Class 9 > Unit # 09:Thermal Properties of Matter > Boiling vs Evaporation

Talha's Physics Academy

Evaporation vs Boiling & Cooling Mechanism

Q.7 Differentiate between Boiling and Evaporation and explain why evaporation causes cooling with examples.

Difference Between Evaporation and Boiling

Evaporation	Boiling
1. It takes place without requiring an external heat source.	1. It takes place only with the continuous supply of an external heat source.
2. It occurs at any temperature below the boiling point.	2. It occurs only at a specific fixed temperature called the boiling point.
3. It causes cooling.	3. It does not cause cooling.
4. It is a relatively slow process.	4. It is a relatively fast process.
5. It takes place only at the liquid surface.	5. It takes place throughout the entire liquid.
6. There is no formation of bubbles.	6. Bubbles are formed inside the liquid.

Fig: Comparison showing surface-only evaporation vs. bulk boiling with vapor bubble formation.

Why Evaporation Causes Cooling

"When evaporation occurs, water molecules with greater kinetic energy escape from the surface into the atmosphere. Consequently, molecules with lower kinetic energy are left behind. This results in a net decrease in the thermal energy and temperature of the remaining liquid. Hence, evaporation causes cooling."

Real-World Examples

1. Feeling Cold After Bathing in a Breeze:

   When you step out of water into a heavy breeze, water molecules with higher kinetic energy rapidly escape from your skin surface. The molecules with lower kinetic energy remain behind, lowering the temperature of the water film on your skin and making you feel cold.

2. Cooling Effect of Methylated Spirit:

   Methylated spirit is a volatile liquid with a low boiling point. When a few drops are placed on your hand, it evaporates instantly. To change from liquid to vapor, it absorbs the required latent heat directly from your skin, causing your hand to lose heat and feel intensely cold.

Note on Volatile Liquids

Some liquids have exceptionally low boiling points, allowing them to transform from liquid to vapor very easily and rapidly at ordinary room temperatures. Such substances are referred to as volatile liquids (e.g., alcohol, ether, petrol).

© 2026 Talha's Physics Academy. All rights reserved.