Which statement correctly describes diffusion?

• A. Molecules move from high concentration to lower concentration
• B. Molecules move from low concentration to higher concentration
• C. Energy is required for diffusion
• D. Diffusion only occurs in gases

Answer: (A)

Diffusion is the passive spread of particles due to their random motion, so the net movement goes from areas of higher concentration to areas of lower concentration. This occurs without any input of energy, because it relies on the natural kinetic energy of molecules pushing them to mix. Over time, substances diffuse through gases, liquids, and even, very slowly, through solids as particles move and spread out until the concentration is even. How quickly diffusion happens depends on factors like the temperature (more motion at higher temperatures), the steepness of the concentration gradient, the surface area available for movement, and the size of the molecules (larger particles diffuse more slowly). The idea that diffusion requires energy is incorrect, and diffusion is not limited to gases—it also happens in liquids and solids, though at different rates.