Gas, in physics and chemistry, any substance that expands and spreads indefinitely when not confined in a container. A gas thus has no shape of its own, but takes on the shape of its container. The gaseous state is one of the three basic states, or forms, in which all matter exists. (The two other states are solid and liquid.) Gases and liquids are both fluids and have certain properties in common.

A gas can be changed into a liquid or solid by being cooled or compressed, or both; a solid or liquid can be changed into a gas by the application of heat. A substance that is in the solid or liquid state at ordinary temperatures is called a vapor when it is in the gaseous form. Steam is water vapor at a high temperature.

A gas consisting of one kind of chemical element is an elementary gas. At ordinary temperatures and pressures, there are 12 such gases: argon, chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen, ozone, radon, and xenon.

A gas consisting of molecules made up of atoms of more than one kind of element is a compound gas. Ammonia and carbon monoxide are common compound gases.

Molecules or atoms of gases can be changed into ions (electrically charged molecules or atoms) by being heated. A mixture of positively charged gas ions and electrons is called a plasma.

Properties of Gases

Gases vary widely in their physical and chemical characteristics. Some, such as oxygen, have neither color nor odor. Chlorine, on the other hand, is yellowish-green and has a distinct odor. Some gases burn in the presence of oxygen, others do not. The six inert (or noble) gases—argon, helium, krypton, neon, radon, and xenon—consist of single atoms that do not ordinarily combine with other elements to form chemical compounds.

Despite these variations, gases have certain properties in common. These include the following:

Molecular Arrangement

A gas is made up of molecules or single atoms. (Single gas atoms are regarded as molecules by chemists and physicists.) Gas molecules are much farther apart than are molecules of solids or of liquids and move about at higher speeds. According to Avogadro's Law, a hypothesis first stated by the Italian physicist and chemist Amedeo Avogadro in 1811, equal volumes of different gases, under the same conditions of temperature and pressure, contain the same number of molecules.

Avogadro's Law has been verified experimentally. Under standard conditions of temperature (0° C.) and pressure (760 mm of mercury), the volume occupied by one grammolecular weight of any gas is 22.4 liters (about 0.79 cubic foot). The number of molecules contained in this volume has been determined to be about 6.023 X 1023, or 602,300 followed by 18 more zeroes. This figure is called Avogadro's number.

Diffusion and Pressure

Because of the rapid motion of its molecules, a gas will diffuse, or spread uniformly. In diffusing, it can mix with another gas or with certain liquids and solids.

When a gas is confined to a container, the moving gas molecules continually strike the container's inside walls, exerting pressure. The molecules move in such a way that the pressure is the same at every point on the inside walls of the container. Increasing the amount of gas without changing the size of the container increases the pressure. Heating the gas also increases the pressure, because the molecules move more rapidly as the temperature increases.

Compression and Expansion

Outside pressure applied to a gas decreases the volume of the gas (the space it occupies). This is because the molecules are pressed closer together. If the temperature is constant, the volume of the gas decreases in inverse proportion to the pressure applied. Thus the greater the pressure exerted on the gas, the less the volume of the gas. This principle is called Boyle's Law. The law does not hold true for all gases at all temperatures.

A volume of gas can be expanded (1) by increasing its temperature (causing the molecules to move more rapidly and thus travel further); or (2) by decreasing the pressure applied to the gas. All gases expand at the same rate under the same conditions. The relation between temperature and the volume of a gas is given by Charles' Law.

Liquefaction

A gas can be liquefied by being cooled or compressed or both. Each gas has its own critical temperature—the temperature above which it cannot be liquefied, no matter how great the applied pressure. A few gases, such as ammonia and nitrous oxide, can be liquefied at room temperature—if enough pressure is applied to the gas. Some gases, such as helium and hydrogen, require extremely low temperatures for liquefaction to occur.