Introduction
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.
Gas Density
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.
Gases Near A Planet
Now imagine the gaseous shroud that surrounds a reasonably large planet, such as our own Earth. Gravitation attracts some gas from the surrounding space. Other gases are ejected from the planet’s interior during volcanic activity. Still other gases are produced by the biologic activities of plants and animals, if the planet harbors life. In the case of Earth, some gases are produced by industrial activity and by the combustion of fossil fuels.
All the gases in the Earth’s atmosphere tend to diffuse, but because there is an essentially unlimited amount of “outer space” and only a finite amount of gas, and because the gravitational pull of the Earth is greater near the surface than far out in space, the diffusion takes place in a different way than inside a small container. The greatest concentration of gas molecules (particle density) occurs near the surface, and it decreases with increasing altitude (see Fig. 10-5b). The same is true of the number of kilograms per meter cubed of the atmosphere, that is, the mass density of the gas.
On the large scale of the Earth’s atmosphere, yet another effect takes place. For a given number of atoms or molecules per meter cubed, some gases are more massive than others. Hydrogen is the least massive; helium is light too. Oxygen is more massive, and carbon dioxide is more massive still. The most massive gases tend to sink toward the surface, whereas the least massive gases rise up high, and some of their atoms escape into outer space or are not captured permanently by the Earth’s gravitation.
There are no distinct boundaries, or layers, from one type of gas to another in the atmosphere. Instead, the transitions are gradual and vague. This is good, because if the gases of the atmosphere were stratified in a defined way, we would have no oxygen down here on the surface. Instead, we’d be smothered in some noxious gas such as carbon dioxide or sulfur dioxide.
Gases In Outer Space
Outer space was once believed to be a perfect vacuum. However, this is not the case. There is plenty of stuff out there, and much of it is hydrogen and helium gas. (There are also trace amounts of heavier gases and some solid rocks and ice chunks as well.) All the atoms in outer space interact gravitationally with all the others. This might be hard to imagine at first, but if you think about it, there’s no escaping it. Even a single atom of hydrogen exerts a gravitational pull on another atom 1 million km away.
The motion of atoms in outer space is almost random but not quite. The slightest perturbation in the randomness of the motion gives gravitation a chance to cause the gas to clump into huge clouds. Once this process begins, it can continue until a globe of gas forms in which the central particle density is significant (see Fig. 10-5c). As gravitation continues to pull the atoms in toward the center, the mutual attraction among the atoms there becomes greater and greater. If the gas cloud has some spin, it flattens into an oblate spherical shape and eventually into a disk with a bulge at the center. A vicious circle ensues, and the density in the central region skyrockets. The gas pressure in the center rises, and this causes it to heat up. Ultimately, it gets so hot that nuclear fusion begins, and a star is born. Similar events among the atoms of the gas on a smaller scale can result in the formation of asteroids, planets, and planetary moons.
Gas Pressure
Unlike most liquids, gases can be compressed. This is why it is possible to fill up hundreds of balloons with a single, small tank of helium gas and why it is possible for a scuba diver to breathe for a long time from a single small tank of air.
Imagine a container whose volume (in meters cubed) is equal to V . Suppose that there are N moles of atoms of a particular gas inside this container, which is surrounded by a perfect vacuum. We can say certain things about the pressure P , in newtons per meter squared, that the gas exerts outward on the walls of the container. First, P is proportional to N , provided that V is held constant. Second, if V increases while N remains constant, P will decrease. These things are apparent intuitively.
There is another important factor—temperature—involved when it comes to gases under pressure when they expand and contract. The involvement of temperature T , generally measured in degrees above absolute zero (the absence of all heat), is significant and inevitable in gases. When a parcel of gas is compressed, it heats up; when it is decompressed, it cools off. Heating up a parcel of gas will increase the pressure, if all other factors are held constant, and cooling it off will reduce the pressure. The behavior of matter, especially liquids and gases, under conditions of varying temperature and pressure is a little complicated, so the entire next chapter is devoted to this subject.
Practice problems of these concepts can be found at: Basic States Of Matter Practice Test
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From Physics Demystified: A Self-Teaching Guide. Copyright © 2002 by The McGraw-Hill Companies, Inc. All Rights Reserved.
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