= Surface chemistry
<Surface chemistry> studies chemical processes at interfaces, where local coordination and composition differ from the bulk. Adsorption can concentrate reactants, change their electronic environment and enable surface-mediated <catalysis>. The active surface can be a solid–gas, solid–liquid or other interface.
For a spherical particle of <radius> $r$, the surface-area-to-volume ratio is $3/r$. Making particles smaller therefore increases the fraction of material exposed to the environment. This geometric change helps explain why fine particles can react differently from a bulk specimen of the same nominal composition. Surface structure, defects and adsorbed species still matter: geometric area alone does not count chemically active sites.
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