The effect of varying the concentration of reactants on the rate of reaction between hydrochloric acid HCl and sodium thiosulfate Na2S2O3 can be explained using the collision theory. According to this theory, the rate of a chemical reaction depends on the frequency of collisions between reactant molecules and the energy with which they collide.When the concentration of either hydrochloric acid or sodium thiosulfate is increased, the number of particles of the respective reactant in the solution also increases. This leads to a higher probability of collisions between the reactant particles, resulting in an increased rate of reaction.In summary, increasing the concentration of either hydrochloric acid or sodium thiosulfate will increase the rate of reaction between the two substances. Conversely, decreasing the concentration of either reactant will result in a slower reaction rate.