Equilibrium  is when the forward and reverse reaction occur at the same rate.

The rates are equal but the quantities (amounts) of reactants and products are not necessarily equal.

• Equilibrium can only occur:

1. In a system in which changes that are taking place are reversible.
2.  In a closed system. A closed system in which nothing is allowed in or out.

• When equilibrium is reached in a system

1. Rate of the forward process is equal to the rate of the reverse process.
2. Concentration (amount) of substances remains constant.

• Types of equilibrium.

1. Physical Equilibrium

A. Phase Equilibrium

B. Solution Equilibrium

2. Chemical Equilibrium 

PHYSICAL EQUILIBRIUM– occurs in certain reversible physical changes.

A. Phase Equilibrium  occurs in a closed system in which phase changes are occurring, at a specific temperature.

e.g 1. Ice/Water equilibrium occurs at 0°C or 273 K (melting and freezing point of water)

H₂O(s) ↔ H₂O (l)

e.g 2. Water/Steam (vapor) equilibrium at 100°C or 373 K (boiling point of water)

H2O(l) ↔ H2O (g)

B. Solution equilibrium occurs in a closed system in which a substance is dissolving in a liquid.

e.g. 3. Solid in Liquid Equilibrium: In a SATURATED SOLUTION.

NaCl(s) ↔Na⁺ (aq) + Cl^– (aq)

• Equilibrium exists between dissolved particles and undissolved particles.
• Rate of dissolving of the solid is equal to the rate of crystallization of the particles.
• Amounts of solid and ions remain constant in the solution.

e.g. 4. Gas in Liquid equilibrium: In a gaseous solution

• Equilibrium exists between dissolved gases in the liquid and undissolved gas above the liquid.
• Rate of dissolving of the gas is equal to the rate of the gas being undissolved.
• Amounts of the undissolved gas (above the liquid) and dissolved gas (in liquid) remain constant.

CO₂ (g) ↔ CO₂ (aq)