Effusion is the process by which gas escapes from a container through a tiny opening. A classic example of effusion can be seen when a balloon filled with helium begins to deflate over time. The helium gas molecules inside the balloon are constantly moving and, due to their speed and small size, some of these molecules will find their way through the small pores in the balloon’s material and escape into the surrounding air.
This movement occurs because the concentration of helium gas inside the balloon is higher than the concentration of helium in the air outside. As a result of this difference in concentration, the gas tends to move from an area of higher concentration (inside the balloon) to an area of lower concentration (the surrounding air). This gradual loss of helium is a perfect illustration of effusion in action.