Gases are a state of matter characterized by their high molecular mobility and their ability to expand to fill the available volume of any container. Their properties include their low density, their ability to flow freely and their compressibility. Gases are commonly described by their four thermodynamic variables: pressure, temperature, volume and amount of substance. The equation of state of an ideal gas, the ideal gas law, relates these variables mathematically.

Diffusion is the process by which gases mix with each other. In the absence of a physical barrier, gases diffuse to equalize their concentrations in a homogeneous mixture. The rate of diffusion depends on the molecular mass of the gas, temperature and pressure. Graham’s law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular mass.

Maxwell’s distribution describes the distribution of molecular velocities in a gas. The distribution is based on the assumption that the particles in an ideal gas move randomly and in all directions, and that the magnitude of their velocity is randomly distributed. The velocity distribution can be expressed in terms of the temperature and molecular mass of the gas.

Diffusion of gases is important in many natural and technological processes, including respiration, combustion, photosynthesis, chemical synthesis, gas separation and dispersion of pollutants in the air.

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