Movement of particles from high to low concentration.

• A. Osmosis
• B. Diffusion
• C. Dialysis
• D. Filtration

Answer: (B)

Diffusion is the net movement of particles from regions of higher concentration to regions of lower concentration, driven by the random thermal motion of molecules. This random motion causes particles to spread out until the concentrations balance, and it happens without needing external energy input. The rate of diffusion increases with higher temperature (more kinetic energy) and with smaller particles or a less viscous medium, while moving more slowly for larger molecules or through denser media.

Osmosis is specifically about the movement of water across a selectively permeable membrane, typically from a region of lower solute concentration to higher solute concentration, so it’s a solvent movement rather than a general solute diffusion. Dialysis is a practical technique that uses diffusion of small solutes across a semipermeable membrane to separate substances; it’s diffusion in a membrane context, not the broad concept itself. Filtration relies on applying pressure to move a mixture through a barrier and separates components by size, not by a concentration gradient. So the broad, defining idea that matches “movement of particles from high to low concentration” is diffusion.