A chemical reaction is characterized by the change of reactant chemicals (or simply reactants) into product chemicals (or products). The rate at which this change takes places is called the reaction rate. Some reactions occur slowly - like the rusting of the steel body of an automobile. This is a low rate of reaction. Other reactons - like the combustion (i.e., explosion) of hydrogen gas. Hydrogen combustion is a reaction that occurs at a high rate. 

Reaction rates can be altered and controlled by changing a few variables. Some variables affecting the rate of a reaction are concentration of aqueous-state reactants, temperature at which the reaction occurs, the exposed surface area of solid-state reactants, and whether or not a catalyst is used. 
 

The rate at which a reaction takes place depends on several variables. The amount of surface area is one of those variables. When solids react, the reaction takes place on the solid's surface. As more atoms are exposed to the other reactant - that is, as surface area increases - the rate of reaction increases. Grinding or pulverizing a solid makes considerably more particles of the solid available for reaction. The interior particles of a tablet are unable to react. Thus the unground tablet reacts more slowly.