Chemical Equilibrium
In a chemical process, chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. 
•	Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction. The reaction rates of the forward and reverse reactions are generally not zero but, being equal; there are no net changes in any of the reactant or product concentrations. 
•	This process is called dynamic equilibrium
Explaining the term "dynamic equilibrium"
•	The reaction has reached equilibrium in the sense that there is no further change in the numbers of blue and orange squares. However, the reaction is still continuing. For every orange square that turns blue, somewhere in the mixture it is replaced by a blue square turning orange.
•	This is known as a dynamic equilibrium. The word dynamic shows that the reaction is still continuing.
•	You can show dynamic equilibrium in an equation for a reaction by the use of special arrows. In the present case, you would write it as:
A  B
•	It is important to realise that this doesn't just mean that the reaction is reversible. It means that you have a reversible reaction in a state of dynamic equilibrium.
•	The "forward reaction" and the "back reaction"
•	The change from left to right in the equation (in this case from blue to orange as it is written) is known as the forward reaction. The change from right to left is the back reaction.
Characteristics of systems at equilibrium
•	The macroscopic or observable properties of the system become constant. Macroscopic properties would include colour , pressure, volume, and concentration 
•	The equilibrium can be approached starting with only reactants, only products or a combination of both. 
Classes of Equilibrium
Equilibrium can be classified into two broad groups based on the states of the substances involved 
a) Homogeneous equilibrium: All the substances are in the same state.
Example : H2 (g) + I2 (g) 2 HI (g)
b) Heterogeneous equilibrium's: Equilibrium in which one or more of the reactant and products are in different states
Example : H2O (l) H2O (g)