What is osmotic pressure?

• A. The force driving water through a semipermeable membrane
• B. The pressure required to maintain a solution
• C. The pressure required to prevent water flow across a semipermeable membrane
• D. The force exerted by solute particles in a solvent

Answer: (C)

Osmotic pressure is best described as the pressure required to prevent the flow of water across a semipermeable membrane due to osmosis. In the context of a solution, this phenomenon occurs when there is a concentration gradient between two solutions separated by a semipermeable membrane that allows only solvent (usually water) to pass through while restricting solute molecules.

When a solute is present, water tends to move from an area of lower solute concentration (hypotonic solution) to an area of higher solute concentration (hypertonic solution) to equalize the solute concentrations on both sides of the membrane. This movement of water generates osmotic pressure, and to counteract this flow and prevent water from moving, an external pressure must be applied.

This concept is fundamental in various biological and chemical processes, particularly in maintaining cell turgor in plants and regulating fluid balance in cells. Thus, the correct identification of osmotic pressure relates directly to the required pressure to stop water flow induced by differences in solute concentration across a semipermeable barrier.