What effect does increasing concentration have on the rate of reaction?

Increasing the concentration of reactants in a chemical reaction leads to a higher number of reactant molecules present in a given volume. This increase in particles leads to a greater frequency of collisions between the reactant molecules. According to the collision theory, the rate of a chemical reaction is proportional to the number of effective collisions per unit time.

When the concentration is higher, the likelihood that molecules will collide—particularly in the correct orientation and with enough energy to react—also increases. This results in a higher rate of reaction. For example, if you double the concentration of the reactants, you can often expect the rate of reaction to also increase, potentially doubling under ideal conditions.

In contrast, factors such as decrease in concentration would reduce the rate due to fewer particles being available for collisions, and some reactions may be less affected by changes in concentration depending on their specific order or mechanism, which does not apply universally to all reactions. The specific effect observed in reactions that involve solids does not detract from the general principle that increasing concentration typically leads to an increase in reaction rates for the bulk of chemical processes.