Carbon dioxide does not exists as a liquid under normal atmospheric pressure, but solid CO₂ (also known as dry ice) sublimes at -78.5 °C (Figure 1). Dry ice (Figure 2) is commonly used as a refrigerant for food or biological sample preservation.

Figure 1: The phase diagram for carbon dioxide.

Figure 2: Photograph of a solid block of dry ice.

Note:

When dry ice is placed in water (especially heated) sublimation is accelerated, and a low-sinking dense cloud of fog (smoke-like) is created. This is used in fog machines, at theaters, concerts, haunted houses, and nightclubs for dramatic effects (Figure 3). Fog from dry ice hovers above the ground unlike other artificial fog machines (that use partial combustion of oil) where the fog rises like smoke.

Figure 3: Dry ice generated smoke being used during Iron Maiden’s Somewhere Back in Time World Tour. Copyright: Pyrotek.

Carbon dioxide is a linear molecule due to π-localization. The bonding in CO₂ involves 2 σ-bond and 2 sets of 3 center π-bonds (Figure 4). The C-O bond length of 1.2 Å should be compared to the value observed for organic carbonyls (e.g., ketones, esters, aldehydes) of 1.2 – 1.3 Å.

Figure 4: The carbon dioxide molecular orbital diagram.

Although CO₂ has no dipole moment it is very polar (dielectric constant = 1.60 at 0 °C, 50 atm) and consequently dissolves in polar solvents such as water up to a concentration of 0.18% (0.04 M). Most of it (+99%) is present as solvated CO₂ (Figure 5), and only ca. 0.2% is reacted to form carbonic acid, Equation 3, with subsequent equilibria resulting in the formation of bicarbonate (HCO₃^-) and carbonate (CO₃^2-).

(3)

Figure 5: Typical water solvation of carbon dioxide.

The overall reaction involves a series of equilibria. The first equilibrium is the formation of carbonic acid, Equation 4. The reaction rates, Equation 5, are on the magnitude of 1 second (i.e., slow), and as a consequence when carbon dioxide is carried in the body an enzyme is present to speed up the reaction.

(4)

(5)

The 2^nd equilibrium is as a consequence of first ionization of carbonic acid to form bicarbonate (HCO₃^-), Equation 6. In contrast to the first reaction, Equation 4, this reaction is very fast with a K_eq = 1.6 x 10^-4 @ 25 °C.

(6)

The 3^rd equilibrium involves the formation of the carbonate ionEquation 7, and has a K_eq = 4.84 x 10^-11. Carbonate (CO₃^2-) is a delocalized ligand, which can act as a mono or bidentate or bridging group.

(7)

The formation of carbonic acid is the reason that even in the absence of pollutants (such as SO₂) natural rain water is slightly acidic due to dissolved CO₂. The equilibrium associated with carbonic acid is also responsible for the buffering of the pH in blood.

Photosynthesis in plants reduces CO₂ to organic matter but similar reactions have yet to be developed in non-living systems.

Grignards react readily with carbon dioxide to form the carboxylate, which yields the associated carboxylic acid upon hydrolysis, Equation 8. Similar reactions occur with other organometallic compounds. In addition, CO₂ reacts with alkali metal salts of phenols (phenolates) to yield the hydroxy-carboxylate, Equation 9.

(8)

(9)

A number of complexes of CO₂ with transition metals are known in which the coordination can occur via the central carbon (Figure 6a) or the C=O bond (Figure 6b). Alternatively, CO₂ can bridge two metal centers.

Figure 6: Bonding modes of CO₂ to transition metals.

Global warming is the process of the observed increase in the average temperature of the Earth's near-surface air and oceans since the mid-20^th century. Global surface temperature increased 0.74 °C (1.33 °F) between the start and the end of the 20^th century (Figure 7). It is generally agreed that the majority of this temperature increase has occurred since the middle of the 20^th century and was caused by increasing concentrations of greenhouse gases resulting from burning fossil fuels (the generation of additional CO₂) and deforestation (the loss of a mechanism for the consumption of CO₂), see Figure 7. While it is appreciated that natural phenomena (including solar radiation and volcanoes) produced most of the warming from pre-industrial times, the magnitude of the changes brought on by global industrialization is more significant.

Figure 7: Correlation of Earth average temperature with carbon dioxide and global population.

The Earth’s atmosphere has two functions. First, the ozone (O₃) in the upper atmosphere screens harmful UV from reaching the surface of the Earth. Second, as solar radiation penetrates the atmosphere a portion of the heat is then retained as a consequence of the CO₂ in the atmosphere. It is this process that modulates the surface temperature and provides a stable environment for life. The failure or alteration of either of these processes can have a dramatic effect on the livability of a planet.

Consider the relative position of Venus, Earth, and Mars to the Sun (Figure 8). The closer a planet is to the sun the greater the UV radiation and the greater the heating of the planet; however, the temperature is also greatly modulated by the atmosphere. Venus has an atmosphere comprising 95% CO₂ and has a surface temperature of approximately 450 °C. In contrast, while Mars’ atmosphere is also 95% CO₂, it is only 1% as dense as that of Earth’s, and thus the surface temperatures range from 40 °C during the day (due to radiative heating) to –80 °C at night (due to the lack of retained heat because of the thin atmosphere). These should be compared to Earth’s atmosphere which is 0.038% CO₂, which allows for the correct amount of heat to be retained to sustain life. Clearly any significant change in the CO₂ content of the atmosphere will change the global temperatures of a planet.

Figure 8: The relative size and position of the planets from the sun.