Chapter 13 · Solutions
13.3Solutions of Solids Dissolved in Water
11 min · two checks
Predict
What does “saturated” mean, and does heating always let you dissolve more solid?
The idea
- Define saturated, unsaturated, and supersaturated.
- Describe how ionic solids dissolve.
An unsaturated solution can dissolve more solute. A saturated solution is at equilibrium with undissolved solute: dissolving and recrystallizing happen at the same rate. A supersaturated solution holds more than the equilibrium amount, usually because it was cooled carefully with no crystals present. It is unstable. A seed crystal or a scratch can make the extra solute crash out.
Ionic solids dissolve when ion–dipole attractions to water compete with the ionic lattice. Compounds that do this freely are the soluble ones from the solubility rules. Solubility is a specific number, grams of solute per 100 g of water at a stated temperature, read from a curve. Do not confuse “soluble” in the qualitative rules with “infinitely soluble.” Even “soluble” salts have a limit.
Keep these
- Saturated: equilibrium with undissolved solute.
- Supersaturated: temporarily past that limit, ready to crystallize.
- Solubility values need a temperature.