Understanding osmosis: Through a perfect semi-permeable membrane separating a dilute solution from pure solvent, which species diffuses and in what direction?

Introduction / Context:
Osmosis is central to membrane separations, biology (cell turgor), and food preservation. A semi-permeable membrane ideally permits only solvent molecules to pass, not solute, creating a driving force due to chemical potential differences across the membrane.

Given Data / Assumptions:

• Membrane is ideal and perfectly semi-permeable (passes solvent, retains solute).
• Solution is dilute; solute does not cross.
• No applied external pressure (i.e., not reverse osmosis).

Concept / Approach:
The solvent’s chemical potential is lowered in the solution due to solute presence. To equalise chemical potentials, solvent flows from the pure solvent side (lower solute concentration) into the solution side (higher solute concentration) until osmotic pressure balances or equilibrium is reached.

Step-by-Step Solution:

Verification / Alternative check:
Van’t Hoff relation for dilute solutions: π = i * C * R * T predicts the pressure difference needed to halt solvent flow, consistent with observed osmosis direction.

Why Other Options Are Wrong:

• Solvent from high to low concentration: That would decrease the chemical potential difference; it is opposite to osmosis.
• Solute diffuses through the membrane: Contradicts the definition of a semi-permeable membrane.
• None of these: Incorrect because option (a) is correct.

Common Pitfalls:
Confusing osmosis with diffusion of solute, or with reverse osmosis (where applied pressure forces solvent the other way).

Final Answer:
The solvent moves from the low solute concentration side to the high solute concentration side.