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A reversible reaction is a chemical reaction that can proceed in both forward and backward directions. This means that the reactants can form the products, and then the products can convert back into the reactants under certain conditions. In other words, a reversible reaction does not reach completion, as both reactions are occurring simultaneously. The chemical equation for such reactions usually has a double arrow, which indicates the reversibility.

Reversible reactions are often in a dynamic equilibrium, meaning the rate of the forward reaction is equal to the rate of the backward reaction. As a result, the concentrations of both reactants and products remain constant over time, even though the reactions are still taking place on a molecular level.

One common example of a reversible reaction is the synthesis and decomposition of ammonia (NH3). Nitrogen gas (N2) and hydrogen gas (H2) can react together to form ammonia gas according to the following equation: N2(g) + 3H2(g) ⇌ 2NH3(g) In this reaction, nitrogen and hydrogen are the reactants, and ammonia is the product. However, the ammonia can also decompose back into nitrogen and hydrogen, indicated by the double arrow in the equation. This means the reaction is reversible and can proceed in both directions. In this example, increasing the pressure or lowering the temperature will favor the forward reaction, shifting the equilibrium towards the right and producing more ammonia. Conversely, decreasing the pressure or raising the temperature will favor the backward reaction, shifting the equilibrium towards the left and decomposing more ammonia back into nitrogen and hydrogen.