Diffusion
![A diffusion is a process in physics. Some particles are dissolved in a glass of water. At first, the particles are all near one corner of the glass. If the particles all randomly move around (](/Images/godic/202501/09/Diffusion.svg0210.png")
![Time lapse video of diffusion of a dye dissolved in water into a gel.](/Images/godic/202501/09/-Diffusion_v2_20101120.ogv0210.jpg")
![Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diffuses to fill the whole container. Top: A single molecule moves around randomly. Middle: With more molecules, there is a statistical trend that the solute fills the container more and more uniformly. Bottom: With an enormous number of solute molecules, all randomness is gone: The solute appears to move smoothly and deterministically from high-concentration areas to low-concentration areas. There is no microscopic force pushing molecules rightward, but there appears to be one in the bottom panel. This apparent force is called an entropic force.](/Images/godic/202501/09/DiffusionMicroMacro0210.gif")
![3D rendering of diffusion of purple dye in water.](/Images/godic/202501/09/Blausen_0315_Diffusion0210.png")
Diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential). This is also referred to as the movement of a substance down a concentration gradient. A gradient is the change in the value of a quantity (e.g., concentration, pressure, temperature) with the change in another variable (usually distance). For example, a change in concentration over a distance is called a concentration gradient, a change in pressure over a distance is called a pressure gradient, and a change in temperature over a distance is a called a temperature gradient.