Which process is illustrated by the spreading of a gas like ammonia through air in a closed room?

• A. Diffusion
• B. Osmosis
• C. Evaporation
• D. Condensation

Answer: (A)

Diffusion is the principal concept here. It is the spreading of particles from regions of higher concentration to regions of lower concentration due to their random motion. Ammonia molecules in a closed room are constantly moving and colliding with air molecules; when some are concentrated in one area, these random motions push more molecules into less crowded areas, gradually equalizing the distribution. Temperature and molecular size affect how fast this happens—the warmer the air, the quicker the diffusion, and smaller molecules tend to diffuse more rapidly. While air currents can speed up the process, the core idea is diffusion driven by the concentration gradient. Other processes don’t fit this scenario: osmosis involves water across a semipermeable membrane; evaporation is liquid turning into gas at a surface; condensation is gas turning into liquid. So diffusion best explains the spread.