Explain the common-ion effect on solubility
Use equilibrium shifts to explain why a sparingly soluble salt becomes less soluble when a common ion is added.
Section 25 · Equilibria
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 25. Use the equilibrium expressions and data stated in the question; written practice is self-checked and human subject-teacher review remains pending.
Before you begin
- Use AS equilibrium expressions, mole calculations, logarithms, and Brønsted–Lowry acid–base ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which ion is common when AgCl dissolves in NaCl solution?
What you will learn
- Identify an ion shared by the solid and added solution
- Predict the equilibrium shift after adding the common ion
- Distinguish solubility change from Ksp at fixed temperature
A common ion shifts the dissolution equilibrium
For AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq), adding NaCl increases [Cl⁻], a common ion. The equilibrium shifts left, reducing the amount of AgCl that dissolves compared with pure water.
At a fixed temperature, Ksp remains constant. The ion concentrations change in a way that restores the Ksp relationship; the salt's molar solubility, however, decreases in the solution containing the common ion.
State the comparison and the condition
The common-ion effect compares solubility in pure water with solubility in a solution already containing one of the dissolved ions. Identify the shared ion and explain the shift using the dissolution equation.
Do not say the added ion makes the solid's Ksp smaller. Ksp is temperature-dependent; the changed solubility is the equilibrium response to altered ion concentration.
Pause and explain
At fixed temperature, what happens to Ksp when NaCl is added to saturated AgCl?
Worked example
Explain why AgCl is less soluble in 0.10 mol dm⁻³ NaCl than in pure water at the same temperature.
Show the worked solution
- Write AgCl(s) ⇌ Ag⁺ + Cl⁻.
- NaCl adds Cl⁻, a common ion.
- Predict the shift and distinguish Ksp from solubility.
Answer Added Cl⁻ shifts AgCl dissolution to the left, so less AgCl dissolves. Ksp remains unchanged at the same temperature; the solubility is lower in NaCl solution.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the common-ion effect when BaSO₄(s) is placed in a solution containing Na₂SO₄.
Give me a hint
Compare SO₄²⁻ in the salt dissolution with the added solution.
Compare my reasoning
- BaSO₄ releases Ba²⁺ and SO₄²⁻.
- Na₂SO₄ adds SO₄²⁻.
- Use Le Chatelier's principle to predict the shift.
Added SO₄²⁻ shifts BaSO₄(s) ⇌ Ba²⁺ + SO₄²⁻ to the left, reducing BaSO₄ solubility at the same temperature.
Look for these in your work
- Common ion identified
- Dissolution equilibrium used
- Solubility shift stated
A saturated AgCl solution receives a small amount of AgNO₃. Predict the effect on AgCl solubility.
Give me a hint
AgNO₃ supplies one of the ions in the AgCl dissolution equilibrium.
Compare my reasoning
- Identify Ag⁺ as the shared ion.
- Adding a product ion shifts dissolution left.
- State what happens to solubility and Ksp.
Added Ag⁺ shifts AgCl dissolution to the left, so its solubility decreases while Ksp stays constant at fixed temperature.
Look for these in your work
- Common cation identified
- Shift direction correct
- Ksp distinguished
A learner says adding NaCl lowers the Ksp of AgCl because less AgCl dissolves. Correct the explanation.
Give me a hint
Keep the equilibrium constant separate from the equilibrium amount dissolved.
Compare my reasoning
- Ksp depends on temperature.
- NaCl increases [Cl⁻] and shifts dissolution left.
- The solubility falls but Ksp does not change at fixed temperature.
The common ion Cl⁻ lowers AgCl solubility by shifting dissolution left. Ksp is unchanged at the same temperature; the solution concentrations adjust to satisfy it.
Look for these in your work
- Cause of shift stated
- Solubility identified as changed
- Equilibrium constant corrected
Common mistakes
- Claiming Ksp decreases when a common ion is added
- Calling Na⁺ the common ion for AgCl
- Predicting more solid dissolves when a product ion is added
- Ignoring the fixed-temperature condition
What can you explain now?
Which ion is common when AgCl dissolves in NaCl solution?
Compare with the explanation
Cl⁻
Both AgCl dissolution and NaCl provide chloride ions.
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Make it stick
Why can a common ion reduce solubility while leaving Ksp unchanged at a fixed temperature?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.