Boiling and evaporation

Each atom in a liquid is held by its neighbours. To leave, it needs enough kinetic energy to climb out of that attraction. At any temperature a few fast atoms escape from the surface — that is evaporation. In a bulk liquid, boiling occurs when its vapour pressure reaches the surrounding pressure and vapour bubbles can grow throughout the liquid. This tiny model measures the change from a liquid cluster to dispersed gas; it does not resolve bulk boiling bubbles.

Notice the heating curve: as the liquid becomes gas, the potential energy per atom rises even though the thermostat holds the temperature. That energy goes into pulling atoms apart, not into making them faster. This illustrates the microscopic energy cost behind the latent heat of vaporisation; a bulk constant-pressure latent heat also includes expansion work.