In AP Chemistry, precipitation is the formation of an insoluble solid (a precipitate) when dissolved ions combine in solution; it happens when the ion product Q exceeds Ksp, and it's the reverse of dissolution, so adding a common ion drives more precipitation (Topic 7.12).
Precipitation is what happens when two dissolved ions meet in solution and form a compound that can't stay dissolved. The solid that falls out is called the precipitate. Think of it as dissolution running in reverse. A salt like BaSO₄ sits in an equilibrium between its solid form and its dissolved ions, and Ksp tells you how far that equilibrium leans toward dissolved.
The AP exam treats precipitation as an equilibrium question, not just a 'mix two solutions, get a solid' fact. You compare the ion product Q (the actual concentrations of ions in solution, multiplied together) to Ksp. If Q > Ksp, the solution holds more ions than equilibrium allows, so a precipitate forms until Q drops back down to Ksp. If Q < Ksp, everything stays dissolved. This is also where the common-ion effect lives (EK 7.12.A.1). If a solution already contains one of the salt's ions, the equilibrium shifts toward the solid by Le Châtelier's principle, so the salt is less soluble and more of it precipitates.
Precipitation anchors Topic 7.12 (Common Ion Effect) in Unit 7: Equilibrium. Learning objective AP Chem 7.12.A asks you to identify a salt's solubility, or its Ksp, based on the concentration of a common ion already in solution, and precipitation is the observable result of that math. When a common ion is present, solubility drops and the precipitate grows. This is Le Châtelier's principle applied to a dissolution equilibrium, which means precipitation problems quietly test your entire Unit 7 toolkit: writing equilibrium expressions, comparing Q to K, and predicting which way a system shifts. It also reaches back to Unit 4, where precipitation reactions show up as a type of chemical change with net ionic equations, and it appears in lab-based FRQs as a way to measure how much of something is in a solution.
Common Ion Effect (Unit 7)
This is precipitation's home topic. Adding an ion the salt already contains (like adding extra SO₄²⁻ to a PbSO₄ system) shifts the dissolution equilibrium toward the solid, so less salt dissolves and more precipitates. It's Le Châtelier's principle wearing a solubility costume.
Double Replacement Reaction (Unit 4)
Most precipitation reactions you write are double replacement reactions, where two ionic compounds swap partners and one new pairing is insoluble. Unit 4 teaches you to write the net ionic equation for it; Unit 7 teaches you to predict whether it actually happens using Q and Ksp.
Molar Solubility (Unit 7)
Molar solubility is the flip side of precipitation. It tells you how much salt can dissolve before the solution is saturated, and precipitation kicks in the moment you push past that limit. Both come from the same Ksp expression.
Supersaturation (Unit 7)
A supersaturated solution has Q > Ksp but hasn't precipitated yet. It's an unstable state where the solution is holding more dissolved ions than equilibrium allows, and precipitation is how it relaxes back to Q = Ksp.
Multiple-choice questions usually hand you a Ksp and ion concentrations, then ask whether a precipitate forms. The move is always the same. Calculate Q from the actual concentrations and compare it to Ksp (Q > Ksp means precipitation occurs). Watch for the dilution trap: when two solutions are mixed, each ion's concentration drops before you compute Q. Other stems test the common-ion effect qualitatively, asking which experimental step minimizes solubility, like adding excess sulfate to recover more Pb²⁺ as PbSO₄. On the free-response side, the 2021 exam (Q3) built an entire FRQ around using precipitation to determine the molar concentration of a CuSO₄ solution, so be ready to connect precipitation to lab procedure: filtering, drying, massing the precipitate, and converting grams of solid back to moles of dissolved ion. You should be able to write the dissolution equilibrium, set up the Ksp expression, and explain shifts using Le Châtelier's principle in full sentences.
These overlap but aren't the same thing. A double replacement reaction is the general pattern where two compounds swap ions (AB + CD → AD + CB). Precipitation is one possible outcome of that swap, the one where a product is insoluble and drops out of solution. Not every double replacement makes a precipitate (some make water or a gas instead), and in Unit 7 precipitation is treated as a standalone equilibrium process governed by Ksp, no swap required. Dissolving solid BaSO₄ and watching it re-precipitate involves no double replacement at all.
Precipitation is the formation of an insoluble solid from dissolved ions, and it's the reverse of dissolution.
A precipitate forms when the ion product Q is greater than Ksp; if Q is less than Ksp, everything stays dissolved.
When mixing two solutions, recalculate each ion's concentration after dilution before comparing Q to Ksp.
The common-ion effect (EK 7.12.A.1) means a salt is less soluble in a solution that already contains one of its ions, which you can explain with Le Châtelier's principle.
To maximize precipitation of a target ion in a lab setting, add excess of the other ion to push the equilibrium toward the solid.
Precipitation shows up in lab FRQs as a quantitative tool, where the mass of dried precipitate lets you calculate the concentration of an ion in the original solution.
Precipitation is the formation of an insoluble solid (a precipitate) when dissolved ions combine in solution. In Unit 7 it's treated as an equilibrium process, the reverse of dissolution, governed by Ksp.
Calculate the ion product Q using the actual ion concentrations and compare it to Ksp. If Q > Ksp, a precipitate forms until the concentrations fall back to equilibrium; if Q < Ksp, no solid forms.
No. Even if the product salt is 'insoluble,' a precipitate only forms if Q exceeds Ksp after mixing. Dilute enough solutions can stay fully dissolved, which is exactly what MCQs test with the dilution trap.
Precipitation is the event (solid forming from ions), while the common-ion effect is one cause of it. Adding an ion the salt already contains shifts the dissolution equilibrium toward the solid, reducing solubility and forcing more precipitation.
Not exactly. Precipitation reactions between two solutions are usually double replacement reactions, but precipitation itself just means a solid forming from dissolved ions. A salt can precipitate from a supersaturated solution with no ion swap happening at all.