How to think about extensive and intensive properties?

[tellmesomething]

https://www.physicsforums.com/threads/confusion-over-intensive-properties-thermo.585648/post-3809940

Can someone explain this explanation given by Rap. Its an old thread so i cannot comment and ask him directly that is if he's still around.

 

[Borek]

Which part of the post you don't understand, it seems reasonably straightforward.

 

[Mayhem]

Intensive properties are consistent for a population or a sample, extensive properties are only consistent for the sample.

For example, if I have 1 L of 1 M hydrochloric acid (population) and I take out 500 mL (half), my sample will still have the concentration of 1 M. Therefore, it is an intensive property.

If I instead consider the amount of substance, my sample of 500 mL will only contain half the substance of the population.

You can replace population/sample with whole/part.

 

[tellmesomething]

Intensive properties are consistent for a population or a sample, extensive properties are only consistent for the sample.

For example, if I have 1 L of 1 M hydrochloric acid (population) and I take out 500 mL (half), my sample will still have the concentration of 1 M. Therefore, it is an intensive property.

If I instead consider the amount of substance, my sample of 500 mL will only contain half the substance of the population.

You can replace population/sample with whole/part.

Thankyou!! But can you expand on the pressure example why can I not think of it as the moles being doubled in a container whose volume is constant? I.e increasing pressure by increasing mass.

Or to consider it to be a valid property like you mentioned I would have to compare the pressure of a part of the gas sample with the whole gas sample?

 

[Borek]

Not every property classifies as intrinsically extensive or intensive.

Mass - obviously extensive.

Density (edit: of liquid or solid) - obviously intensive.

Length? Well, you can add some restrictions so that the length becomes an extensive property (say "length of a gauge 32 wire made of material"), but in general length is impossible to classify. Similarly with pressure - you can add restrictions to the setup that will make the pressure an intensive property (when you split tank in half, the pressure is identical in each part of the tank), but you can also make a setup in which pressure becomes an extensive property (adding more ideal gas to the tank increases the pressure linearly, so the value depends on the amount of material).

 

[tellmesomething]

Not every property classifies as intrinsically extensive or intensive.

Mass - obviously extensive.

Density - obviously intensive.

Length? Well, you can add some restrictions so that the length becomes an extensive property (say "length of a gauge 32 wire made of material"), but in general length is impossible to classify. Similarly with pressure - you can add restrictions to the setup that will make the pressure an intensive property (when you split tank in half, the pressure is identical in each part of the tank), but you can also make a setup in which pressure becomes an extensive property (adding more ideal gas to the tank increases the pressure linearly, so the value depends on the amount of material).

But textbooks ask questions like state the extensive and intensive properties among so & so options which includes pressure. In that case...?

 

[Mayhem]

Pressure is dependent on temperature (for gases, at least) and temperature is a statistic of average kinetic energy of a particle. If the temperature is uniform in your container, then so is the pressure. Therefore, any part of such a gas should have the same pressure if partitioned into a smaller container.

 

[Borek]

But textbooks ask questions like state the extensive and intensive properties among so & so options which includes pressure. In that case...?

In that case you need to take into account the description of the case, as the answer can be either intensive or extensive, depending on the setup.

If there is no precise description the question is poorly asked (and most likely expects you to answer "intensive", even if in general that's not always the correct answer).

 

[tellmesomething]

Pressure is dependent on temperature (for gases, at least) and temperature is a statistic of average kinetic energy of a particle. If the temperature is uniform in your container, then so is the pressure. Therefore, any part of such a gas should have the same pressure if partitioned into a smaller container.

I see okay... Thankyou!

 

[tellmesomething]

In that case you need to take into account the description of the case, as the answer can be either intensive or extensive, depending on the setup.

If there is no precise description the question is poorly asked (and most likely expects you to answer "intensive", even if in general that's not always the correct answer).

Okay Thankyou!