How exactly is density an intensive property?

[September24]

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This concept seems to confuse me a bit.

Density is an intensive property so it doesn't change depending on quantity. But look at EK Physics 526

A rigid container holds air (p=1.3) at 1 atm. If pressure is increased to 2 atm by adding air, what is the density of the air?

Answer is:2.6

I understand that air is added so mass increases...so density increases (in other words. PV=nrt....pressure and moles are proportional).

This makes sense, but since density changed on account of a changed "quantity", how is density an intensive property?

 

[QuantumJ]

Density being an intensive property means that the ratio of mass/volume stays constant. So mass can change but volume will change accordingly to keep density constant.

 

[September24]

Thanks for the help. That sort of makes sense. Plus, looking back at the question, it says that a "rigid container" is used. That means that mass is added at a constant volume which means density has to change.

 

[BigBoss]

Thanks for the help. That sort of makes sense. Plus, looking back at the question, it says that a "rigid container" is used. That means that mass is added at a constant volume which means density has to change.

Intensive and Extensive properties only apply when adding together two systems with identical thermodynamic properties or when looking at a system by itself in a steady state.

Examples:
1. You have two balloons with volumes of 1 L and and pressures at 1 atm and you combine them. When combined, the volume will double but the pressure will remain at 1 atm. (In your question, both the container and the source of additional pressure have different thermodynamic properties and thus, these rules don't apply).

2. You have a balloon with with a volume of 1 L and 1 atm. You reduce the volume of the balloon to 0.5 L by letting out air, but the pressure at this volume is still 1 atm. (This examples applies to the rules above because you're looking at one system by itself, without the interference of another system with different thermodynamic properties).