A reversible reaction is one that can go in both directions: the reactants form the products, and the products can react to re-form the reactants. The symbol ⇌ shows that a reaction is reversible.The reaction from left to right is the forward reaction, and from right to left is the backward (or reverse) reaction.Example 1. Heating blue hydrated copper(II) sulfate drives off water and leaves white anhydrous copper(II) sulfate. Adding water to the white solid turns it blue again, and the mixture gets hot:CuSO4·5H2O(s) ⇌ CuSO4(s) + 5H2O(l) Example 2. Heating ammonium chloride forms ammonia and hydrogen chloride. In a cooler part of the tube, the gases recombine to form white solid ammonium chloride:NH4Cl(s) ⇌ NH3(g) + HCl(g) 
Example 3. The Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g).If the forward reaction is exothermic, the backward reaction is endothermic, with an enthalpy change of the same size but opposite sign. For example, if ΔH for the forward reaction is −92 kJ mol−1, ΔH for the backward reaction is +92 kJ mol−1. For copper(II) sulfate, the dehydration is endothermic (it needs heating), so the rehydration is exothermic.In principle, all reactions are reversible. Many, such as combustion, have a backward reaction so slow or so unfavourable that they are treated as going to completion.
Example 3. The Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g).If the forward reaction is exothermic, the backward reaction is endothermic, with an enthalpy change of the same size but opposite sign. For example, if ΔH for the forward reaction is −92 kJ mol−1, ΔH for the backward reaction is +92 kJ mol−1. For copper(II) sulfate, the dehydration is endothermic (it needs heating), so the rehydration is exothermic.In principle, all reactions are reversible. Many, such as combustion, have a backward reaction so slow or so unfavourable that they are treated as going to completion.