Final answer:
If the forward and reverse reactions are happening at the same rate, then the reaction is at equilibrium. The concentrations of reactants and products are not changing, but reactions are still occurring in both directions. The reaction at equilibrium is indicated by a double-headed arrow.
Step-by-step explanation:
If the forward and reverse reactions are happening at the same rate, the reaction is said to be at equilibrium or dynamic equilibrium. At this point, the concentrations of A, B, C, and D are not changing (or, are constant) and we would see no difference in our reaction container, but reactions are still occurring in both directions. It is important to point out that having constant amounts of reactants and products does NOT mean that the concentration of the reactants is equal to the concentration of the products. It means they are not changing. These reactions appear to have stopped before one of the reactants has run out.
Chemists use a double-headed arrow, ⇌, to show that a reaction is at equilibrium. We would write the example reaction as:
A+B=C+D.