# Effusion and Diffusion

How does this fragrance move across a room?

We’ve probably all experienced sitting next to someone who has applied way too much perfume or body spray. Maybe you tried to move farther away from this person and realized that you could smell their fragrance. Maybe you have experienced sitting in the locker room after a game when a teammate sitting across the room sprayed body spray. You would not smell the fragrance immediately, however, given a few minutes the smell would soon move across the room. The smells of some fragrances can be more overwhelming than others, however, all gases move to fill a space in a similar way.

Diffusion is the tendency of the particles of a gas to move from an area of high concentration to an area of low concentration. In other words, diffusion is the tendency of the particles of a gas to spread out and fill a room.

Another process that involves movement of the particles of a gas is effusion. Effusion is the tendency of the particles of a gas to escape through a tiny hole in a container.

In 1846, Thomas Graham found that the rate at which a gas effusions if related to its molar mass. Heavy gases effuse slowly and light gases effuse quickly. Graham’s law of effusion is used to compare the rates at which two gases effuse. Graham’s law of effusion states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass. We can express this mathematically with the following equation,

Graham’s Law of Effusion