Intermediate Topic 482 of 1102 Kinetics & Equilibrium
Precipitation Prediction
A precipitate forms when the ion product Q exceeds Ksp.
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In-depth explanation of Precipitation Prediction
A precipitate forms when the ion product Q for a sparingly soluble salt exceeds its solubility product Ksp, because the solution is then supersaturated with respect to the solid. The ion product is calculated from the concentrations present after mixing, so the volumes of the solutions must be taken into account and the concentrations diluted. If Q is less than Ksp, the solution is unsaturated and no solid forms. If Q equals Ksp, the solution is just saturated and precipitation is at the point of starting. This comparison allows chemists to predict whether a precipitate will form before carrying out an experiment.
Going deeper: Precipitation Prediction
Key ideas: Procedure: compute concentrations after mixing with the total volume, form Q with stoichiometric powers, compare with Ksp. To find the concentration at which precipitation begins, solve for the ion concentration needed so Q = Ksp. This is also how the pH for hydroxide precipitation is found, using [OH-] = (Ksp/[M])^(1/n) for M(OH)n.
Common mistakes and connections: The classic mistake is to skip dilution, which can change the outcome by a factor of two for each ion. Students also compare Q with Ksp for the wrong salt formula, or omit the powers. A solution can sit supersaturated without forming solid if nucleation is slow. Q less than Ksp means that any solid would dissolve. Links to reaction quotient and selective precipitation.
Visualization of Precipitation Prediction
A precipitate forms when Q exceeds Ksp: mixed 1e-3 M Ag+ and Cl- give Q = 1e-6, over 5000 times greater than Ksp = 1.8e-10. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Mixing equal volumes of 2 x 10^-3 M Ag+ and 2 x 10^-3 M Cl- gives 1 x 10^-3 M of each ion, so Q = 1 x 10^-6, which exceeds Ksp of 1.8 x 10^-10 and a precipitate forms.
Frequently asked questions about Precipitation Prediction
What is Precipitation Prediction?
A precipitate forms when the ion product Q exceeds Ksp.
Is Precipitation Prediction a beginner, intermediate or advanced topic?
Precipitation Prediction is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Precipitation Prediction?
Study these earlier topics in Kinetics & Equilibrium first: Equilibrium and Free Energy, Solubility Product, Common Ion Effect.
What comes after Precipitation Prediction?
The next topics in Kinetics & Equilibrium are Selective Precipitation and Complex Ion Equilibria.
Can I practise Precipitation Prediction with quizzes?
Yes. The interactive lesson includes a self-check question on Precipitation Prediction, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Precipitation Prediction fits in Kinetics & Equilibrium
Precipitation Prediction is topic 63 of 91 in Kinetics & Equilibrium.