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Does the term “substance” refer to any sample of matter that is tangible? In other words is “substance” a macroscopic concept or can we also refer to individual particles such as atoms, molecules, etc. as substances?

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It depends on the context

"Substance" is not a tightly defined term so the argument is largely linguistic not about chemistry. Unless, that is, you are talking about observable chemical properties and use the word. Then, the context of use matters and you might need to be careful in how the word is used.

With gases it doesn't much matter how much stuff you have as the individual molecules don't interact much. But for solids and liquids when you talk about "substance" the apparent chemical properties may depend on how many molecules or atoms you have. So you might need to be careful how many of the components are present. The reason why I say this is that some collections of small numbers of components have different properties to larger collections.

Nanoparticles of gold, for example, have different properties to bulk metallic gold. Some show much stronger colours that metallic gold (eg small colloidal gold clusters are used to impart a red colour to some specialty glass products). So the small clusters have different properties to bulk gold and you don't see the bulk properties until the amount of gold is large enough.

So although the substance of both bulk gold and colloidal clusters of gold atoms might be said to be the same, it is probably better to be more specific and talk about colloidal clusters as being a different "substance" to bulk gold. But this isn't universally agreed as usage of substance is not precisely defined.

In short, if you are talking purely about what something is made from then the amount doesn't matter and you can say substance for any amount but if you are talkingg about the properties of the material it can be better to be specific as the properties do change with the amount of the components involved.

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  • $\begingroup$ So if I were to say that “sodium” is a reactive substance would I be referring to sodium atoms or sodium metal? $\endgroup$
    – user70490
    Commented Nov 27, 2018 at 20:25
  • $\begingroup$ @Interestinggg For sodium it would make no difference. Pure metals contain atoms not molecules of their element so, in reactions, act like they react one atom at a time in simple reactions. $\endgroup$
    – matt_black
    Commented Nov 27, 2018 at 21:16
  • $\begingroup$ Alright so elements are substances “made up of one type of atom”. And sodium is an element. So why can we say that sodium “has one valence electron” when sodium is a substance not a type of atom? $\endgroup$
    – user70490
    Commented Nov 28, 2018 at 7:43
  • $\begingroup$ @Interestinggg Context. And note that just because sodium is a substance doesn't mean we can't talk about sodium atoms. Sodium is both a type of atom and a substance. If we talk valence electrons the context tests us we are talking about atoms; if we talk "soft, reactive metal" we are talking about the bulk substance. $\endgroup$
    – matt_black
    Commented Nov 28, 2018 at 10:06
  • $\begingroup$ Could you just explain what the “made up of one type of atom” thing means. This would clear a lot of confusion. Regards. $\endgroup$
    – user70490
    Commented Nov 28, 2018 at 12:38

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