The liquid-to-gas phase transition, broadly known as vaporization, is a fundamental process governing the water cycle and cooking techniques. Vaporization can happen in two distinct ways: evaporation and boiling. Understanding the difference between these two processes—one slow and surface-limited, the other rapid and volume-wide—is central to comprehending how water changes state. The question of whether boiling water evaporates touches upon the core physics that separate these two phenomena.
Evaporation is a Surface Process
Evaporation is a form of vaporization that happens exclusively at the surface of a liquid and can occur at nearly any temperature above the freezing point. The molecules within a liquid are constantly in motion, possessing a wide distribution of kinetic energies. Only those molecules with sufficient energy near the surface can overcome the cohesive forces of the liquid and escape into the surrounding air as a gas.
Because only the highest-energy molecules transition to the vapor phase, the average kinetic energy of the remaining liquid decreases. This reduction in energy results in a cooling effect on the liquid left behind. Evaporation does not require the liquid to reach a specific temperature and continues until the surrounding air becomes saturated with vapor.
Boiling is a Bulk Process
Boiling is a rapid form of vaporization that involves the entire volume of the liquid. This process only begins when the liquid reaches a specific temperature known as the boiling point. At this temperature, the internal vapor pressure of the liquid becomes equal to the external atmospheric pressure acting on the surface.
When this pressure balance is achieved, water molecules throughout the liquid gain enough energy to form vapor bubbles internally. These bubbles grow and rise to the surface, where they release their gas into the atmosphere. Because the phase change occurs throughout the entire body of water, boiling is classified as a bulk phenomenon and requires a constant external source of heat to sustain the rapid transition.
The Relationship Between Evaporation and Boiling
Both evaporation and boiling are classified under the umbrella term of vaporization, which describes the change from a liquid to a gas state. Evaporation is generally a slow process, while boiling is significantly faster, transforming a large mass of liquid into gas quickly once the boiling point is reached. The distinction lies in the location and mechanism of the phase change.
When water is actively boiling, it is held at the boiling point, which means the rate of surface-level evaporation is also maximized. Therefore, evaporation is technically still occurring at the surface. However, the massive, rapid phase transition happening throughout the bulk of the liquid is the dominant mechanism for mass loss. The presence of vapor bubbles throughout the liquid volume confirms that boiling is the primary process. This makes the contribution of surface evaporation practically insignificant by comparison.