Describe the reason why a reaction between two substances comes to an end.

Answer: A reaction between two substances comes to an end for several reasons, but the most common one is that one or more of the reactants are used up, or entirely consumed. When this happens, there is no further reactant to continue the reaction, and so the reaction naturally halts. This situation is described as the reaction reaching completion.

Another reason why a reaction might come to an end is because it has achieved a state of dynamic equilibrium. In reversible reactions, the forward reaction, where reactants convert to products, is balanced by the reverse reaction, where products convert back into reactants. At equilibrium, both reactions occur at the same rate, so the concentrations of reactants and products remain constant over time, even though the reactions are still ongoing. This can give the appearance that the reaction has stopped, but in truth, both forward and reverse reactions are still happening.

Environmental conditions can also cause a reaction to stop. For example, if the temperature changes, a reactant is depleted, or a catalyst is lost or deactivated, these conditions can halt a reaction.

Chemical reactions also might naturally reach a completion point due to the formation of a precipitate, the evolution of a gas, or the production of a weakly dissociating substance like water—conditions that remove reactants from the active mixture and halt the further progression of the reaction.