# 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.

Chemistry (university) · Part 07: Chemical equilibrium and solubility · Card 9 of 9 · Last updated 2026-10-19

## Analogy

Adding more guests to a full room makes people leave.

## Example

**AgCl in 0.10 M NaCl**

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.

## Related concepts

- [Solubility product](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-7/chemistry-university-part-7-concept-8)

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