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I learned that in thermodynamics, pressure is regarded as an intensive variable, while properties that you can add up such as mass or volume, are considered extensive.

However, to me, Dalton's law seems to contradict this idea. Dalton's law states that the total pressure of a mixture of gases is the sum of the partial pressures of each gas.

Given this, my question is: if the pressure can be added up, why is pressure intensive? What am I understanding wrong? Why is Dalton's law not a valid argument for pressure being an extensive property?

To give a simple example, let's say that I have a "mixture" of two same gases (i.e. oxygen gas), at a total pressure of 1 bar. Now, if I "divide" the gas into two mole fractions and determine their partial pressures, the sum of those partial pressures would also be 1 bar. So clearly, in this example, I'm adding the two pressures to get a larger, total pressure. So, to me, this makes it appear that pressure can be an extensive property, since it can be added up. However, I know that I'm wrong somewhere but don't know where, so can someone explain why my example would not prove that pressure is extensive?

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  • $\begingroup$ If you double the total volume and double the mass of each gas, its partial pressure will not change $\endgroup$ – Chet Miller Sep 5 at 0:44
  • $\begingroup$ @ChetMiller Thanks for the reply; that makes sense. However, I'm curious as to why my reasoning using Dalton's law is invalid. I know pressure is intensive, but I'm having a hard time understanding why Dalton's law makes it appear to be extensive. $\endgroup$ – F16Falcon Sep 5 at 0:57
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    $\begingroup$ The way I see it is that the definition implicitly assumes that when you add the systems you put them next to each other and in contact, but you do not mix them. $\endgroup$ – Wolphram jonny Sep 5 at 1:08
  • $\begingroup$ @Wolphramjonny That's a great way to think about "addition" in extensive systems; your comment helped, thanks! $\endgroup$ – F16Falcon Sep 5 at 2:24
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Given this, my question is: if the pressure can be added up, why is pressure intensive? What am I understanding wrong? Why is Dalton's law not a valid argument for pressure being an extensive property?

I think it's not valid because what is really being "added up" in Dalton's law is the effect of adding the masses of the individual gases that comprise the mixture. The partial pressure of each gas is the pressure that it would exert, for a fixed volume and temperature, if all the other gases were removed. The greater the number of gas molecules (mass) that occupy a fixed volume the greater the number of collisions per unit area of the walls of the container and thus the greater the pressure.

Thus, in my view at least, it is the extensive property of mass that is responsible for the "adding up" of the partial pressures per Dalton's Law, as opposed to pressure itself being an extensive property. We know this has to be the case because if pressure were an extensive property, the pressure a gas in a room would be cut in half if we simply divided the room in half.

Hope this helps.

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  • $\begingroup$ This cleared up my confusion; so when we add partial pressures together, we are basically adding mass and thus creating a completely new "system" with a different state. That makes a lot of sense, thanks a lot! $\endgroup$ – F16Falcon Sep 5 at 2:23
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I think your idea of what extensive and intensive means might be confusing the question a little. Specifically it's this idea of what you mean by "adding up" a property. If you have a balloon full of gas there's a difference between pumping in a second helping of gas (as with your line of thinking using Dalton's Law) and imagining a second, identical balloon of gas appearing out of thin air. With two balloons things like the temperature, density, and pressure stays the same (intrinsic quantities) while things like the number of particles, mass, energy, and entropy double (extrinsic quantities).

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