Solubility Equilibria and When a Precipitate Appears
A sparingly soluble salt sitting on the bottom of a beaker is at equilibrium with its ions, and one constant describes the whole situation. Comparing the ion product with that constant is how a chemist predicts a precipitate before mixing anything.
What a learner can do afterwards
- Write the solubility product expression for a salt and relate it to the salt's solubility
- Predict whether mixing two solutions produces a precipitate by comparing the ion product with the constant
- Explain the common ion effect and use it to account for selective precipitation
1 · Read
A spoonful of silver chloride sitting in water is not idle. Ions leave the solid for the water while other ions rejoin the solid. When both trips run at equal rates, the system sits at equilibrium. The constant describing it is the solubility product, Ksp, the multiplied ion concentrations with each raised to its coefficient. The solid itself never appears, since a pure solid counts as constant.
You can tie a salt's solubility to its Ksp with one letter, x, for moles dissolved per liter. For a 1:1 salt like silver chloride, Ksp equals x squared. For a 1:2 salt like magnesium hydroxide, Ksp equals 4x cubed. Because stoichiometry bends the math, always write the balanced dissolution equation first. Salts with matching ion counts can be ranked by Ksp directly.
To predict a mixture, compute the ion product Q with the same formula but the actual concentrations after mixing. Below Ksp means unsaturated, so more salt can still dissolve. Equal to Ksp means exactly saturated. Above Ksp means a precipitate forms until Q sinks back to Ksp.
Pour extra chloride into dissolved silver chloride and less of it stays dissolved. The common ion pushes equilibrium back toward the solid, which is the common ion effect. Chemists exploit it for selective precipitation, dropping one metal out while leaving another dissolved. The same equilibria govern kidney stones and magnesium drawn from seawater.
Compare Q with Ksp to predict a precipitate, and link Ksp to solubility through x.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.