Factors Influencing the Solubility

Solubility depends on several factors: the chemical nature of the solute and solvent, the temperature, and (for gases) the pressure.

Nature of solute and solvent ("like dissolves like"). Sodium chloride, an ionic compound, dissolves readily in a polar solvent such as water but not in non-polar solvents such as benzene or toluene. Many organic compounds show the reverse pattern -- readily soluble in organic solvents, poorly soluble in water. Even different gases dissolve to different extents in the same solvent: ammonia, for instance, is far more soluble in water than oxygen is.

Effect of temperature -- solid solute in liquid solvent. Generally, the solubility of a solid in a liquid increases with rising temperature. Higher temperature raises the average kinetic energy of both solute and solvent molecules, which helps solvent molecules overcome the intermolecular forces holding the solid together, so more solute dissolves. At saturation, an equilibrium exists between undissolved and dissolved solute:

By Le Chatelier's principle, raising the temperature shifts this equilibrium toward more dissolution if the dissolution process is endothermic, and toward less dissolution if it is exothermic.

Reading the solubility-versus-temperature graph (Figure 9.1) for four salts confirms this is not a simple rule:

• Sodium chloride's solubility barely changes with temperature -- only about a 10% rise from 0$^\circ$C to 100$^\circ$C.
• Ammonium nitrate's dissolution is endothermic, so its solubility rises steeply with temperature.
• Ceric sulphate's dissolution is exothermic, so its solubility actually falls as temperature rises.
• Calcium chloride's dissolution is exothermic too, yet its solubility still rises moderately with temperature -- here, entropy also plays a significant role in determining where the equilibrium settles, so a simple "exothermic = lower solubility at higher T" rule is not universal.

Effect of temperature -- gaseous solute in liquid solvent. Here the trend reverses: solubility decreases as temperature rises. A dissolved gas's molecules are held in solution by comparatively weak intermolecular forces with the solvent; raising the temperature raises kinetic energy enough to break these weak forces, releasing gas molecules back to the gaseous state. Moreover, dissolving most gases in a liquid is itself an exothermic process, so by Le Chatelier's principle, raising temperature further disfavours dissolution.

Activity: opening a soda bottle and stretching a balloon over its mouth shows the balloon slowly inflate as dissolved CO$_2$ escapes; repeating this with the bottle placed in hot water makes the balloon inflate noticeably faster -- direct evidence that gas solubility falls as temperature rises. This has a real ecological consequence: rivers that receive hot water discharged from industrial plants hold less dissolved oxygen, and aquatic life is less able to thrive there. …