Evaporation & Cooling

• Evaporation — surface phenomenon; liquid → vapour below boiling point.
• Factors — surface area, temperature, humidity, wind speed.
• Cooling effect — high-energy particles escape → average KE of liquid decreases.
• Applications: sweating, sprinkling water on roof, earthen pot (matka).

1. Surface phenomenon — only top-layer particles with enough energy escape.
2. Rate increases with — larger area, higher T, low humidity, faster air flow.
3. Cooling — remaining liquid has lower average kinetic energy → lower temperature.
4. vs Boiling — boiling at definite T throughout bulk; evaporation at any T from surface.
5. Diagram (text) — wet cloth on forehead cools body as water evaporates.
6. NCERT Ch. 1 — spirit on palm feels cold; drying clothes on windy day.
7. Real world — desert cooler; acetone before injection; salt recovery from sea water.
8. Humidity — moist air slows evaporation (fewer spaces for vapour).

Why do we feel cool when sweat evaporates from skin?

Step 1 — Sweat (liquid water) on skin surface.
Step 2 — High-energy water molecules escape as vapour.
Step 3 — Average KE of remaining sweat decreases → temperature drops.
Step 4 — Heat flows from body to sweat → body cools.
Step 5 — Fan increases evaporation rate by removing saturated air.

• Saying evaporation and boiling are the same process.
• Misconception: evaporation only occurs at high temperature (occurs at all T, slower when cold).
• Thinking humidity helps drying (high humidity slows drying).
• Confusing evaporation with condensation.
• Forgetting evaporation causes cooling of the remaining liquid.

• Define evaporation. How differs from boiling?
• List four factors affecting evaporation rate.
• Why are clothes dried faster on a windy day?
• Why does an earthen pot keep water cool?

Open the Practice tab for graded questions on Evaporation & Cooling.