Every molecule within a solution, solvent or
solute, interacts with its neighbors.

Every molecule within a solution, solvent or
solute, interacts with its neighbors. The
average overall momentum delivered to a
molecule by its neighbors is zero, otherwise,
the molecule will move and follow the
direction of the non-zero momentum,
according to Newton’s second law. Therefore,
according to Newton’s third law, the average
overall momentum that a molecule delivers to
its neighbors is also zero. Non-zero
momentum fluctuations do exist, however,
and generate the Brownian movement.

A solute molecule located nearby the semipermeable
membrane interacts with the
membrane and delivers momentum to it in a
some specific direction. Therefore, it must
deliver a non-zero momentum to its neighbor
molecules in other directions. The overall
average momentum, delivered by the
molecule to both the membrane and neighbor
molecules, must be zero. The momentum
delivered to the neighbors transfers further to
next neighbor molecules through the solution,
finally reaches the solution boundaries, and
generates the osmotic pressure.

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