Equilibrium in saturated solution

You've seen that if you take something like salt, and put it into water, the solid material breaks down, disappears and goes into solution. We call that process solution, dissolution, or dissolving.

You have also seen, in the supersaturation demonstration, how material can crystallize, that is, come out of solution and form crystals.

Because you've seen ionic materials dissolve and you've also seen ionic materials crystallize, you know that the process is not a one-way process. It can go in either direction.

Let's go over the process step by step.

When you put an ionic material into water and watch it dissolve, you can see that the solid is getting smaller. You know that the ionic material is breaking up and going into solution.
You also know that you can add so much of that solid that it will not all dissolve. That is, you can have a saturated solution.
When you have reached the saturation point, the dissolving process does not really stop. But what happens is that the crystallization process is occurring at the same rate that the dissolution process is occurring.
At the saturation point, there is a balance between the amount of material that is dissolving, and the amount of material that is crystallizing at the same time.

The amount of material in each condition (or state) remains the same but the process of change from one state to the other continues. Solid material continues to dissolve, and material that is in solution continues to crystallize. Dissolution and crystallization are occurring at the same rate. There is a balance between those two opposite reactions, and we call that kind of balance a dynamic equilibrium -- equilibrium because there is a balance, dynamic because there are changes taking place.

A saturated solution is considered to be in dynamic equilibrium because there is molecular change that is occurring in opposite directions at the same rate, such that there is no net observable change. All you can see is that there is a certain amount of solid and a certain amount of solution, you cannot see that one is changing into the other. Thus, a saturated solution in contact with undissolved solute is an example of a dynamic equilibrium.

Shifting Equilibrium

We can alter this situation in a number of ways and doing so will cause a shift in the equilibrium balance.

Let's look first at what would happen if we added more water or solvent. The concentration of the solute would be less than it was and it would no longer be saturated. More of the solute would be able to dissolve.

On the other hand, if we evaporated away some of the water or solvent, the solute would become more concentrated and if it was already saturated, some of the solute would have to crystallize out of the solution. Or perhaps it would become a supersaturated solution.

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