Reaction rates of solids are a special problem because mixing of reactants is limited by the solid state. Reactions only take place at the outer surface because that is the only area where collisions take place. To increase solid reaction rates, solids are ground to provide a larger surface area exposed to other reactants. Two solids may be ground together. A solid and liquid react faster when a ground solid is dispersed in the liquid. A solid and gas react faster when a ground solid is dispersed in the gas as a fine powder or dust.