Precipitation prediction is the AP Chem skill of comparing the reaction quotient Q to the solubility product Ksp for a salt: if Q > Ksp the solution is supersaturated and a precipitate forms, if Q < Ksp more salt can dissolve, and if Q = Ksp the solution is exactly saturated (at equilibrium).
Precipitation prediction answers one question: if you mix two solutions, will a solid actually crash out? You answer it with math, not solubility rules. Write the dissolution equilibrium for the possible precipitate, plug the actual ion concentrations in the mixed solution into the Ksp expression, and call that number Q (the reaction quotient). Then compare. If Q > Ksp, the solution holds more dissolved ions than equilibrium allows, so it's supersaturated and a precipitate forms until Q drops back down to Ksp. If Q < Ksp, the solution is unsaturated and no solid forms (any solid present would keep dissolving). If Q = Ksp, you're sitting exactly at saturation.
This is the predictive payoff of treating dissolution as an equilibrium system (EK 7.11.A.1). Ksp is a fixed equilibrium constant at a given temperature; Q is a snapshot of your specific solution right now. The whole game is figuring out which side of Ksp your snapshot lands on. One trap to watch: when you mix two solutions, the volumes add, so every ion concentration gets diluted before you calculate Q. Skipping that dilution step is the single most common way to get the wrong verdict.
This lives in Topic 7.11 (Introduction to Solubility Equilibria) in Unit 7: Equilibrium, supporting learning objective 7.11.A, which asks you to calculate solubility from Ksp. Precipitation prediction is where Ksp stops being an abstract constant and starts making real predictions about mixtures. It also quantifies the solubility rules you memorized back in Unit 4 (EK 7.11.A.3): a salt with Ksp > 1 is what those rules call "soluble." Conceptually, it's the same Q-vs-K logic from earlier in Unit 7, just applied to dissolving salts, so mastering it reinforces the central idea of the entire unit: comparing where a system is to where equilibrium says it should be tells you which direction the system will shift.
Keep studying AP® Chemistry Unit 7
Solubility Product Constant, Ksp (Unit 7)
Ksp is the benchmark in every precipitation prediction. It tells you the maximum ion product a saturated solution can sustain, and your calculated Q is measured against it. No Ksp comparison, no prediction.
Reaction Quotient Q (Unit 7)
Precipitation prediction is just the Q-vs-K comparison from earlier in Unit 7 wearing a solubility costume. Q > K means the reaction shifts toward reactants, and for dissolution, 'toward reactants' means ions recombining into solid. Same logic, new context.
Precipitation Reaction (Unit 4)
Unit 4 taught you to predict precipitates qualitatively with solubility rules. Unit 7 upgrades that to a quantitative test. The rules say 'most carbonates are insoluble'; Q vs Ksp tells you whether THIS carbonate precipitates at THESE exact concentrations.
Stoichiometric Coefficients (Units 4 & 7)
Coefficients in the dissolution equation become exponents in both Q and Ksp. For PbCl₂, Q = [Pb²⁺][Cl⁻]², so forgetting to square the chloride concentration gives you the wrong Q and possibly the wrong verdict on whether a solid forms.
On multiple choice, expect a stem that gives you ion concentrations after mixing (or initial concentrations plus volumes) and a Ksp value, then asks whether a precipitate forms. The trap answers usually come from skipping the dilution step or forgetting to raise a concentration to its stoichiometric power. On free response, this shows up as one part of a larger solubility equilibrium problem: you might calculate molar solubility from Ksp in part (a), then justify whether a precipitate forms in part (b). The justification matters. Full credit requires the comparison itself, something like "Q = 4.2 × 10⁻⁸ > Ksp = 1.6 × 10⁻¹⁰, so a precipitate forms." Stating the conclusion without showing Q earns nothing. No released FRQ uses the phrase "precipitation prediction" verbatim, but the Q-vs-Ksp comparison is a standard move in solubility FRQs.
Ksp is a constant. It only changes with temperature and describes a saturated solution at equilibrium. Q is a snapshot of your actual solution at this moment and can be anything. Mixing them up wrecks the logic: you calculate Q from your real concentrations, and Ksp is the fixed line you compare it to. If you find yourself "calculating Ksp" from a freshly mixed solution that isn't at equilibrium, you're actually calculating Q.
To predict precipitation, calculate Q using the actual ion concentrations in the solution and compare it to Ksp.
If Q > Ksp, the solution is supersaturated and a precipitate forms until Q falls to equal Ksp; if Q < Ksp, no precipitate forms and more salt could still dissolve.
When two solutions are mixed, dilute each ion concentration using the new total volume before calculating Q.
Stoichiometric coefficients from the dissolution equation become exponents in Q, so a salt like PbCl₂ requires squaring the chloride concentration.
Ksp is fixed at a given temperature, while Q changes with conditions; the comparison between them tells you which way the dissolution equilibrium shifts.
This is the same Q-vs-K logic used throughout Unit 7, applied to the dissolution of salts (EK 7.11.A.1).
It's using the comparison between the reaction quotient Q and the solubility product Ksp to decide whether a solid forms when solutions are mixed. If Q > Ksp, the solution is supersaturated and a precipitate forms; if Q < Ksp, everything stays dissolved.
Yes, in the AP model. Q > Ksp means the ion product exceeds what a saturated solution can hold, so ions combine into solid until Q drops back to Ksp. (Real solutions can briefly stay supersaturated, but on the exam, Q > Ksp means precipitation.)
Ksp is a fixed equilibrium constant for a saturated solution at a given temperature. Q uses the same expression but with your solution's actual current ion concentrations, which may not be at equilibrium. You calculate Q; you look up or are given Ksp.
Solubility rules (Unit 4) give a qualitative yes/no for whole categories of salts. The Q-vs-Ksp test is quantitative and depends on your exact concentrations. A "slightly soluble" salt might not precipitate at all if the ion concentrations are low enough to keep Q below Ksp. EK 7.11.A.3 links the two: Ksp > 1 roughly corresponds to salts the rules call soluble.
Yes, always. Mixing 50.0 mL of one solution with 50.0 mL of another doubles the total volume, cutting each ion's concentration in half before you calculate Q. Skipping this step inflates Q and is the most common error on these problems.
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