The effect of changing the concentration of reactants on the rate of the reaction between hydrochloric acid HCl and magnesium Mg can be explained using the collision theory. According to this theory, the rate of a chemical reaction depends on the frequency of collisions between the reacting particles and the energy with which they collide.When the concentration of hydrochloric acid is increased, there are more HCl molecules in the same volume of the solution. This leads to a higher probability of collisions between HCl molecules and magnesium atoms, resulting in an increased rate of reaction. Conversely, if the concentration of hydrochloric acid is decreased, there will be fewer HCl molecules in the same volume, leading to a lower probability of collisions and a slower rate of reaction.The reaction between hydrochloric acid and magnesium can be represented by the following balanced equation:2HCl aq + Mg s MgCl2 aq + H2 g In summary, increasing the concentration of reactants in this case, hydrochloric acid will generally increase the rate of the reaction between hydrochloric acid and magnesium, while decreasing the concentration will slow down the reaction rate.