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My textbook, Atkins' Physical Chemistry, 11th edition, by Atkins, de Paula, and Keeler, says the following:

Although it is good practice to use only SI units, there will be occasions where accepted practice is so deeply rooted that physical quantities are expressed using other, non-SI units. By international convention, all physical quantities are represented by oblique (sloping) letters (for instance, $m$ for mass); units are given in roman (upright) letters (for instance m for metre).

I'm confused about what the difference here is between "physical quantities" and "units"?

I would appreciate it if people would please take the time to clarify this.

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    $\begingroup$ I think that passage explains it pretty well, but to give another example energy is a physical quantity that can be expressed using different units like Joules and calories. Physical quantities describe some property of an object, units are used to express a particular amount of that property. $\endgroup$
    – Tyberius ♦
    Feb 18, 2020 at 13:40
  • $\begingroup$ @Tyberius So how does the different uses of $m$ and m in the textbook excerpt relate to this? $\endgroup$
    – The Pointer
    Feb 18, 2020 at 13:42
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    $\begingroup$ That's basically just telling you how to read expressions they write in the book. If you see $F=ma$, you know that m is referring to mass. If you see $1600$ m, it's talking about that many meters, not that number times mass. $\endgroup$
    – Tyberius ♦
    Feb 18, 2020 at 13:44
  • $\begingroup$ @Tyberius Oh, so that's what they were referring to. It wasn't even clear to me until you used the $F = ma$ example. I think this could have been expressed more clearly; they managed to make such a simple point into something much vaguer than reasonable. Anyway, thank you for the clarification. $\endgroup$
    – The Pointer
    Feb 18, 2020 at 13:45

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Mass is a physical quantity. Its unit may be gram or kilogram. Volume is a physical quantity. Its unit may be the liter or the cubic meter

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  • $\begingroup$ Yes, it wasn't clear to me what they meant until Tyberius clarified it with his example in the comments. $\endgroup$
    – The Pointer
    Feb 18, 2020 at 14:05

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