What defines an isotonic liquid?

An isotonic liquid is defined by its ability to have nearly the same number of solute particles in solution per unit of liquid as the surrounding environment, typically referring to the comparison with body fluids. This ensures that there is no net movement of water across cell membranes due to osmotic pressure differences. In physiological contexts, isotonic solutions help maintain cellular shape and function, as they do not cause cells to swell (as in hypotonic solutions) or shrink (as in hypertonic solutions).

The characteristics of high viscosity, a significantly higher number of particles, and rapid absorption pertain to different properties of liquids but don't accurately describe isotonicity. Viscosity relates to the thickness and flow characteristics of a liquid, while the concept of particle count is more related to hypertonic and hypotonic conditions rather than isotonic. Rapid absorption is often linked to the formulation of specific solutions for medicinal or hydration purposes but does not directly correlate with isotonic solutions. Thus, the definition hinges on the balance of particle concentration, making the correct answer clear.