AP® Chemistry · Unit 7 · Topic 7.12
The Common-Ion Effect
The common-ion effect is the drop in a salt's solubility that happens when the solution already contains one of the ions the salt would produce. Le Chatelier's principle pushes the dissolving equilibrium backward, but Ksp itself never changes.
Silver chloride dissolves a little in pure water, settling at AgCl(s) <→ Ag+ + Cl-. Drop that same solid into 0.10 M NaCl instead, and far less of it dissolves. The salt water already has a flood of Cl- ions sitting in solution, and Le Chatelier's principle says the equilibrium shifts left to relieve that stress, pulling Ag+ and Cl- back together as solid AgCl. Less solid dissolves overall. That's the common-ion effect: an ion already present in solution, one that also appears in the dissolution equilibrium, suppresses how much of the solid can dissolve.
The key thing to hold onto is that Ksp never moves. Ksp is an equilibrium constant, and equilibrium constants depend only on temperature. What changes is the equilibrium position — where the system settles — not the constant that describes it. So in 0.10 M NaCl, [Cl-] is fixed near 0.10 M (since NaCl is a strong electrolyte that dissociates completely, and it dwarfs whatever trace of Cl- comes from the AgCl itself). Plugging that into Ksp = [Ag+][Cl-] and solving for [Ag+] gives the molar solubility directly.
This connects straight back to Ksp and ICE tables: the common-ion effect is just an ICE table where the initial concentration of one ion isn't zero. It also sets up buffers and selective precipitation later in the unit, where deliberately adding a common ion is the whole strategy for controlling how much of something stays dissolved.
Key terms
- Common-ion effect
- The decrease in a salt's solubility caused by an ion already present in solution that also appears in its dissolution equilibrium.
- Ksp
- The equilibrium constant for a solid dissolving into its ions, equal to the product of the ion concentrations at equilibrium.
- Molar solubility
- The number of moles of a solid that dissolve per liter of solution before equilibrium is reached.
- Le Chatelier's principle
- The idea that a system at equilibrium shifts to partially counteract any change imposed on it.
Watch it explained
- How it worksHow does an ion already in solution push a dissolving equilibrium backwards?
- Worked exampleFind the molar solubility of AgCl in 0.10 M NaCl given a Ksp of 1.8e-10.
- Common mistakeWhy isn't the value of Ksp changed by the presence of the common ion?
- Going furtherWhat happens to the solubility of CaF2 when the solution is made acidic instead?
Check yourself
End of topic.