Phase Transitions

Boiling point depends on the strength of intermolecular forces. Substances with stronger intermolecular forces require more heat energy to escape the liquid phase, resulting in higher boiling points. For example, water has strong hydrogen bonds and boils at a higher temperature than acetone, which exhibits only weak dipole-dipole forces.

Temperature remains constant during a phase transition, even though heat is continuously supplied or removed. This occurs because the energy goes into breaking intermolecular forces rather than increasing molecular motion. Once the transition completes, temperature begins to change again as the substance remains in a single phase.

In solids, particles pack closely in fixed, ordered positions and vibrate in place. In liquids, particles remain closely spaced but randomly arranged and free to move past neighbors. In gases, particles are widely separated with weak attractive forces, allowing complete freedom of movement.

Molar enthalpy is the energy required for one mole of a substance to completely undergo a phase transition without changing temperature. Endothermic transitions (melting, vaporization) have positive enthalpy values because they absorb heat. Exothermic transitions (freezing, condensation) have negative values because they release heat.

Dynamic equilibrium occurs when a substance is held at a transition point in a closed system. The opposing transition processes—such as vaporization and condensation—occur at equal rates, creating a stable state where both phases coexist without net change in composition.

Internal energy, the total kinetic energy of molecules, is lowest in solids and highest in gases. Solids and liquids are condensed phases with strong intermolecular forces holding particles close together. Gases have weak interparticle forces and high internal energy, allowing particles to move freely and occupy large distances.

Molecules transition between phases when their internal energy allows them to exist in a different state. Changes in temperature or pressure alter the strength of intermolecular forces relative to molecular motion. When thermal energy overcomes intermolecular attractions, particles escape to a less-condensed phase.