Phase Change of a Pure Substance

During the phase change of a pure substance, the substance undergoes a change in its physical state from one phase to another. For example, water changes its state from solid (ice) to liquid and then to gas (vapor) as the temperature increases.

dG = 0

The correct answer is option 'A', i.e., dG = 0. During a phase change, the Gibbs free energy of the substance remains constant, and hence dG = 0.

Explanation:

Gibbs free energy (G) is defined as the energy available to do work in a system at constant temperature and pressure. It is given by the equation:

G = H - TS

where H is the enthalpy (heat content) of the system, T is the absolute temperature, and S is the entropy (degree of disorder) of the system.

During a phase change, the enthalpy and entropy of the substance change, but the temperature and pressure remain constant. As a result, the change in Gibbs free energy (dG) is zero.

This can be explained using the phase diagram of a substance, which shows the relationship between its pressure, temperature, and phase. On the phase diagram, the phase change occurs along a line called the coexistence curve, where two phases coexist in equilibrium. Along this curve, the Gibbs free energy of both phases is equal, and hence dG = 0.

Conclusion:

In conclusion, during the phase change of a pure substance, the Gibbs free energy of the substance remains constant, and hence dG = 0. This is because the temperature and pressure of the substance remain constant, while its enthalpy and entropy change.

Temperature remain constant....during phase change