeze that in to the same volume as the original iron? Or do you put another 1g slug of iron on top of/next to the first one?

 

[chiddler]

If you increase the number of moles by cramming more into the same volume, the pressure increases. However, it's more natural to think about taking another equivalent "chunk" of gas, and placing it alongside the original.

Compare to another intrinsic property, mass. If you start with a 1g mass of iron, and add a second gram of iron, do you squeeze that in to the same volume as the original iron? Or do you put another 1g slug of iron on top of/next to the first one?

mass is an extensive property.

i'm reading this.

so it seems that even though you CAN change pressure by changing amount of substance, that you can change it without amount of substance is enough to make it an intensive property. seems like playing with the definition and i don't like it.

can anybody offer a more satisfying explanation?

 

[phltz]

mass is an extensive property.

i'm reading this.

so it seems that even though you CAN change pressure by changing amount of substance, that you can change it without amount of substance is enough to make it an intensive property. seems like playing with the definition and i don't like it.

can anybody offer a more satisfying explanation?

You're right, of course. I meant to say "density", but for some reason I typed "mass". Nonetheless the logic is sound.

 

[Mountaineurons]

mass is an extensive property.

i'm reading this.

so it seems that even though you CAN change pressure by changing amount of substance, that you can change it without amount of substance is enough to make it an intensive property. seems like playing with the definition and i don't like it.

can anybody offer a more satisfying explanation?

Mass is most certainly an extensive property, I'm also confused by phltz's question.

But in any case, chiddler, your definition is interesting but I'm not sure it works because volume is an extensive property and yet you can change it without changing amount of substance - so according to your definition volume would be an intensive property which is false.

As much as the perfectionist in me would like to nail this down, I wonder if it's even relevant for the MCAT.

 

[411309]

Pressure is definitely intensive. I remember in gen chem they taught us whenever you divide two extensive property you always get an intensive one ( i.e. density). P= F/A force and area= extensive

 

[Mountaineurons]

Yup - just trying to figure out why Pressure is intensive, not that it is.

 

[theseeker4]

Yup - just trying to figure out why Pressure is intensive, not that it is.

Imagine a vessel containing n moles of gas. Measure the entire vessel, and pressure is P. What is the pressure of the top third of the vessel? How about the bottom tenth? How about the three molecules of gas in the exact center of the vessel? The pressure in the vessel is always P, independently of how much of the gas you are measuring.

Now take mass, an extensive property. Mass of the gas in the entire vessel is m1, mass of the top third is m2, mass of the bottom tenth is m3, etc. None of them equal one another, therefore mass is extensive, while pressure is intensive.

 

[chiddler]

Imagine a vessel containing n moles of gas. Measure the entire vessel, and pressure is P. What is the pressure of the top third of the vessel? How about the bottom tenth? How about the three molecules of gas in the exact center of the vessel? The pressure in the vessel is always P, independently of how much of the gas you are measuring.

Now take mass, an extensive property. Mass of the gas in the entire vessel is m1, mass of the top third is m2, mass of the bottom tenth is m3, etc. None of them equal one another, therefore mass is extensive, while pressure is intensive.

thank you

 

[umdnjmed]

would density be considered an intensive property when dealing with gases?

 

[milski]

Density is intensive property, regardless of phase.

 

[umdnjmed]

If more gas is forced into a container with a fixed volume wouldn't that lead to a change in density? therefore density of gases would vary based on the number of molecules and would therefore be extensive.

 

[milski]

If more gas is forced into a container with a fixed volume wouldn't that lead to a change in density? therefore density of gases would vary based on the number of molecules and would therefore be extensive.

If it was forced in the same container, yes, that would increase the density but that's not how you determine if a property is extensive or intensive.

You can think about it in two ways:
- take any subpart of the existing system - if you measure that property in the subsystem and it's still the same, it is intensive. Density, pressure, color are some of those.
- put two system in exactly the same state next to each other. Again, if the property does not change, it is intensive.

Forcing the two systems in the same place is not preserving their state and you cannot say anything about intensive/extensive if you start changing the state.

 

[umdnjmed]

If it was forced in the same container, yes, that would increase the density but that's not how you determine if a property is extensive or intensive.

You can think about it in two ways:
- take any subpart of the existing system - if you measure that property in the subsystem and it's still the same, it is intensive. Density, pressure, color are some of those.
- put two system in exactly the same state next to each other. Again, if the property does not change, it is intensive.

Forcing the two systems in the same place is not preserving their state and you cannot say anything about intensive/extensive if you start changing the state.

Thank you 🙂