Abstract: The study explores the scientific meaning of “directionality” in the Second Law of Thermodynamics, revealing two distinct interpretations: the unidirectionality of spontaneous changes and the bidirectionality of state changes. Through an analytical framework of the interactions between a closed system and its surrounding, the study elucidates the spontaneous changes’ unidirectionality as defined by the Second Law. Additionally, it highlights the bidirectional characteristics of state changes under different conditions (isothermal, isothermal-isochoric, isothermal-isobaric). A comprehensive analysis of the path characteristics of state changes in both forward and reverse directions is provided. Finally, practical examples demonstrates the essential role of thermodynamic laws in guiding the understanding of actual change processes, with the analysis serving as a reference for physical chemistry education.

Key words: The Second Law, State change, Direction of change, Spontaneity, Path characteristics