Species, Components, and Constituents

It is now time to amplify our comments in Chapter 3 on the terms used to describe the composition of solutions. 

To use classical Thermodynamics we need absolutely no knowledge of such things. We need only measure macroscopic properties of the bulk solution, such as its bulk composition, and from the.se we will be able to calculate molar properties of dissolved substances (partial molar properties) just as we do for pure substances. 

Note too, that although H2O is the other component in this two-component system, and although it makes up the major part of the solution phase, it does not appear in equation (10.24). We do not write 

Having made the point that we can write reactions using components, as in (10.24), we should next point out that reactions are also often written between reacting species, e.g. 

In (10.25) we are concerned with what really happens to the aqueous or dissolved calcite, rather than with its bulk properties. Actually, what really happens includes several other reactions as well, as mentioned earlier, such as 

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