= Permeating solute
A <permeating solute> can cross the specified membrane appreciably during the observation interval. This is a property of a <solute>-membrane-time comparison, not an immutable label attached to a <molecule>. Passage can reduce a <concentration> difference through <passive transport> while <water> is also moving; it does not require <water> to wait.
For an original dilute ideal example, stipulate a flexible model <biological cell> with $20\,\mathrm{mmol\,L^{-1}}$ retained particles and an external reservoir with $10\,\mathrm{mmol\,L^{-1}}$ retained particles plus $10\,\mathrm{mmol\,L^{-1}}$ freely permeating neutral <solute>. Initially the total <osmolarities> are equal. Suppose the permeating <solute> eventually has equal internal and external <concentrations>, there is negligible <pressure> difference, no <active transport>, and the internal retained particle amount is conserved. The permeating contributions cancel from the final osmotic balance; the internal retained <concentration> must fall from $20$ to $10\,\mathrm{mmol\,L^{-1}}$. The required <volume> is twice its initial value. This illustrates an initially equal-<osmolarity> but <hypotonic solution>; it assumes the boundary can survive that expansion and predicts no universal time course.
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