Phase Transitions: Vaporization and Condensation

In a liquid, when the thermal motions of molecules overcome the intermolecular forces holding them together, the molecules break free and enter the gas state. This transition from liquid to gas is known as vaporization and can occur under two conditions: non-boiling or boiling. Under the non-boiling condition, vaporization occurs only at the surface and below the liquid’s boiling point. This is called surface evaporation and occurs without the formation of vapor bubbles in the bulk liquid. In contrast, if vaporization occurs at the liquid’s boiling point, vapor bubbles form in the bulk liquid, and the process is called boiling. Boiling is not a surface phenomenon and occurs at all points inside the liquid. Vaporization is a temperature-dependent endothermic process: the greater the heat supplied, the higher the vaporization rate.  The amount of energy required to vaporize one mole of a liquid is called its molar heat of vaporization or its molar enthalpy of vaporization. As vaporization is an endothermic process, its enthalpy value is always positive. Intermolecular forces influence the molar enthalpy of vaporization. For instance, due to the strong network of hydrogen bonds between water molecules, one mole of water requires a substantial amount of heat energy — around 40.65 kilojoules — to transform into water vapor. In comparison, the weaker dipole–dipole forces between acetone molecules can be overcome with merely 31.3 kilojoules per mole of heat energy. The reverse of vaporization, that is, the transition from gas to liquid, is called condensation. When gas molecules collide with cooler liquid or solid surfaces, they lose heat. Multiple collisions result in a significant loss of heat, and the molecules ultimately condense. Condensation is, therefore, an exothermic process. Although the enthalpy of condensation is negative, its magnitude is the same as the enthalpy of vaporization. When the opposing transitions — vaporization and condensation — occur in a closed system, the system reaches a state of dynamic equilibrium called the vapor–liquid equilibrium.