Final answer:
The double-ended arrow in a chemical reaction signifies a reversible reaction, which can proceed in both directions and reach a dynamic equilibrium where reactants and products exist in a constant ratio.
Step-by-step explanation:
The double-ended arrow in a chemical reaction represents a reversible reaction. This means that the reaction can proceed in both directions simultaneously: the reactants can combine to form products, and those products can also decompose back into the original reactants. When the forward and reverse reactions occur at the same rate, the system is at a state of dynamic equilibrium. At equilibrium, the proportions of reactants and products remain constant over time, even though the individual molecules continue to react with one another.