Many chemical reactions go only one way, but some are reversible. In a reversible reaction the products can react to re-form the original reactants. In an equation this is shown with the symbol ⇌ instead of a single arrow.
A reversible reaction can be made to go in the opposite direction by altering the reaction conditions. A good example is hydrated copper(II) sulfate, CuSO4·5H2O, which forms blue crystals. When these are heated, water is driven off and white anhydrous copper(II) sulfate is left. When water is added to the white anhydrous solid, the blue hydrated crystals form again and the mixture gets hot.
The energy changes in the two directions are linked. If the forward reaction is endothermic, the reverse reaction is exothermic, and the same amount of energy is transferred in each direction. In the copper sulfate example, heating (the forward reaction) is endothermic, so adding water (the reverse reaction) is exothermic.