1. Phase: A phase is defined as "an homogeneous, physically distinct and mechanically separable portion of system, which is separated from other such parts of the system by deinite boundary surfaces"
   For e.g.:$CaCO_{3{(s)}}\rightleftharpoons CaO_{(s)}+CO_{2\ (g)}$
   It consists of three phases i.e. 2 solid and 1 gaseous
2. Component: It is defined as "the smallest number of independent variable constituents, taking part in the state of equilibrium, by means of which the composition of each phase can be expressed in the form of chemical equation"
   e.g.:$Ice_{(s)} \rightleftharpoons Water_{(l)} \rightleftharpoons Vapour_{(g)}$
   the chemical composition of all the three phases is H2O
   Hence it is one component system
3. Degree of freedom: It is stated as "the minimum number of independently variable factors, such as temperature, pressure and composition of the phases, which must be arbitrarily specified in order to represent perfectly the condition of a system"
   For e.g.: For a system consisting of water in contact with its vapour,
   $Water_{(l)}\rightleftharpoons Water\ vapour _{(g)}$
   We must state either the temperature or pressure to define it completely. Hence, degree of freedom is one or system is invariant.
   Degree of freedom can also be calculated using the formula$F=C-P+2$
   Where,
   F = Degree of freedom;
   C = Component of system;
   P = Phase of the system.

For the equilibrium,

$H_2O_{(s)}\rightleftharpoons H_2O_{(l)}\rightleftharpoons H_2O_{(vapour)}$

1. No. of phases: 2
2. Components: The chemical composition of all the three phases is$H_2O$
   Hence it is one component system.
3. Degree of freedom: For a system consisting of water in contact with its vapour we must either state the temperature or pressure to be defined completely. Hence degree of freedom is one.