For a chemical system to achieve dynamic equilibrium, certain conditions are necessary:

1. Reaction must be reversible.
   
   
2. System must be closed.
   
   
3. Temperature and pressure must remain constant.

1. Reaction must be reversible.

   
   Until now in your study of chemistry, you may have assumed that all chemical reactions are quantitative, which means that the limiting reagent is consumed completely and that the reaction proceeds only in the forward direction. However, evidence obtained from numerous reactions proves that many reactions are reversible. A reversible reaction can proceed in either direction depending on the conditions. For example, as you will see in the following investigation, even after the reaction appears to have stopped, there is evidence that both reactants are still present to some extent in the reaction mixture.

   
   Reversibility of reactions can be explained by the collision-reaction theory. On one hand, the reactants are colliding to form products; on the other hand, products are colliding to form reactants.
   
   

1.  
2. System must be closed.

   
   A closed system is a system in which no matter can enter or exit. For a chemical system to achieve dynamic equilibrium, the system must be closed so that the reactants and products cannot escape from the reaction container.  If no gas is consumed or produced in the reaction however, a physical boundary is unnecessary. For example, a beaker containing a reaction in solution can be considered "closed" if no gas is consumed or produced in the reaction.

1.  
2.  
3. Temperature and pressure must remain constant.

   
   Constant temperature and pressure is a condition of dynamic equilibrium. You will learn how temperature and pressure affects equilibrium when you study Le Chatelier's Principle.