# Why is pressure an intensive property?

#### AAMAIK

Here is how I thought about it
Consider a surface on which atoms bump into, and if I increase the number of atoms and at the same time allow the surface area to increase as well the pressure is still the same because these atoms occupy have size and thus occupy a certain area , and If they are distributed equally and their size remains fixed namely (made from the same substance).
I also thought to myself that If I consider a rigid tank with a valve that allows mass to enter, then as mass flows into the rigid tank, pressure must increase. this led to me thinking that in classifying of whether a certain property is intensive or extensive I should not put constraints such as my previous example that volume normally would increase as well and in that way pressure would remain the same.

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#### HallsofIvy

Homework Helper
Are you clear on the definitions of "intensive" and "extensive"? A property is "intensive" if it can be defined at each point on a surface or each point of a three dimensional region. It is "extensive" if it must be defined over a surface or three dimensional region. The total force on a surface is "extensive", the pressure, the "force per cc" at each point on the surface, is "intensive".

#### AAMAIK

Are you clear on the definitions of "intensive" and "extensive"? A property is "intensive" if it can be defined at each point on a surface or each point of a three dimensional region. It is "extensive" if it must be defined over a surface or three dimensional region. The total force on a surface is "extensive", the pressure, the "force per cc" at each point on the surface, is "intensive".
Based on the definition given in the lecture notes I am studying from, an intensive property is independent of mass and an extensive property varies with mass. The definitions you gave for intensive and extensive properties are more reasonable to me.

#### mjc123

The definition in your notes is a poor one, because it does not specify what else changes when you change the mass. Implicitly it is assuming that the substance is in the same state (defined by P, Vm, T), and you just have "more of the same". If you have twice the mass in twice the volume, you have the same density and pressure. If you double the mass in the same volume (halving Vm), you double the density and pressure, but you have a different thermodynamic state.

#### 256bits

Gold Member
The definition in your notes is a poor one, because it does not specify what else changes when you change the mass. Implicitly it is assuming that the substance is in the same state (defined by P, Vm, T), and you just have "more of the same". If you have twice the mass in twice the volume, you have the same density and pressure. If you double the mass in the same volume (halving Vm), you double the density and pressure, but you have a different thermodynamic state.
The definition in the notes is the usual one given out and is correct. for any thermodynamic course.
An intensive property does not change due to the amount of matter present. They can be considered as any bulk property of the material, such as temperature, density, color to name a few, or any other property than can be used to describe a particular item without changing the item.
An extensive property does. Examples here would be weight, volume, mass.

"Why is pressure an intensive property?"

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