Passive diffusion is the process by which molecules move from a region of high concentration to a region of low concentration without the input of energy. This process occurs due to the random motion of molecules, and does not require the presence of a membrane. For example, if you place a drop of food coloring in a glass of water, the food coloring molecules will spread out and evenly distribute throughout the water over time due to passive diffusion.

The rate of passive diffusion is determined by several factors, including:

- The concentration gradient: The greater the difference in concentration between two regions, the faster the rate of diffusion.

- The temperature: The higher the temperature, the faster the rate of diffusion.

- The size of the molecules: Smaller molecules diffuse faster than larger molecules.

- The viscosity of the medium: The more viscous the medium, the slower the rate of diffusion.

Passive diffusion is an important process in biology, as it allows molecules to move across cell membranes and into and out of cells. It is also essential for the exchange of gases in the lungs and the uptake of nutrients from the digestive tract.