A gas is one of the phases of matter. Gases are, like liquids, fluids: they have the ability to flow and do not resist deformation. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but instead expand to fill whatever space they occupy. The kinetic energy in a gas is the second greatest of the states of matter (after plasma). Because of this increased kinetic energy, gas atoms and molecules tend to bounce off of one another, more so as the kinetic energy is increased.
The word "gas" was probably made by a Flemish chemist as a re-spelling of his pronunciation of the Greek word chaos.
Gases are also released from magma that either remains below ground (for example, as an intrusion) or is rising toward the surface.
The gases spread from an erupting vent primarily as acid aerosols (tiny acid droplets), compounds attached to tephra particles, and microscopic salt particles.
The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide, and hydrogen fluoride.
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