Osmosis and Diffusion in Solutions

About this topic

Diffusion is the net movement of particles from high to low concentration until evenly spread. Osmosis is the diffusion of water across a semipermeable membrane from a region of lower solute concentration to higher solute concentration. The pressure needed to stop it is the osmotic pressure, π = iMRT.

The membrane lets water through but blocks larger solutes, so water moves toward the more concentrated side to even out concentrations. In an isotonic solution, there is no net movement; in a hypotonic solution, cells take in water and may burst; in a hypertonic solution, cells lose water and shrivel. Diffusion needs no energy input, driven by random motion, and is faster at higher temperature and for smaller particles.

Examples: a red blood cell placed in pure water swells and bursts, in salt solution stronger than 0.9 percent it shrinks, so intravenous fluids are made isotonic at about 0.9 percent saline. Plants wilt when overly salted and stand firm when well watered. Salting meat preserves it because bacteria lose water. Kidney dialysis machines and reverse osmosis for desalination apply these principles.

A misconception is that osmosis moves solute; it is the water that moves. Another is that water moves toward the more dilute side; it moves toward the more concentrated one.