Safety boundary
Use solubility curves, supplied datasets, and simulations only. Do not perform unsupervised precipitation chemistry or mix household chemicals.
Explicit chemistry instruction
Adding an ion already present in a dissolution equilibrium changes Qsp immediately.
The system can respond by favoring the solid phase, reducing additional dissolution in the simplified common-ion model.
This is an application of equilibrium response, not a separate law.
Vocabulary: common ion; coupled equilibrium; Qsp; shift; solubility.
Worked example
Adding X⁻ to MX(s)⇌M⁺+X⁻ raises Qsp and can favor formation of more solid MX.
Guided question
Why can adding a soluble salt containing X⁻ reduce the solubility of MX?
Independent work
- Predict three common-ion effects.
- Connect each to Qsp/Ksp.
- Identify one case with no common ion.
Misconception check
The common-ion effect changes equilibrium composition; it does not change Ksp at constant temperature.
Formative check
What stays constant during a common-ion perturbation at fixed temperature?
Reveal response
Ksp.
Mentor evidence conference
Ask Archaemenes to audit saturation language, Ksp/Qsp distinctions, and whether the data justify the classification.
Teacher guidance
Foundation: Use before/after ion tables.
Core: Explain common-ion shifts.
Extended: Analyze coupled equilibria qualitatively.