Unravelling tonicity: Causes of confusion and pathways to clarity.

Tonicity is the most confusing concept in teaching about osmosis in physiology, biology, and many clinical disciplines. A total of seven causes (four superficial and three deep) have led to this confusion but have never been thoroughly clarified. In this article, we systematically address and resolve these causes through logical reasoning, which not only thoroughly clarifies what tonicity is, but also leads to an understanding of its physical nature and properties. Several key concepts are introduced in order to resolve the causes of confusion and lay a new theoretical foundation for studying osmosis. This article not only advances the teaching and learning of tonicity and osmosis but also provides new insights into how osmosis across the cell membrane should be studied.