 We will demonstrate the diffusion of hydrogen by a single displacement reaction of zinc metal with hydrochloric acid. The hydrogen gas is used to fill the beaker surrounding the porous cup. The hydrogen diffuses through the walls of the porous cup faster than the air can diffuse out because the hydrogen gas has a lower molar mass than the nitrogen and oxygen in the air. The pressure inside the porous cup increases. This forces the air down the tube and we can see it bubbling out in the green water. When removed the beaker, the hydrogen gas diffuses out of the porous cup faster than the air can diffuse into the cup. This creates a low pressure inside the cup. Atmospheric pressure then forces the green water up the tube and into the cup.