A phase change, or transition is the transformation of thermodynamic systems from one phase or state of matter to another. A phase in a thermodynamic system and the state of matter has uniform physical properties. During a phase transition, certain properties of the medium change discontinuously. This results in come external condition such as temperature and pressure. Phase transitions are commonly observed in nature. The most commonly used transition is between solid, liquid and gaseous states of matter.
Phase transitions are classified based on the behavior of the thermodynamic free energy as a function of other thermodynamic variables. First-order phase transition exhibits a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable. Second-order phase transitions are continuous in the first derivative but exhibit discontinuity in the second derivative of free energy. This method is called the Ehrenfest’s classification. Although this classification method is useful, it is found to be inaccurate because it does not take into consideration where the derivative of free energy diverges.