Diffusion is the transport of mass by atomic motion; many processes rely on diffusion within a solid or between phases.

• A. It is the transport of mass by atomic motion; many processes rely on diffusion within a solid or between phases
• B. It is only relevant to liquids
• C. It does not affect treatment of materials
• D. It requires chemical reactions to occur

Answer: (A)

Diffusion is the transport of mass by atomic motion, which means atoms move from regions of higher concentration to regions of lower concentration due to random thermal motion. In solids this happens through mechanisms like vacancies and interstitial diffusion, and it underpins many important processes such as alloying, phase transformations, surface treatments, and diffusion bonding. It is not limited to liquids; diffusion in solids is essential for how materials are heat-treated and how they interact across interfaces. It does not require chemical reactions to occur—concentration gradients alone drive the movement, although reactions can occur alongside diffusion. This is why the statement in the option is the best choice.