The given chemical reaction is between calcium carbonate and hydrochloric acid to produce calcium chloride, carbon dioxide and water. This reaction can be represented as,
[tex]CaCO_{3}(s) + 2HCl(aq)-->CaCl_{2}(aq)+H_{2}O(l)+CO_{2}(g)[/tex]
When the concentration of the reactants is increased, the number of collisions that take place between the reactant molecules increases. This speeds up the reaction. Therefore, when the concentration of HCl is doubled as long as there is enough calcium carbonate to react with HCl, the number of effective collisions giving products increases, this causes the rate of forward reaction to increase.
