Why do liquids and solids have constant concentrations?

I don't understand why liquids and solids have constant concentrations. Can anyone explain this to be in simple terms?




If you think about what concentration, it usually refers to the molar concentration in $\mathrm{mol/l}$ (at least in the field of chemistry). To calculate this concentration we need to find how many moles of a certain chemical are in 1 liter of the liquid or solid.


I will start with an example and describe the general principle afterwards. Let's say we have pure water at room temperature and atmospheric pressure. This means that the density is $998\,\mathrm{g/l}$. We also know that the molar mass of water, consisting of 2 hydrogen and 1 oxygen, is $18.016\,\mathrm{g/mol}$. If you look at the units of the molar mass and the density you can already see that we can combine these into something that has units $\ce{mol/l}$. If we do that we get that the concentration of water is $\frac{998\, \mathrm{g/l}}{18.016\,\mathrm{g/mol}}=55.4\,\mathrm{mol/l}$. So the concentration of water is $55.4\,\mathrm{mol/l}$. Notice that I have only used the density and the molar mass of water, which are (at least for equal temperature and pressure) constant.


In general you can do this for any solid or liquid (or gas) and find the concentration as $c=\frac{\rho}{M}$, where $\rho$ is the density, $M$ the molar mass and $c$ the molar concentration. For a given chemical, the molar mass is constant.


Now about the ‘fact’ that the molar concentration is constant for liquids and solids: strictly speaking it is not. That is because the density of liquids and solids is temperature and pressure dependent. The reason that the molar concentration is often called constant is twofold: 


1. the density of liquids and solids has a much weaker dependence on the temperature than gases do therefore it can be regarded as approximately constant
2. if you look at solutions, the variation of concentration can be
orders of magnitude, whereas for e.g. water the concentration
variation when going from $0$ to $100\,\mathrm{^\circ C}$ is only a few percent.
Therefore, again, the concentration of the liquid is approximately
constant.


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In liquids and solids, particles have intermolecular interactions that keep them at a certain distance. As a result, the density of liquids and solids is not too sensitive to pressure and temperature changes.


The concentration (technically: amount of substance concentration) of a pure solid or liquid is defined as the chemical amount of the substance per volume of the sample. You can calculate it from the molar mass and the density of the substance:


$$ c = \frac{n}{V} = \frac{m / M}{m / \rho} = \frac{\rho}{M}$$


with $\rho$ the density of the sample and and $M$ the molar mass of the substance. For liquids and solids, the concentration does not depend much on pressure or temperature. 


For gases, on the other hand, the concentration is proportional to the pressure and anti-proportional to the temperature (ideal gas law). For substances in solution, the concentration is variable because you can add more or less solute to the solvent.


If you consider a chemical reaction at constant temperature and pressure, pure solid or liquid reactants and products will not change their concentrations, whereas reactants or products that are gases (if the reaction is in an enclosed space) or are in solution typically will.


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In simple terms, under normal conditions, one can only "fit" a **definite amount of a solid or a liquid in a definite space**but you can fit a
**variable amount of a gas in a definite space.** Think about the large amount of compressed gas that can be stored in deodorant spray cans, but on the other hand a glass of water, or a brick can only occupy an unchanging amount of space.


Since, in chemistry we're concerned only with molecules, we express amount of a substance by moles. Thus the concentration of a substance [ (amount in moles)/(volume) ], pure solids and pure liquids have definite a constant concentration, for a given volume, whereas gases have a variable concentration.


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My explanation is that the molarity (concentration and also the activity in case of an ideal solution) of a pure liquid (or solid) won't change considerably in a solution (in which a chemical equilibrium exists) even if the concentration of the solute in the binary solution with the said pure liquid changes. The reactants of the solvation equilibrium are the pure solvent and solute and the product would be the solvated solute. Hence their concentrations are constant and therefore their activities (a thermodynamic concept which means concentration in case of ideal solutions) can be taken as unity itself instead of its actual concentration in the binary solution. Also, the molarity of a pure solid or liquid doesn't affect the concentration of the solute/electrolyte present in the solution and remains constant giving rise to an active mass (but not the concentration and, here, the active mass is what we are actually interested in) of unity which does not affect the equilibrium of the solution.


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Vaguely,
the reason is the incompressibility of solids and liquids. They cannot have variable volume for the same amount of substance. When their amount increases, so does their volume and when amount decreases, volume decreases too in such a way that this amount to volume ratio (concentration) remains constant.
However, their concentration does change, but the change is quite small and hence negligible.


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