Evaporation
Definition and meaning of Evaporation in chemistry.
Evaporation is the process by which molecules at the surface of a liquid gain enough kinetic energy to overcome intermolecular forces and escape into the gas phase, occurring at any temperature below the liquid's boiling point.
In more detail
Because molecules in a liquid have a range of kinetic energies (described by the Maxwell-Boltzmann distribution), a small fraction near the surface at any moment move fast enough to break free as vapor, even well below the boiling point. Since the fastest, highest-energy molecules leave first, the average kinetic energy, and therefore the temperature, of the remaining liquid drops, a phenomenon called evaporative cooling. The rate of evaporation increases with higher temperature, greater surface area, air movement over the surface, and lower humidity or vapor pressure in the surrounding air.
Key facts
| Field | Physical Chemistry |
|---|---|
| Process type | Phase change, liquid to gas |
| Location | Occurs only at the liquid's surface |
| Key effect | Evaporative cooling of the remaining liquid |
A puddle of water on a sidewalk gradually disappears over several hours in the sun, even though its temperature never reaches 100°C (373 K), because surface water molecules continuously escape into the air as vapor.
Frequently asked questions
How is evaporation different from boiling?
Evaporation happens only at the liquid's surface and can occur at any temperature, while boiling occurs throughout the entire liquid at a specific temperature where the liquid's vapor pressure equals the surrounding atmospheric pressure.
Why does evaporating liquid feel cold?
The molecules with the highest kinetic energy leave the liquid first, which lowers the average kinetic energy of the molecules left behind, and temperature is a measure of that average kinetic energy.