What marks the beginning of phase change? Why does phase change at constant temperature?

Phase change begins at the set point when the two phases establish thermodynamic equilibrium. The thermodynamic equilibrium involves the transition of a substance from one state to another. The thermal equilibrium at thermodynamic equilibrium between two phases is the compelling factor in fixing the phase change temperature constant.

In a single-component system, the degree of freedom (F) is determined by the number of components (C), the number of phases present (P), and a constant value of 2. This equation, F = C - P + 2, allows for the prediction that F is fixed at one. Consequently, the intensive independent variable, typically temperature, dictates the phase change by setting the temperature and pressure at the phase change point. Pressure, as a dependent variable, automatically adjusts accordingly.

The set point temperature of phase change is the saturation temperature of the substance.

Details

Phase change happens at a constant temperature due to the saturation of the existing phase with its total energy, leading to a halt in temperature increase. This halt occurs because at saturation point the liquid phase has no more heat capacity to absorb heat to raise its temperature further. This saturation point is crucial for the phase transition to occur.

At saturation point, the thermal equilibrium between the two phases does not allow any heat transfer between the two phases. It is the saturation temperature of the liquid phase that decides the phase change temperature.

During a phase change, the substance undergoes significant internal rearrangement as molecules shift from one phase to another. This rearrangement requires a precise amount of energy to break existing intermolecular bonds and form new ones. This energy, known as the latent heat of fusion or vaporization, sustains the phase change process and is essential for the transition to occur.

Moreover, during the phase change from liquid to vapor, the expanding vapor requires additional energy for expansion, contributing to the overall energy budget of the phase change process. As the intermolecular bonds break and the vapor expands, entropy is generated, further driving the transition from one phase to another.

At phase change, with the onset of thermodynamic equilibrium, the free energy change reaches a minimum.

Thermodynamic equilibrium ensures that both phases involved in the phase change remain in balance, with minimal free energy and constant temperature. This equilibrium allows for the efficient exchange of energy between the two phases, facilitating the transition while maintaining thermal stability.

Three important points

1. The beginning of thermodynamic equilibrium is the set point for phase change.

2. Phase change always occurs at thermodynamic equilibrium at minimum free energy

3. It is the thermal equilibrium between two phases that makes the phase change temperature constant.