Evaporation and boiling

Two ways a liquid becomes vapour

Water in an open container slowly loses its level. Vapour leaves the surface silently and steadily. This is evaporation, and it happens at all temperatures. Warmer water evaporates faster, because more molecules escape.

Keep heating and the molecules reach the most kinetic energy they can have as a liquid. Further heat turns liquid water into gas at constant temperature. This is boiling, with bubbles forming throughout the liquid.

External pressure matters. Lower pressure lets escaping molecules drift away, so evaporation speeds up. Higher pressure pushes them back. Lower pressure also lowers the boiling point.

Your book: water boils at 100 °C in Karachi but at 98 °C in Murree, where air pressure is lower. At Murree's height, water actually boils nearer 92 °C. The idea is the same: lower pressure, lower boiling point.

Evaporation is a few fast molecules slipping out of the surface. Boiling is the whole liquid turning to gas, once its vapour pressure matches the pressure outside.

Key terms

Surface phenomenon

A change that happens only at the top of the liquid.

Bubbles

Pockets of vapour forming inside the liquid when it boils.

External pressure

The push of the air on the liquid; it sets the boiling point.

Cooling effect

Evaporation takes energy from the liquid, so what is left gets cooler.

Short questions with model answers

1. Q1. Rice is cooked in boiling water in Karachi and in Murree. Where is the boiling water hotter?

   • Compare pressure.
   • Pressure and boiling point.

   In Karachi: boiling water there is at 100 °C, but below 100 °C in Murree, so food cooks more slowly in Murree.

2. Q2. Why do we feel comfortable wearing cotton clothes in summer? (Quick Check 14.2, p. 5)

   • Sweat.
   • Evaporation cools.

   The sweat held by cotton evaporates and takes energy from the skin, which cools us.

Common mistakes

• ✗ Water always boils at 100 °C.

  ✓ Only at 1 atm. At lower pressure, such as in the hills, it boils below 100 °C.

• ✗ Higher pressure makes a liquid evaporate faster.

  ✓ Higher pressure pushes molecules back and slows evaporation.

• ✗ Both evaporation and boiling happen only at the surface.

  ✓ Only evaporation is a surface process; boiling forms bubbles throughout.

MCQs

1. 1. Evaporation is described as a:

   1. (a) Process throughout the liquid
   2. (b) Surface phenomenon
   3. (c) Process only at 100 °C
   4. (d) Process only in closed containers
   Show answer

   (b) Table, point 1 (p. 6): it occurs slowly and only at the surface.

2. 2. Which process produces cooling?

   1. (a) Boiling
   2. (b) Condensation
   3. (c) Freezing
   4. (d) Evaporation
   Show answer

   (d) Table, point 3 (p. 6).

3. 3. When the external pressure on water is lowered, its boiling point:

   1. (a) Rises
   2. (b) Stays at 100 °C
   3. (c) Falls
   4. (d) Disappears
   Show answer

   (c) p. 7: the boiling point decreases at lower atmospheric pressure (Karachi and Murree).

4. 4. Increasing the external pressure on an open liquid makes evaporation:

   1. (a) Faster
   2. (b) Slower
   3. (c) Unchanged
   4. (d) Turn into boiling
   Show answer

   (b) p. 7: higher pressure forces escaping molecules back to the surface, making evaporation difficult.

5. 5. How does evaporation depend on the attraction between the molecules of a liquid?

   1. (a) It decreases as the attraction gets stronger
   2. (b) It increases as the attraction gets stronger
   3. (c) It does not depend on the attraction
   4. (d) It first increases, then decreases
   Show answer

   (a) Exercise MCQ 6 (p. 14): stronger attraction holds molecules back, so fewer escape.

Quick revision

• Evaporation is slow, at the surface, at all temperatures, and it cools the liquid.
• Boiling happens throughout the liquid at one temperature for a given pressure.
• Lower external pressure speeds evaporation and lowers the boiling point.