vincikai said:
Why do we just ignore the concentration of solid or solution solvent and replace it as one M? I always thought the reason is because the concentration of solid or water solvent does not change much so in practice we just move it to the Kc side such as Kw = Kc[H2O]=[H][OH]??? Now I am not so sure
Simply put, the reason why one assigns a numerical value of 1 to the solvent of a dilute solvent system is because most of the thermodynamical concepts were derived from the assumption of ideal gas system and all of the thermodynamical concepts applied to non-ideal system are nothing more than rough tinkering of the ideal system based models.
Numerous tinkering schemes were developed in order to utilize the ideal models to actual systems. One famous and well-known tinkering scheme is van der waal's equation for non-ideal gases. Another famous tinkering is the concept of "activity" which was developed to apply the ideal chemical potential function(a partial derivatives of general Gibb's function with respect to a component of the system) to a non-ideal system such as solution.
This concept of activity, which was developed by G.N. Lewis, is perhaps more important and useful to modern chemistry than van der waal's. But this is rarely taught in gen. chem due to the requirement of rigorous mathematical background(at least some MODERN understanding of differentials which inevitable requires studying of functional analysis or above) in explaining why such a thing is required, although the concept of activity itself does not require any mathematical background. In order to avoid exposition of this cumbersome concept and its origin, most of the gen. chem texts put nonsenses such as
"the concentration of solid or water solvent does not change much so in practice we just move it to the Kc side such as Kw = Kc[H2O]=[H][OH]"
and so on.
Anyway, it can be mathematically shown that the general function of chemical potentional can be used without tinkering in two extreme cases of idealized real solution system. One extreme is ideal solution(basically solvent and solute is indistinguishable in interacting property) and the other extreme is ideally dilute solution(fancy name for infinitely dilute solution). In the ideally dilute solution the chemical potential term for solvent component just becomes negligible in calculation of the overall chemical potential of the system. This results in the assignment of number value of 1 to solvent when calculation for equilibrium constant is performed.
I know that this is quite an inadequate answer, but a truly satisfying answer can be obtained only if you are sufficient in mathetical concepts, especially differentials. You might wanna consult any p-chem in your library but most of the p-chem texts and thermodynamic books treats differential using really antiquated interpretation by making one to think differential as infinitely small changes. If you do think such an effort is worthless, just accept this fact as an axiom rather than some consequence of a fundamental concept.
And about your question in the title of this post.
A pure liquid has a defined concentration. From the definition of molarity, you know that molarity is equal to ratio of number of moles of a substance in question to 1 liter of solvent. Since in pure liquid, liquid becomes both the solute and solvent you can obtain its molarity(a form of concentration) straight from its original definition. For example, in x gram of pure water, there is (x/18) moles of water molecules and there is (x/1000) liter of water, assuming water's density is relatively independent of pressure and its density, d = (1 kg)/(1 liter). When you put this two quantites in the defined ratio [(x/18)/(x/1000)], you get approximately 55.6 M.
For pure solid, there is no definition for concentration in general.
