Solids and Liquids in Equilibrium Systems

It doesn’t matter whether you are solving math problems or discussing Le Châtelier’s Principle, you need to be on the lookout for solids and (pure) liquids in the system because, quite simply they play by a different set of rules from gases and aqueous solutions.

At its heart, the issue is one of concentration. Doing mathematical applications of equilibrium we work directly with concentration and, when dealing with Le Châtelier’s Principle we are concerned about how changes in concentration affect the rate of either the forward or backward reaction. 

So why do solids and liquids play differently? Let’s take a close look at concentration with respect to a solid.

Imagine that you had a container of table salt and were asked, “what’s the concentration of NaCl in the container?”

This would be a new experience. Generally, we find the concentration of a solution by determining the number of moles of solute (say NaCl) and divide it by the liters of the solution.

But, in this container, there is only salt - no solvent, no solution. So, how would you find the concentration?

A simple way to do this would be to fill a 1.00 L volumetric flask with salt, determine the mass of the salt, and then convert that mass to moles. That amount would be the moles of salt in a liter...molarity. 

Specifically, my 1L volumetric flask would hold 2160. g of NaCl. Converting to moles and then to molarity:

But what if I didn’t have a 1 L volumetric flask? What if the only flask I had was a 50.00 mL flask? Then, of course, I couldn’t get nearly as much salt into the flask. In fact, I would only be able to get 108.0 g of NaCl into the flask. Let’s try the same math as before:

It turns out that no matter what size flask I have, the concentration does NOT change. This should, hopefully, make sense. For a pure substance (rather than a solution) the molarity is essentially a measure of density, albeit with very different units. We can even convert between them:

The lesson

The concentration of a solid is a constant.

Why do we care? Two reasons:

1. When we first created an equilibrium constant (here) we rearranged the rate laws to put the constant values on the left and the concentration (variables) on the right. Following the same logic, whenever we write the equilibrium expression for a system, we do NOT include the solid concentrations, since they are not variables.

2. When we work with Le Châtelier’s Principle, adding or removing a solid has NO effect on the equilibrium since you cannot change the concentration.

Pure Liquids

We can make the same argument with regards to pure liquids that we did with solids. Imagine a 1 L flask filled with pure water. The water would weigh 1000.0 g. If we convert that to moles:

A 50.00 mL flask would contain 50.00 g of water, giving:

Just like solids, the concentration of a pure liquid is constant.

The lesson with liquids is a little more complex:

1. We do NOT include liquids in the formula for the equilibrium expression since the concentrations are constant

2. Liquids that are pure do not affect the system according to Le Châtelier’s Principle, BUT if the liquid (often water) is the solvent for something in the system, it can affect equilibrium through the concept of dilution.