Effect of Concentration on Speed of Reaction

This video shows the reaction of magnesium with hydrochloric acid of different concentrations. We can see that the speed of reaction increases as the concentration of the reactants increases.

This can be explained using the Collision Theory. When concentration increases, number of particles per unit volume increases. Frequency of effective collisions between the reacting particles increases. Since frequency of effective collision is proportional to speed of reaction, speed of reaction increases.