During a phase transition, a substance is indeed undergoing a change in its macroscopic properties as it transitions from one phase to another (such as from solid to liquid or liquid to gas). However, when a substance is at equilibrium during a phase transition, it means that the rates of transformation between the two phases are balanced, resulting in no net change in the overall amount or composition of the substance.
At equilibrium, the substance exists in a state where the forward and reverse processes of the phase transition occur at equal rates. For example, during the melting of ice, as heat is continuously applied, ice molecules gain energy and transition into the liquid phase. At the same time, an equal number of liquid molecules lose energy and transition back into the solid phase. This dynamic equilibrium maintains a constant amount of ice and liquid coexisting, without a net change in the overall system.
The properties of the substance are changing at the molecular level during a phase transition, as bonds are broken or formed, and the arrangement of particles changes. However, at the macroscopic level, the substance remains in equilibrium as long as the transition is occurring under constant conditions (such as temperature and pressure).
It's important to note that equilibrium during a phase transition refers to a dynamic equilibrium, where there is continuous movement and transformation at the molecular level, even though there is no observable change in the macroscopic properties of the substance as a whole.