Grade 8 physics · Thermodynamics
Vaporization, boiling and air humidity
The same liquid can quietly evaporate from its surface, boil throughout its volume, or return from vapor to liquid by condensation. Energy, pressure and humidity determine which changes we observe.
1. Three related phase processes
Vaporization is the general change from liquid to gas. It includes evaporation and boiling. Evaporation occurs only at the free surface and can happen at any liquid temperature. Faster surface particles sometimes have enough energy to escape the attraction of neighboring particles.
Boiling occurs throughout a liquid when vapor bubbles can grow inside it. At the boiling temperature, the saturated-vapor pressure in the bubbles matches the external pressure. Condensation is the reverse change: vapor becomes liquid and transfers energy to its surroundings.
| Process | Where? | Temperature condition | Energy transfer |
|---|---|---|---|
| Evaporation | Surface only | Any temperature | Liquid absorbs energy |
| Boiling | Throughout the liquid | At the boiling point for that pressure | Liquid absorbs energy |
| Condensation | Where vapor meets cooler matter | When vapor becomes saturated or is compressed | Vapor releases energy |