Chemistry · Year 12

Active learning ideas

Common Ion Effect

Active learning helps Year 12 students grasp the common ion effect by turning abstract equilibrium shifts into observable phenomena. This topic benefits from hands-on experiments and problem-solving where students see how adding a common ion visibly reduces solubility, reinforcing Le Chatelier’s principle through concrete evidence.

• ACARA Content Descriptions
• ACSCH095

• 25–50 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Simulation Game
• 50 min · Small Groups

Lab Rotation: Solubility Tests

Prepare solutions of sparingly soluble salts like PbCl₂. Add common ion solutions at three concentrations and a control. Filter precipitates, dry, and weigh to quantify solubility reduction. Groups record and graph results.

Analyze how the common ion effect impacts the solubility of a salt.

Facilitation Tip

During Lab Rotation: Solubility Tests, have students record initial and final precipitate masses in a shared table so the class can compare trends directly.

What to look for

Provide students with the Ksp for calcium fluoride (CaF₂) and ask them to calculate its molar solubility in pure water. Then, ask them to predict qualitatively whether the solubility will increase, decrease, or stay the same if sodium fluoride (NaF) is added, and to briefly explain why.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 02

• Simulation Game
• 35 min · Pairs

Pairs Calculation: Ksp Predictions

Provide ICE tables for salts like Ag₂CrO₄ with common ions. Pairs calculate new solubilities, predict observations, then test predictions with quick qualitative mixes. Share findings in a class gallery walk.

Predict the change in solubility of a sparingly soluble salt when a common ion is added.

What to look for

Pose the question: 'How could a chemist intentionally reduce the amount of silver ions remaining in a solution after precipitating silver chloride?' Guide students to discuss the role of adding more chloride ions and the underlying equilibrium principles.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 03

• Simulation Game
• 25 min · Whole Class

Whole Class Demo: Fluoride Precipitation

Demonstrate CaF₂ solubility with and without NaF using turbidity tubes. Students note changes, then in pairs justify using Le Chatelier and link to toothpaste formulations. Follow with exit ticket predictions.

Justify the application of the common ion effect in industrial processes like water treatment.

What to look for

Students are given a scenario: 'A solution contains saturated silver chromate (Ag₂CrO₄). If potassium nitrate (KNO₃) is added, what happens to the solubility of Ag₂CrO₄? Explain your reasoning using equilibrium concepts.' Students write their answer and justification.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 04

• Simulation Game
• 40 min · Small Groups

Station Challenge: Industrial Scenarios

Set stations with case studies like boiler scale prevention. Groups design experiments or calculations to apply common ion effect, present solutions, and critique peers.

Analyze how the common ion effect impacts the solubility of a salt.

What to look for

Provide students with the Ksp for calcium fluoride (CaF₂) and ask them to calculate its molar solubility in pure water. Then, ask them to predict qualitatively whether the solubility will increase, decrease, or stay the same if sodium fluoride (NaF) is added, and to briefly explain why.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

A few notes on teaching this unit

Teach this topic by starting with a whole-class demo to show the dramatic effect of common ions, then move to structured calculations to build confidence. Avoid rushing through ICE tables—instead, let students work step-by-step in pairs to uncover how stoichiometry impacts results. Research shows that pairing visual demonstrations with guided calculations solidifies understanding of dynamic equilibria.

Students will accurately explain how the common ion effect decreases solubility using equilibrium principles and calculations. They will measure precipitate changes in lab rotations, solve Ksp problems with peers, and apply these ideas to real-world scenarios during station challenges.

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Watch Out for These Misconceptions

• During Lab Rotation: Solubility Tests, watch for students who believe adding a common ion increases solubility because they see no immediate change in the solution.

  Have students compare the mass of precipitate in tubes with and without the common ion, then discuss why the precipitate forms more readily with the common ion, reinforcing that solubility decreases.

• During Pairs Calculation: Ksp Predictions, watch for students who think the equilibrium shifts completely to the left, leaving no ions in solution.

  Guide pairs to use their calculated Ksp values to show that some ions remain in solution, shifting the equilibrium partially but not eliminating all ions.

• During Station Challenge: Industrial Scenarios, watch for students who ignore the stoichiometry of the salt when predicting outcomes.

  At the ICE table station, have peers check each other’s work to ensure they account for the correct ion ratios, using CaF₂ as the primary example.

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Methods used in this brief

• Simulation Game