Physical Properties of Gases Behaviour of Gases

Compressibility Gas particles are far apart from one another, so they can move closer to one another to occupy a smaller volume of space. DEMO - Cartesian diver, a device that contains a pocket of air and has an opening at the bottom. Squeeze the bottle and observe the Cartesian diver sinking when pressure is exerted on the bottle. Release the bottle and observe that the Cartesian diver rises when the pressure is reduced. Question: Why does the Cartesian diver sink when pressure is exerted on the bottle? Answer: the gas inside of the diver is compressed by the liquid when the bottle is squeezed, which decrease its volume and therefore increases the density of the Cartesian diver causing it sink. Question: According to the kinetic theory of gases, what makes gases compressible? Answer: because of the large spaces between the particles.

Expansion Because gases do not have a shape or volume they can fill any space indefinitely. The amount of air that can fill a space varies according to the atmospheric pressure; any given amount of gas will occupy a different volume at different pressures.

Diffusion Gas particles collide with each other and with the walls of any container they occupy. The random dispersion of gas particles is called diffusion. The ability of a gas to diffuse (due to the random collisions between particles) allows for a uniform distribution of a mixture of gas particles in a container. The rate of diffusion is slow and depends on the velocity of the particles; since smaller particles can move faster, they will diffuse faster than particles that are larger.

Effusion The flow of a gas across a barrier through a small opening.   The duration of effusion is the rate of diffusion. We know that the mean kinetic energy of two gases is the same according to the kinetic theory of gases. Using this hypothesis, we can derive an equation that will allow us to demonstrate the relationship between the rate of diffusion or effusion of two gases and the masses (or molar masses) or the gas. (see next slide for the derivation of the equation)

GRAHAM’S LAW Under identical conditions (temperature and pressure), the relative rates of diffusion and effusion, v, of two gases is inversely proportional to the square roots of their masses, m (or molar masses, M).