In most reactions the reactants turn into products and that is the end of it. In some reactions, though, the products can react to produce the original reactants. These are called reversible reactions and they are shown with a double arrow: A + B ⇌ C + D. The reaction that makes the products is the forward reaction. The reaction that turns the products back into the reactants is the reverse reaction.
The direction of a reversible reaction can be changed by changing the conditions. Heating can drive the reaction one way and cooling or adding something can drive it back the other way.
Ammonium chloride is a white solid. When it is heated it breaks down into ammonia and hydrogen chloride gas: NH4Cl → NH3 + HCl. When the two gases cool down, they react together and form solid ammonium chloride again: NH3 + HCl → NH4Cl. We write this as NH4Cl ⇌ NH3 + HCl.
Hydrated copper sulfate is blue. Heating it drives off the water and leaves white anhydrous copper sulfate. If water is added to the white solid, it turns blue again, so the reaction has gone in the reverse direction: CuSO4.5H2O ⇌ CuSO4 + 5H2O.
