Some chemical reactions simply proceed in one direction until the reactants are used up. These reactions are known as irreversible.
For example, a combustion reaction, in which methane reacts with oxygen in the air:
CH4 (gas) + 2O2(gas) →CO2 (gas) + 2H2O (gas)
However, other reactions are classified as reversible. In a reversible reaction, reactants become products, but also products become reactants.
Reagents ↔ Products
For example, the reaction of nitrogen and hydrogen to form ammonia:
3H2(gas)+N2 (gas)↔ 2NH3(gas)