On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid, or else the liquid would turn into vapour quickly. When the molecules collide, they randomly bump into each other and transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to escape from the attractive pull of the liquid.
Volatile liquids (alcohol, petrol) need less energy for molecular escape
If the faster molecules escape, the slower, cooler ones are left behind. Evaporation (sweating) causes cooling.
Evaporation happens at all temperatures, but faster in:
• Warm (average energy greater in the liquid, increasing probability of escape)
• Dry (allows molecules to escape into the air without competing for molecules already there)
• Windy conditions (evaporated molecules swept away, making room for more) This is why we blow on a drink to cool it
Boiling is forced evaporation and happens ONLY at one temperature.
Think about this applied to:
- a puddle on a wet day (damp humid days puddles last longer)
- washing drying (best if warm, dry and windy)
- the shape of coffee cups (those with large surface area will cool more quickly than cofffe in little espresso cups)
This is a nice little interactive animation