The Gaseous Phase Help
The gaseous phase of matter is similar to the liquid phase insofar as a gas will conform to the boundaries of a container or enclosure. However, a gas is much less affected by gravity than a liquid. If you fill up a bottle with a gas, there is no discernible surface to the gas. Another difference between liquids and gases is the fact that gases generally are compressible.
The density of a gas can be defined in three ways, exactly after the fashion of liquids. Mass density is defined in terms of the number of kilograms per meter cubed (kg/m3) that a sample of gas has. The weight density is defined in newtons per meter cubed (N/m3) and is equal to the mass density multiplied by the acceleration in meters per second squared (m/s2) to which the sample is subjected. The particle density is defined as the number of moles of atoms per meter cubed (mol/m3) in a parcel or sample of gas, where 1 mol ≈ 6.02 × 10 23 .
Diffusion In Small Containers
Imagine a rigid enclosure, such as a glass jar, from which all the air has been pumped. Suppose that this jar is placed somewhere out in space, far away from the gravitational effects of stars and planets and where space itself is a near vacuum (compared with conditions on Earth anyhow). Suppose that the temperature is the same as that in a typical household. Now suppose that a certain amount of elemental gas is pumped into the jar. The gas distributes itself quickly throughout the interior of the jar.
Now suppose that another gas that does not react chemically with the first gas is introduced into the chamber to mix with the first gas. The diffusion process occurs rapidly, so the mixture is uniform throughout the enclosure after a short time. It happens so fast because the atoms in a gas move around furiously, often colliding with each other, and their motion is so energetic that they spread out inside any container of reasonable size (Fig. 10-5a).
Fig. 10-5 . (a) Distribution of gas inside a container, (b) Distribution of gas around a planet with an atmosphere, (c) Distribution of gas in a star as it is forming. Darkest shading indicates highest concentration.
What would happen if the same experiment were performed in the presence of a gravitational field? As you can guess, the gases would still mix uniformly inside the jar. This happens with all gases in containers of reasonable size.
Planetary atmospheres, such as that of our own Earth, consist of mixtures of various gases. In the case of our planet, approximately 78 percent of the gas in the atmosphere at the surface is nitrogen, 21 percent is oxygen, and 1 percent is made up of many other gases, including argon, carbon dioxide, carbon monoxide, hydrogen, helium, ozone (oxygen molecules with three atoms rather than the usual two), and tiny quantities of some gases that would be poisonous in high concentrations, such as chlorine and methane. These gases blend uniformly in containers of reasonable size, even though some of them have atoms that are far more massive than others. Diffusion, again, is responsible.
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