Which of the following processes is not a surface phenomenon?

Analyze the question: We need the process that does NOT happen only at a surface (that is, one that involves the bulk of the substance, or is not confined to the surface of a liquid or solid).

Concepts:

• Evaporation: Only the fastest molecules at the surface of a liquid escape into the vapour. It is a surface phenomenon.
• Photoemission (photoelectric emission): Light hits the metal surface and ejects electrons from the surface layer. It is a surface phenomenon.
• Thermionic emission: A heated metal gives off electrons from its surface. It is a surface phenomenon.
• Condensation: Vapour changes to liquid. It can occur throughout the body of the vapour, for example in fog, clouds or mist, where droplets form on dust particles in the air. It does not need a liquid or solid surface, so it is not strictly a surface phenomenon.

Working through the options:

• B, C and D all depend on particles leaving a surface.
• A (condensation) takes place within the vapour itself, so it is the odd one out.

Answer: A. Condensation.

Tip: Remember that "emission" and "evaporation" mean particles leaving a surface. Condensation is the reverse process, where particles join together, and it can occur anywhere in the vapour. If a question says "not a surface phenomenon," look for the process that does not depend on escape from a surface.