Gas is a basic state of matter, along with solids, liquids, and plasma. Gases have higher energy than solids or liquids, and are characterized by molecules that move freely, and by their capacity to expand to fill any container they are placed in. Many of the gases we encounter in everyday life are actually mixtures of gases: the atmosphere, for example, is a mixture of nitrogen, oxygen, carbon dioxide, water vapour, and trace amounts of other gases.
Gases have several common properties. First, gas molecules have enough energy that they move very quickly, and thus are well-spaced apart. (By contrast, molecules in liquids can move but remain close together, and molecules in solids can only vibrate.) Because they consist of spaced-apart moving molecules, they also have the lowest density of all the states of matter, and are most susceptible to changes in temperature and pressure. Whereas it is difficult or impossible to compress solids and liquids through changes in pressure, gases respond quickly to such changes, and will also shrink or expand more noticeably as a result of changes in temperature.
Several important laws and principles apply to gases. First, Boyle's Law - defined in the 1600s by Irish chemist Robert Boyle - states that the product of the pressure times the volume of a gas will always be constant, assuming that the number of particles has not changed: in other words: if the pressure applied to a gas increases, then the volume of that gas will decrease, and vice versa. Later, French chemists Charles and Gay-Lussac added that the volume divided by the temperature was also constant for gases: that is, if the volume of the gas increases, its temperature will also increase.
Secondly, Avogrado's Law states that all gases which occupy a specific volume at a specific temperature will consist of the same number of individual molecules. This allows the application to gas volume of the mole, or Avogrado's constant - the mole, or 602 sextillion (6.02 x 10^23) molecules, which is the number of molecules contained in exactly twelve grams of carbon-12. Specifically, at standard (room) temperature and normal atmospheric pressure, one mole of a gas will take up exactly 22.4 litres of volume. This number (22.4 L) is therefore called a "molar volume."
When cooled sufficiently, a gas will shrink in pressure and its molecules will slow down until it reaches the condensation point, when the molecules are no longer moving quickly enough to sustain gaseous form and the sample instead collapses or "condenses" into a liquid. In the classical formulation of the states of matter, gases have no upper temperature limit. However, twentieth-century science revealed that gases heated to sufficiently high temperatures (thousands or millions of degrees) become ionized and form a separate state of matter called plasma. Observable everyday plasmas are uncommon, but one example is the Sun, which is heated to extraordinarily high temperatures by internal processes of hydrogen fusion.