This model illustrates the process of gas diffusion through a semi-permeable membrane. Molecules of some gases will pass through the membrane, while molecules of other gases will not. Phenomena that arise in systems with semi-permeable membranes are called osmotic phenomena. When several gases are present in the same vessel, each gas contributes to the total pressure on the walls of the vessel. Partial pressure is a concept that refers to the individual pressures that are exerted by the individual gases. Dalton's law states that the pressure in a mixture of gases that do not chemically interact between each other is equal to the sum of their partial pressures P = P1 + P2 + ... The partial pressures of each gaseous component behave in accordance with the equation of the ideal gas:
where V is the volume of the gas mixture, T is the absolute temperature, mi is the mass of a certain gas component in the mixture, and μi is the molar mass of this component. This model illustrates the process of pressure difference appearing in a mixture of two gases as they diffuse through a semi-permeable membrane. At the outset of the experiment, the two different gases are located in different parts of the vessel, and are divided by a semi-permeable membrane. Their temperature and their initial pressure are the same. Throughout the process of diffusion of one of the gases through the semi-permeable membrane, the pressure in one of the vessel's halves increases while decreasing in the other half, in accordance with Dalton's law.