fluid
Phase and Surface Area Effects When two reactants are in the same fluid phase, their particles collide more frequently than when one or both reactants are solids or when they are in different fluids that do not mix . If the reactants are uniformly dispersed in a single homogeneous solution, then the number of collisions per unit time depends on concentration and temperature, as we have just seen. If the reaction is heterogeneous, however, the reactants are in two different phases, and collisions between the reactants can occur only at interfaces between phases. The number of collisions between reactants per unit time is substantially reduced relative to the homogeneous case, and, hence, so is the reaction rate. The reaction rate of a heterogeneous reaction depends on the surface area of the more condensed phase. Automobile engines use surface area effects to increase reaction rates. Gasoline is injected into each cylinder, where it combusts on ignition by a spark from the spark plug. The gasoline is injected in the form of microscopic droplets because in that form it has a much larger surface area and can burn much more rapidly than if it were fed into the cylinder as a stream. Similarly, a pile of finely divided flour burns slowly or not at all , but spraying finely divided flour into a flame produces a vigorous reaction Figure 14.3 "The Effect of Surface Area on Reaction Rates" . Similar phenomena are partially responsible for dust explosions that occasionally destroy grain elevators or coal mines.