The substance, state, and presence of a membrane are the main differences between diffusion and osmosis. Diffusion involves the movement of particles from an area of high concentration to an area of lower concentration. Osmosis is a type of diffusion, but involves only the movement of water across a semipermeable membrane from an area containing a low solute concentration to an area containing a high solute concentration.
Diffusion and osmosis are both forms of passive transport, in which a system does not require energy to carry out the process. In diffusion, this process can occur with essentially any substance in any state, including solids, liquids, and gasses. For example, the helium released from a balloon can diffuse through the air, while a drop of dye in water will gradually diffuse throughout the container. Solids can experience diffusion, as in the case of sodium chloride dissolving and diffusing in water. Experiments have also found that solids can diffuse into other solids, but this is not easily observed because the diffusion rates are much slower than those of liquids and gasses.
Osmosis only describes the movement of water. It specifically involves the diffusion of water across a semipermeable membrane, such as a cell membrane. Osmosis also requires that water be in its liquid state rather than solid ice or gaseous water vapor.
During the osmosis process, water is treated as a solvent molecule, and all other substances are treated as solutes. The water travels from a region containing low amounts of solute across a membrane to an area with higher amounts of solute. Generally, the solutes are separated by the membrane, which remains permeable to water but impermeable to the solute materials, meaning that they cannot cross through it. Diffusion and osmosis differ in this way, as diffusion describes the movement of the solutes through a solvent, while osmosis describes the movement of the solvent itself.
Another difference between diffusion and osmosis include the fact that osmosis must take place over the membrane while diffusion can occur anywhere in any sized space. Osmosis also frequently occurs at a slower rate than diffusion on average, since diffusion includes gaseous movement that is much faster than liquid movement. Diffusion also observes the motion of particles in any direction, while osmosis only focuses on the movement of water in one direction across the membrane.