Common ion effect
Adding an ion that is already part of a dissolving equilibrium pushes the equilibrium back toward the solid. The salt becomes less soluble. This is Le Chatelier's principle applied to Ksp.
Last updated
Analogy
Adding more guests to a full room makes people leave.
Example
Ksp = [Ag⁺][Cl⁻] = 1.8×10⁻¹⁰[Cl⁻] ≈ 0.10 M[Ag⁺] = 1.8×10⁻¹⁰ / 0.10= 1.8×10⁻⁹ M (10,000× less)
In practice
Silver chloride is far less soluble in salt water than in pure water.
Practice
Flashcards
- How does a common ion change solubility?It lowers the solubility of the salt.
Interview questions
- Calculate the molar solubility of AgCl (Ksp = 1.8×10⁻¹⁰) in water and in 0.10 M NaCl, and explain the difference.
- In water s = √Ksp = 1.3×10⁻⁵ M
- In NaCl [Cl⁻] = 0.10 M
- s = Ksp/0.10 = 1.8×10⁻⁹ M
- Common ion effect
Questions people ask
Why does the common ion lower solubility?
Dissolving AgCl produces Ag⁺ and Cl⁻. Adding Cl⁻ from another source raises the product [Ag⁺][Cl⁻] above Ksp, so some Ag⁺ and Cl⁻ recombine to form solid. The system adjusts until the product is back to Ksp, leaving less silver chloride dissolved than there would be in pure water.
Can a different ion increase solubility?
Yes. If an ion removes one of the dissolved ions, such as H⁺ removing the carbonate from CaCO₃ or ammonia forming a complex with Ag⁺, the equilibrium shifts forward to replace it. More solid dissolves. This is why acids dissolve limescale.