Carbon dioxide is a common compound found in the air; its molecule consists of one carbon atom and two oxygen atoms.

At standard atmospheric pressure, it is a colorless, odorless, non-oxidizing, and non-flammable gas. Carbon dioxide is soluble in water, and a small portion of it reacts with water to form carbonic acid.

Carbon dioxide is typically produced by the combustion of organic compounds, cellular respiration, and microbial fermentation. Plants absorb carbon dioxide in the presence of sunlight and use it in their chloroplasts for photosynthesis, producing carbohydrates and oxygen. This oxygen is then used by other organisms for respiration; these processes are part of the carbon cycle.

Carbon dioxide is one of the greenhouse gases; it absorbs infrared radiation emitted by Earth’s surface, slowing the loss of heat to space. When the concentration of carbon dioxide in the atmosphere increases, it strengthens the greenhouse effect, causing the Earth’s average temperature to rise. This process contributes to global warming.

The solid form of carbon dioxide is dry ice. At atmospheric pressure, dry ice sublimates directly into a gas. Carbon dioxide can exist as a liquid only under sufficient pressure; its triple point, where solid, liquid, and gas coexist, occurs at approximately 5.1 times atmospheric pressure and −56.6°C. Carbon dioxide can be injected into beverages to create bubbles, enhancing the drinking experience; examples include soda and beer.

Liquid carbon dioxide and dry ice can both be used for rapid food freezing. Liquid carbon dioxide absorbs heat as it expands and vaporizes, while dry ice absorbs heat as it sublimates directly from a solid into a gas.

Carbon dioxide is denser than air and does not support ordinary combustion; consequently, many fire extinguishers release carbon dioxide to suppress fires. CO₂ extinguishers discharge pressurized liquid carbon dioxide, which expands rapidly into gas and helps extinguish flames. They also leave no solid residue, making them useful for certain electrical equipment and flammable-liquid fires. However, they are not suitable for every type of fire.

Carbon dioxide can also be used as a shielding gas in welding. Unlike inert gases such as argon, carbon dioxide can react in the welding arc, affecting weld penetration and spatter. It is often less expensive than argon and is widely used in certain welding processes.

By Michael Chiu, MWS, CI – Updated and Revised in 2026