Which process involves molecules moving from areas of higher concentration to lower concentration without energy input?

• A. Mass flow
• B. Osmosis
• C. Diffusion
• D. Capillary action

Answer: (C)

Diffusion is the spontaneous spread of molecules from areas of higher concentration to areas of lower concentration, driven by random molecular motion and requiring no external energy input. In soils, this means solutes or gases move down their concentration gradient simply because molecules are constantly bouncing around and mixing, continuing until concentrations are even. Osmosis is a related passive process but refers specifically to water moving across a semi-permeable membrane to balance water potential; it’s a special case of diffusion focused on water. Mass flow describes bulk movement of fluid driven by pressure differences and may involve energy or external forces, not just a concentration gradient. Capillary action involves liquid rise or movement in narrow spaces due to adhesive and cohesive forces, not primarily diffusion down a concentration gradient. Therefore, the process described is diffusion.