Question

During evaporation particles of a liquid change into vapour :

• A. From the surface
• B. From the bulk together
• C. From the bottom
• D. From all over the liquid

Hint:

Key difference:
{Evaporation:} A {surface} phenomenon. Happens at any temperature (below boiling point). No bubbles. (e.g., a puddle drying up).
{Boiling:} A {bulk} phenomenon (happens throughout the liquid). Happens at a specific temperature (boiling point). Bubbles form. The question is about evaporation, so particles change to vapor {from the surface}.

Answer 1

The Correct Option is A

Concept: Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase at a temperature below its boiling point. It is distinct from boiling, which is a bulk phenomenon. Step 1: Understanding Evaporation Evaporation is a surface phenomenon. Here's why:
Particles in a liquid are in constant random motion and have a range of kinetic energies.
Particles at the surface of the liquid experience weaker intermolecular attractive forces from neighboring molecules compared to particles in the bulk (which are surrounded on all sides).
If a particle at the surface has sufficient kinetic energy to overcome the attractive forces holding it in the liquid phase, it can escape into the vapor (gas) phase.
This process can occur at any temperature at which the liquid exists, though the rate of evaporation increases with temperature. Step 2: Distinguishing Evaporation from Boiling
Evaporation: Occurs only at the surface of the liquid. Can occur at any temperature below the boiling point. No bubble formation within the liquid.
Boiling: Occurs throughout the entire bulk of the liquid when the liquid reaches its boiling point. Bubbles of vapor form within the liquid and rise to the surface. Step 3: Analyzing the options
(1) From the surface: Correct. Evaporation is fundamentally a surface phenomenon where particles escape from the liquid-gas interface.
(2) From the bulk together: This describes boiling, not evaporation.
(3) From the bottom: While heat might be supplied from the bottom (e.g., when heating a pan of water), the actual phase change during evaporation (below boiling point) occurs at the surface. If boiling occurs, bubbles can form at the bottom and rise. The image shows this option circled, but it's scientifically incorrect for evaporation.
(4) From all over the liquid: This also describes boiling (vaporization throughout the bulk). Therefore, during evaporation, particles of a liquid change into vapor primarily from the surface.