Diffusion rates in phases – why do gases diffuse faster than liquids? In physical chemistry and transport phenomena, gases typically diffuse much faster than liquids. Which underlying reason best explains this observation in terms of molecular interactions in the liquid phase?

Introduction / Context:
Diffusion is the spontaneous mixing of molecules driven by random thermal motion and concentration gradients. A common comparison in transport phenomena is that diffusion coefficients of gases are orders of magnitude larger than those of liquids. Understanding the reason helps in designing absorbers, extractors, and reactors where mass transfer resistance matters.

Given Data / Assumptions:

• Gases have weak intermolecular attractions and large mean free paths.
• Liquids have much stronger cohesive forces and far smaller molecular mobility.
• Temperature is comparable in both cases for the qualitative comparison.

Concept / Approach:
Fick's law relates flux to the diffusion coefficient D. Empirically, D for gases at ambient conditions is about 10^-5 to 10^-4 m^2/s, while for liquids it is typically 10^-10 to 10^-9 m^2/s. The principal cause is not merely molecular mass but the much stronger intermolecular forces and close packing in liquids, which hinder molecular rearrangements and random walks. In gases, molecules are widely spaced; collisions randomize direction yet allow longer displacements between interactions, enabling faster diffusion.

Step-by-Step Solution:

Verification / Alternative check:
Measured diffusion coefficients support this: D_air for small gases ~ 0.1–1 cm^2/s, whereas D_liquid for solutes in water ~ 10^-5–10^-6 cm^2/s. This five-order magnitude gap cannot be explained by mass differences alone.

Why Other Options Are Wrong:

• Move faster: Liquid molecules generally move slower than gas molecules at the same temperature due to constraints.
• No definite shape: A macroscopic property unrelated to molecular-level diffusion rate.
• Are heavier: Molecular mass has some effect, but it is not the dominant reason across phases.

Common Pitfalls:
Attributing diffusion solely to temperature or mass while ignoring phase structure and cohesion; overlooking that viscosity and packing dominate in liquids.

Final Answer:
are held together by stronger inter-molecular forces.