Carbon dioxide (CO2) is an odorless, colorless gas formed by the oxidation of hydrocarbons, both naturally, in animals, and artificially, as fuel. It is used by plants in the process of photosynthesis to create complex carbohydrates (cellulose and sugars), using water and the energy from sunlight. The waste product in this process is oxygen. It is one of the human body's major waste products, being excreted through exhalation (breathing).
The Lewis structure indicates it has two double bonds,
At room temperature and atmospheric pressure, carbon dioxide is a gas. It becomes solid at -78 ºC at standard atmospheric pressure, forming dry ice. When it thaws, it turns directly into a gas again (called sublimation).  This can be seen from the phase diagram to the right, where at 1 atmosphere of pressure, there is no liquid state separating the solid and gas as temperature rises.
Carbon Dioxide and Atmospheric Temperature
CO2 is one of the "greenhouse gases". Heat-trapping by CO2 has been observed on Earth and Venus. The extent of its role in the current increase in average global temperatures is a topic of active political and scientific debate. Rising CO2 levels in the atmosphere have been observed to promote acidification of the oceans, through CO2 + H2O => H2CO3
Environmentalists predict catastrophic global warming scenarios at the hands of excessive CO2 released by industrial and commercial activities. A counterargument to this theory is that, the planet naturally gives off carbon dioxide, from natural sources such as volcanoes, and thus, it is not so bad for the Earth.
The effects of an increased level of CO2 is Increased plant growth, as CO2(g) + 12H2O(aq) + energy → C6H12O6(aq)(A complex sugar) + 6O2(gas) + 6H2O(aq), as more products for photosynthesis. This supposedly has beneficial effects for humans, such as greater crop yields and alleviating the effects of a growing human population.
- ↑ If not enough oxygen is present, carbon monoxide will also be produced, in a process called incomplete combustion.
- ↑ "Dry ice is frozen carbon dioxide. A block of dry ice has a surface temperature of -109.3 degrees Fahrenheit (-78.5 degrees C). Dry ice also has the very nice feature of sublimation -- as it breaks down, it turns directly into carbon dioxide gas rather than a liquid." How Stuff Works