Which factor would increase diffusion rate, as an extrinsic factor?

• A. Concentration
• B. Type of bonding
• C. Grain boundary area
• D. Temperature

Answer: (D)

Temperature directly controls how mobile atoms are. Diffusion depends on atoms having enough thermal energy to jump from one site to another, which means increasing temperature raises the number of successful jumps. This is captured by the diffusion coefficient following an Arrhenius relationship, D = D0 exp(-Q/RT): as temperature rises, the exponential term grows and diffusion speeds up. The other factors are tied to the material itself or its structure rather than an external condition: concentration affects the driving force for diffusion but not the intrinsic mobility unless the gradient changes; the type of bonding is an intrinsic chemical property setting the energy barrier; and grain boundary area changes diffusion paths through the microstructure, not by altering external conditions. So, the external condition that increases diffusion rate is temperature.