Osmosis and Osmotic Pressure of Solutions

A semipermeable membrane allows certain substances to pass through while blocking others based on size, shape, polarity, and charge. Biological cell membranes and dialysis tubing are common examples. These selective barriers enable osmosis by permitting solvent molecules to diffuse across while restricting solute passage, creating the foundation for osmotic pressure development.

Osmosis is the diffusion-driven transfer of solvent molecules through a semipermeable membrane toward a solution with higher solute concentration. Since solvent concentration is greater on the pure solvent side, molecules diffuse faster in that direction, creating net solvent movement. This continues until osmotic pressure builds sufficiently to equalize the transfer rates.

Osmotic pressure is the minimum pressure required to halt osmosis, calculated using π = MRT, where M is molarity, R is 0.0821 liter-atmosphere per mole kelvin, and T is absolute temperature in kelvin. For a 1.00 molar sugar solution at 25°C, osmotic pressure equals 24.5 atmospheres. It is a colligative property that increases proportionally with solute concentration.

Isotonic solutions have equal osmotic pressure and show no net water movement across cell membranes. Hypotonic solutions have lower osmotic pressure, causing water to enter cells and potentially rupture them through hemolysis. Hypertonic solutions have higher osmotic pressure, drawing water out and causing crenation, where cells shrivel. Medical IV fluids must be isotonic to prevent these harmful effects.

In hypertonic solutions, the external solute concentration exceeds that inside red blood cells, creating an osmotic gradient. Water exits through the semipermeable cell membrane to equalize concentrations, causing cells to lose volume and shrivel in a process called crenation. This demonstrates how osmotic pressure differences drive water movement across biological membranes.

In hypotonic solutions, the external solute concentration is lower than inside cells, so water moves inward across the semipermeable cell membrane. Cells swell as they accumulate water and eventually rupture in a process called hemolysis. This occurs because osmotic pressure inside the cell exceeds external pressure, driving net water influx until the membrane fails.

Reverse osmosis occurs when external pressure exceeding the solution's osmotic pressure is applied, reversing normal solvent flow and pushing solvent molecules from the solution into pure solvent. This technique is used for large-scale seawater desalination and producing high-purity drinking water. Unlike spontaneous osmosis, reverse osmosis requires energy input to overcome the natural osmotic gradient.