In a chemical reaction, chemical substances interact together and end up forming product substances. Chemical reactions can take place between solids, liquids, or gases, and the products, likewise, can be of any phase. The speed with which a reaction proceeds can vary greatly, from fractions of a second to years, and is affected by several factors, including the surface area of the reactants.
Temperature can have a dramatic effect on the rate of a reaction. For example, a teaspoon of sugar might sit on the bottom of a glass of cold tea, but dissolve rather quickly when poured into a hot glass. Two factors contribute to this behaviour. Temperature is a measure of the kinetic energy of molecules and a higher temperature indicates that the reactant molecules are moving around faster and colliding more frequently. The reactant molecules must also possess enough energy to surmount the activation energy barrier, and they are more likely to do so as the temperature increases.
The concentration of the reactants can have a big effect on the rate of a reaction. As the concentration goes up, the reactant molecules have more chances to collide with each other and react in a given amount of time, since there are simply more of them in close proximity. In the case of a reaction between gases, the partial pressure of each gas serves the same role as concentration, and the rate of reaction goes up as the partial pressures increase.
When one of the reactants is a solid, the amount of its exposed surface area can have a profound effect on reaction rate. If the solid is present in one big mass, then the molecules inside the mass have no potential to interact with reactant molecules, since they are surrounded only by other solid molecules. The reaction will only happen at the surface of the solid, where reactant molecules can meet. Increasing the surface area of the solid by breaking it up into smaller pieces, or grinding it into a powder, will increase the rate of the reaction.
A dramatic example of the effect of surface area on reaction rate is the phenomenon of dust explosions. A number of substances are susceptible, including sawdust, coal dust, grain, flour, or powdered metals. When the powdered substance is suspended in the air in sufficient concentration, it can literally explode when a source of ignition, such as static electricity, is present. For example, the much greater surface area of sawdust, as opposed to a solid piece of wood, allows the suspended particles to react with oxygen molecules in the air, sustaining a combustion reaction.