Common Ion Effect and Miscellaneous Points of Interest

What is the common ion effect?

A term used to describe the effect that two dissolved solutes have on each other when they both contain the same ion. So, if two solutes contain sodium, the solute that will dissociate more easily (i.e. have a higher $K_{sp}$) can suppress the other from dissociating. This is an example of Le Chatelier's principle in which the presence of one molecule shifts the equilibrium in the other direction to ``balance'' out the number of molecules.

For example, say you are at the circus and there's a popcorn vendor. He has two popcorn machines that fill his bin: One can produce 10 bags of popcorn per minute, the other 1 bag per minute. The one that creates more popcorn will ``dominate'' and ``suppress'' the other machine because there is enough common ion effect (i.e. the popcorn) produced by the 10-bag machine.

What is the effect of pH on solubility?

pH is a key player in the solubility of proteins (e.g. casein) and other substances. Weak acids and bases are commonly affected by pH in that the availability of H$^{+}$ or OH$^{-}$ changes the charge of the molecule and thereby changes its ability to stay dissolved or precipitate out of solution.

What are complex ions?

We've actually touched on several, but complex ions are molecular aggregates that consist of a metallic component and one or more electron-donating molecules, e.g. sulfate (SO$_{4}^{-2}$), chloride (Cl$^{-}$) and hydroxide (OH$^{-}$).

What does miscible mean?

The ability to mix (versus immiscible). Remembering that like dissolves like will get you far: non-polar solvents dissolve non-polar solvents, polar solvents dissolve polar solvents, etc.