N₂O₄: A colorless gas that forms when two NO₂ molecules combine.
Le Chatelier's principle:A principle in chemistry that states if a system at equilibrium experiences a change, the system will respond in such a way as to counteract the change and restore equilibrium.
Equilibrium:A state in which the forward and reverse reactions of a chemical reaction occur at equal rates, resulting in no net change in concentrations of reactants or products.