It states that "provided the equilibrium between any number of phases is not influenced by gravity, or electrical, or by magnetic forces, or by surface action and only by temperature, pressure and concentration, then the number of degrees of freedom (F) of the system is related to the number of components (C) and of phases (P) by the phase rule equation,

$F=C-P+2$

for any system at equilibrium at a definite temperature and pressure".

For the given equation, $\text{CaCO}_{3(s)}\rightleftharpoons\text{CaO}_{(s)}+\text{CO}_{2(g)}$

• No. of phases: It consists of three phases i.e. two solids and one gaseous.
• Components: In the themal decomposition of$\text{CaCO}_3$, the composition of each of the three phase can be expressed in terms of atleast any two of the independantly variable constituents,$\text{CaCO}_3$,$\text{CaO}$ and$\text{CO}_2$.
  Thus, it is a two component system.
• Degree of Freedom: It is given by the formula,
  $\begin{align*} F &= C-C+2\\[2ex] &=2-3+2\\[2ex] &=-1+2\\[2ex] &=1\end{align*}$