[blokeybloke]

Hey guys I was wondering why chemistry textbooks use substances and species interchangeably. They are not the same thing are they? My knowledge is that “substance” refers to any defined sample of matter with constant composition whereas a species is more like a “type of atom/ion/molecule/formula unit”. So why does my textbook say things like “an acid is a substance that donates a proton”? Wouldn’t it be appropriate to define it on the microscopic scale as a “species” that donates a hydrogen proton? Apologies if this sounds like pedantry but I am genuinely just trying to establish the differences between the two things.

[mjc123]

This is another of those many instances where language is sometimes used loosely because we think on both molecular and macroscopic scales., like saying does "oxygen" mean the gas around us, or an O₂ molecule, or an O atom? Answer is it may mean any of those, according to the context. You need to develop a feeling for context.

An acid may be considered to be a substance that has certain macroscopic properties (e.g. low pH) that correlate with certain properties on the molecular scale (e.g. propensity to donate a proton). 100mL of acid in a beaker doesn't "donate a proton", but it may be corrosive, react exothermically with alkalis etc. But this is all a result of the proton-donating property of its molecules. "Acetic acid" is the name of a substance; we can also talk of the acetic acid molecule. In an equation, CH₃CO₂H can mean a molecule of acetic acid, a mole of acetic acid, or indeed 1 equivalent on any scale.

[blokeybloke]

This is another of those many instances where language is sometimes used loosely because we think on both molecular and macroscopic scales., like saying does "oxygen" mean the gas around us, or an O₂ molecule, or an O atom? Answer is it may mean any of those, according to the context. You need to develop a feeling for context.

An acid may be considered to be a substance that has certain macroscopic properties (e.g. low pH) that correlate with certain properties on the molecular scale (e.g. propensity to donate a proton). 100mL of acid in a beaker doesn't "donate a proton", but it may be corrosive, react exothermically with alkalis etc. But this is all a result of the proton-donating property of its molecules. "Acetic acid" is the name of a substance; we can also talk of the acetic acid molecule. In an equation, CH₃CO₂H can mean a molecule of acetic acid, a mole of acetic acid, or indeed 1 equivalent on any scale.

Hey there thanks for the reponse. Although in any case it would still be more correct to say that “an acid is a substance whose constituent molecules are able to donate a hydrogen proton” right?

[-_-zzzz]

Hey guys I was wondering why chemistry textbooks use substances and species interchangeably. They are not the same thing are they? My knowledge is that “substance” refers to any defined sample of matter with constant composition whereas a species is more like a “type of atom/ion/molecule/formula unit”. So why does my textbook say things like “an acid is a substance that donates a proton”? Wouldn’t it be appropriate to define it on the microscopic scale as a “species” that donates a hydrogen proton? Apologies if this sounds like pedantry but I am genuinely just trying to establish the differences between the two things.

Yes it would be more correct to say that “an acid is a species that donates a proton” but in this context we can sort of infer what the authors are trying to say. Just to clarify, you are correct in saying that a“chemical species” essentially just refers to any sort of atom/ion/molecule or formula unit. In other words a chemical species is a type of “chemical entity” that makes up substances. You can think of “chemical substances” as materials or building blocks of matter. Just like how brick is the building block of most houses/buildings. We can consider “water” to be a chemical substance as well as a chemical species but we can refer to “water” as a chemical species or substance depending on the context. In summary, a chemical species refers to any sort of chemical entity whereas a chemical substance is a way of referring to any defined sample of matter that exhibits a constant composition (by mass) and characteristic (unique) properties. Hope this helped. Anyone feel free to correct me if I am wrong in any way.

[Babcock_Hall]

I use species to refer to a broader class of things than molecules.  A chemical species could be a molecule or an ion in my usage.  I have not thought about the definition of the word "substance" much.