Boiling is a *phase change* that occurs throughout the entire liquid when it reaches its boiling point \$T_b\$. At this temperature, the liquid’s vapour pressure equals the external pressure, so bubbles of vapour form inside the liquid and rise to the surface. Think of a pot of soup on the stove: when the soup reaches 100 °C (at 1 atm), bubbles start to form everywhere, not just at the bottom. The process is rapid and requires a continuous supply of heat.
Evaporation is the *slow* transition of molecules from the surface of a liquid into the gas phase. It can occur at any temperature, but the rate increases with temperature and decreases with humidity. Imagine a puddle drying on a sunny day: only the molecules at the very top escape into the air. Unlike boiling, evaporation does not require the liquid to reach a specific temperature and does not produce bubbles inside the liquid.
Imagine a crowded party (the liquid). When the music (heat) turns up to a certain volume, everyone starts dancing everywhere and the room fills with energy—this is boiling. If the music is low, only a few people at the edge of the room quietly leave the party (evaporation). Both involve people leaving, but the circumstances and scale are different.
| Aspect | Boiling | Evaporation |
|---|---|---|
| Occurs throughout? | Yes | No (surface only) |
| Temperature requirement | \$T = T_b\$ | Any \$T\$ |
| Pressure condition | \$P{\text{vap}} = P{\text{ext}}\$ | \$P{\text{vap}} < P{\text{ext}}\$ |
| Speed | Fast, visible bubbles | Slow, invisible |
| Heat input | Continuous | Optional |
Exam Tip: When answering questions about boiling and evaporation, always: