CO2g

CO2g is the notation for gaseous carbon dioxide, the CO2 molecule in the gas phase. In chemical notation, the g indicates a gas, as opposed to CO2(s) for solid carbon dioxide or CO2(aq) for dissolved carbon dioxide. CO2 occurs naturally as a trace component of Earth's atmosphere and is also produced by various natural and human processes.

Physically, CO2 is a linear, nonpolar molecule with the structure O=C=O. At standard conditions it is a colorless, odorless gas. It sublimates directly from solid to gas at about −78.5 °C at 1 atmosphere. Its critical point is near 31 °C and 7.4 MPa, above which it exists as a supercritical fluid. In water it dissolves only moderately, forming carbonic acid through a reversible reaction, with much of the dissolved CO2 described by Henry’s law.

Occurrence and use: natural sources include respiration, volcanic outgassing, and ocean release; human activities such as burning fossil fuels and cement production add substantial amounts to the atmosphere. CO2 is widely used in beverages for carbonation, in fire suppression systems, and as a shielding gas in some industrial processes. In climate science, CO2 is a major greenhouse gas that traps infrared radiation and contributes to the warming of the planet. Atmospheric concentrations are on the order of several hundred parts per million by volume in modern times.

Safety and environmental considerations: elevated CO2 levels can displace oxygen and pose asphyxiation risks in enclosed spaces, though CO2 is not acutely toxic at typical environmental concentrations. Monitoring and ventilation are important in workplaces and confined environments.