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Diffusion occurs because regions of higher concentration contain more gas particles than regions of lower concentration. Although molecules move randomly in all directions, the net movement is from high to low concentration areas simply due to probability—more particles exist to move from concentrated regions. This spontaneous mixing continues until concentrations equalize throughout the container.

Mean free path is the average distance a gas particle travels between collisions. As particle density increases, collision frequency increases, resulting in a shorter mean free path. Conversely, decreasing particle density reduces collisions, creating a longer mean free path. This relationship directly influences how quickly gas molecules can move through space.

Lighter gases diffuse faster because root-mean-square (RMS) speed and molar mass are inversely related. Lighter gas molecules move at higher speeds, allowing them to travel greater distances in the same time period. In the ammonia-hydrogen chloride experiment, ammonia's lower molar mass enables it to diffuse farther down the tube than the heavier hydrogen chloride.

Diffusion is the spontaneous spreading of gas molecules throughout a container in response to a concentration gradient. Effusion is the escape of gas molecules through a tiny hole smaller than the mean free path, driven by pressure differences. Both processes depend on molar mass, but effusion specifically requires a small opening, while diffusion occurs throughout a space.

Graham's law states that effusion rate is inversely proportional to the square root of molar mass. Helium has lower molar mass than oxygen, so it effuses approximately 2.8 times faster through balloon pores. This faster escape rate causes helium balloons to deflate more quickly than oxygen-filled balloons at the same pressure.

Although gas molecules travel at hundreds of meters per second, they experience billions of collisions per second at room temperature. These frequent collisions cause constant changes in speed and direction, preventing direct movement toward their destination. The numerous random path changes significantly slow the overall net movement of molecules from high to low concentration regions.

Diffusion rate depends on three main factors: the concentration gradient (the difference in concentration between regions), the surface area available for diffusion, and the distance gas particles must travel. Steeper concentration gradients and larger surface areas increase diffusion rates, while greater distances decrease them. These factors work together to control how quickly gases mix.