Homogeneous system is one which has one phase. It has the same chemical composition and uniform properties throughout. It is formed by particles of molecular size only. Pure solids, liquids, gases and solutions are the examples of homogeneous systems.
A system consisting of only one phase is called a homogeneous system.
Heterogeneous system has at least two phases - a mixture of solids or immiscible liquids constitutes a heterogeneous system.
Any system consisting of two or more phases is called heterogeneous system.
Equilibrium can be established in either type of systems. Since all physical equilibria involve at least two phases, therefore these are all examples of heterogeneous equilibrium. But chemical equilibrium can be homogeneous or heterogeneous in nature. It is homogeneous if both the reactants and products are present only in one phase gas or liquid and heterogeneous if present in more than one phase.
Gas - Phase Homogeneous Systems
Such systems contain only gaseous reactants and products. Since all gaseous mixtures are homogeneous in nature they constitute only one phase. For example,
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
2N2O5 (g) ⇌ 4NO2 (g) + O2 (g)
Liquid - Phase Homogeneous Systems
These are the systems in which both the reactants and products are present in only one liquid phase (as a solution). For example,
CH3COOH (l) + C2H5OH (l) ⇌ CH3COOC2H5 (l) + H2O (l)
KCN (aq) + H2O (l) ⇌ HCN (aq) + KOH (aq)
The systems in which reactants and products are present in more than one phase belong to this type. For example,
Fe (s) + 4H2O (g) ⇌ Fe3O4 (s) + 4H2 (g)
CaCO3 (s) ⇌ CaO (s) + CO2 (g)