A gas is a sample of matter that conforms to the shape of a container in which it is held and acquires a uniform density inside the container, even in the presence of gravity and regardless of the amount of substance in the container. If not confined to a container, gaseous matter, also known as vapor, will disperse into space . The term gas is also used in reference to the state, or condition, of matter having this property.
The atoms or molecules of matter in the gaseous state move freely among each other, and are, in most instances, packed more loosely than the molecules of the same substance in the solid or liquid state. A sample of gaseous matter can be compressed. Examples of gases are oxygen at room temperature (approximately 20 ºC or 68 ºF), hydrogen at room temperature, and water at standard atmospheric pressure and a temperature above 100 ºC or 212 ºF.
When a sample of matter in the gaseous state is heated, the atoms or molecules gain kinetic energy and move more rapidly. When a sample of gaseous matter is cooled, the atoms or molecules lose kinetic energy and move more slowly. If a sample of gaseous matter, confined to a container of fixed size, is heated, the pressure increases. If the sample is cooled, the pressure drops. If a sample of gaseous matter is placed in a sealed container and then the volume of the container is reduced, the compression heats the gas. If the volume of the sealed container increases, the decompression cools the gas.
If the temperature becomes sufficiently high, certain gases, such as hydrogen, will rapidly combine with other gases such as oxygen or chlorine. This is combustion. Some chemical reactions between gases and other substances occur more slowly; an example is the gradual oxidation of iron to form iron oxide (rust). In this case, the oxygen is gaseous at room temperature, while the iron and the iron oxide are solids.
When a sample of matter in the gaseous state is cooled to a low enough temperature, it may become a liquid or a solid. For example, if nitrogen is chilled to a temperature far below zero Celsius , it liquefies. Liquid nitrogen is used by some medical doctors to destroy minor skin lesions such as warts. Another gas, carbon dioxide, skips the liquid phase when cooled at atmospheric pressure, and becomes a solid known as dry ice.