In a reversible reaction the products can react to form the original reactants again. The reaction can go forwards and backwards. In an equation this is shown with the symbol ⇌, which is two arrows pointing in opposite directions.
Heating ammonium chloride is a simple example. The white solid breaks down into two gases, ammonia and hydrogen chloride. Higher up the test tube, where the glass is cooler, the two gases react together and form white ammonium chloride again. NH4Cl ⇌ NH3 + HCl
Another example is hydrated copper sulfate. Heating the blue crystals drives off water and leaves a white solid, anhydrous copper sulfate. Adding water to the white solid turns it blue again.
In these examples the direction of the reaction is altered by changing the reaction conditions. Heating pushes the reaction one way and cooling, or adding water, pushes it back the other way.
