osmosis

Osmosis is the way water moves to even out a difference it cannot reach directly. Picture a barrier that lets water molecules through but blocks the dissolved sugar or salt. Water will flow toward the saltier side, diluting it, because that is the only way the system can balance the concentrations when the solute itself cannot move. The membrane is described as semipermeable: open to solvent, closed to solute.

This is everyday biology and pharmacy at once. Red blood cells swell or shrink depending on the solution around them; that is why injections and eye drops must be balanced so water does not rush into or out of cells. The driving tendency is always the same: solvent moves from the more dilute side toward the more concentrated side until something stops it.

Osmosis differs from ordinary diffusion in that it is specifically the solvent that moves across a selective barrier, not the solute spreading through open space. The strength of the pull depends on the number of dissolved particles, not their identity, which links osmosis directly to the colligative properties of solutions.