Evaporation happens when surface molecules acquire enough kinetic energy to overcome the attractive forces holding them in the liquid.
Why kinetic energy matters
All molecules in a liquid move randomly, but their speeds vary. The distribution of speeds follows the Maxwell‑Boltzmann curve: most molecules have average energy, while a small fraction have much higher energy. Only those high‑energy molecules can break the bonds that keep them in the liquid.
Temperature’s role
Temperature is a measure of the average translational kinetic energy of the molecules. Raising the temperature shifts the entire Maxwell‑Boltzmann distribution toward higher energies and widens the high‑energy tail. Consequently, a larger proportion of molecules reach the threshold needed to escape.
Intermolecular forces set the barrier
Liquids are held together by forces such as hydrogen bonding, dipole‑dipole interactions, and London dispersion. The energy required to overcome these forces is the latent heat of vaporization. A molecule must have kinetic energy greater than this latent heat to evaporate.
Net evaporation and vapor pressure
When a molecule leaves the surface, it adds to the vapor pressure above the liquid. If the surrounding air is already saturated, condensation balances evaporation and the net loss stops. In open air, airflow removes vapor, keeping the vapor‑pressure deficit high and allowing continuous net evaporation.
Factors that modify the rate
- Temperature: higher temperature dramatically increases the number of escaping molecules.
- Surface area: more area provides more opportunities for molecules to reach the interface.
- Air movement: wind or drafts sweep away vapor, maintaining a low local vapor pressure.
- Humidity: higher ambient humidity reduces the vapor‑pressure gradient and slows evaporation.
Evaporative cooling
When the high‑energy molecules leave, the remaining liquid loses a disproportionate amount of kinetic energy, causing its temperature to drop unless heat is supplied from outside. This is why sweating cools the body and why puddles cool as they dry.