Gas is one of the four fundamental states of matter, distinct from solids, liquids, and plasma. A gas can consist of individual atoms, elemental molecules, or compound molecules. Gas mixtures, like air, comprise various pure gases. Gases are characterized by the large distances separating individual particles, which often makes them invisible to the naked eye.
In 1990, the eruption of Mount Redoubt served as an example where real gas effects had a significant impact due to the gases produced during the geological event.