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Based on the following definitions, can anyone explain why volume is an extensive property?

Extensive Property: Properties that depend on the amount of matter in a sample.

Volume: the amount of space an object occupies.

It has clearly been shown that volume depends on the AMOUNT OF SPACE and not the AMOUNT OF MATTER (that is mass!). So why do textbooks describe volume as being an extensive property?

Note: I do not believe it is an intensive property either, such as color. I am just wondering why it is labeled an extensive property?

iceguru
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  • Welcome to Chemistry.SE! Please take a minute to check out the help center and tour page to better understand our guidelines and question policies. – A.K. Sep 03 '18 at 02:08
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    Think of it this way. Extensive properties are like price: you take twice as much, you pay twice as much. This is clearly the case with volume. – Ivan Neretin Sep 03 '18 at 06:18
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    The definition is very short, imprecise and incomplete actually imo. An extensive property is directly proportional to he amount of matter, at constant conditions. Twice the mass, twice the volume. – Karl Sep 03 '18 at 08:30
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    "It has clearly been shown that volume..." this has not been shown, it's is the definition of volume. If two definitions clash, then one of them must be illogical. Or in this case, misunderstood. ;-) – Karl Sep 03 '18 at 08:34
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    I'm voting to close this question as off-topic because it's about elementary logic applied to physics. – Mithoron Sep 03 '18 at 18:45

1 Answers1

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For solids and liquids volume occupied is strictly proportional to mass

Liquids and solids occupy volume. This volume is proportional to the amount of substance (or mass). The volume is determined by the mass and the constant of proportionality is the density which is specific to the substance. Density, mass and volume are all directly related for a given substance (and could be thought of as being different ways to measure the same thing). Therefore all are extensive properties.

This isn't true for gases as they fill whatever container they are in and the volume is not a measure of how much stuff of the substance there is.

matt_black
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    If we maintain a constant external pressure, however, then extensivity holds for gases also. – a-cyclohexane-molecule Sep 03 '18 at 12:03
  • @a-cyclohexane-molecule But, of course, the volume of a gas at constant pressure depends on the number of moles of the particles but not the mass of the particles. – matt_black Sep 03 '18 at 12:29
  • Only for ideal gases. In real gases, volume depends on more factors, but it is still true that volume is proportional to mass for a given gas under certain $P$, $T$ conditions. –  Sep 03 '18 at 14:28