What term describes when a chemical passes through a material on a molecular level?

• A. Diffusion
• B. Osmosis
• C. Permeation
• D. Adsorption

Answer: (C)

Permeation describes a chemical moving through a barrier material at the molecular level. It covers the whole process: the substance sorbs into the material on the entry side, diffuses through the barrier, and desorbs on the far side. This is the term used when discussing how a chemical can penetrate protective barriers, filters, or packaging. Diffusion is part of permeation—how molecules spread from high to low concentration within a medium—or across a barrier, but on its own it doesn’t imply passage through the material to the other side. Osmosis is the flow of water across a semipermeable membrane due to concentration differences, a specific case not general to chemical permeation through arbitrary materials. Adsorption is surface sticking to the material rather than passing through. So the concept best describing a chemical crossing the material itself is permeation.