Flexi Says: By their nature, some reactions occur very quickly, while others are very slow. However, certain changes in the reacting conditions can have an effect on the rate of a given chemical reaction. Collision theory can be utilized to explain these rate effects. An increase in the concentration of one or more reacting substances results in an increase in the rate of reaction. When more particles are present in a given amount of space, a greater number of collisions will naturally occur between those particles. When the pressure of a gas is increased its particles are forced closer together, decreasing in the process the amount of empty space between the particles. For gaseous reactions, an increase in pressure increases the rate of reaction as higher gas pressure leads to a greater number of collisions between reacting particles. An increase in the surface area of a reactant increases the rate of a reaction. Surface area is larger when a given amount of a solid is present as smaller particles. Raising the temperature of a chemical reaction usually results in a higher rate of reaction. Increasing the temperature of a reaction increases the number of effective collisions between reacting particles, so the reaction rate increases.