The effect of changing the concentration of reactants on the rate of a chemical reaction between hydrochloric acid HCl and sodium thiosulphate Na2S2O3 can be explained using the collision theory. According to this theory, the rate of a reaction depends on the frequency of effective collisions between reactant particles.When the concentration of either hydrochloric acid or sodium thiosulphate is increased, the number of particles of the reactants in the solution also increases. This leads to a higher probability of effective collisions between the reactant particles, resulting in an increased rate of reaction.In summary, increasing the concentration of either hydrochloric acid or sodium thiosulphate will increase the rate of the chemical reaction between them. Conversely, decreasing the concentration of either reactant will result in a slower reaction rate.