Carbon Dioxide
Properties
| State | Gas at room temperature (sublimes as dry ice at -78.5°C) |
| Color | Colorless |
| Solubility | Slightly soluble in water (1.45 g/L at 25°C, 1 atm) |
| Melting Point | -56.6°C (at 5.18 atm, triple point) |
| Boiling Point | -78.5°C (sublimation point at 1 atm) |
About Carbon Dioxide
Carbon dioxide is the textbook nonpolar molecule that contains two polar bonds — a linear O=C=O with sp-hybridized carbon, two C=O bond dipoles of about 2.3 D each, and a net molecular dipole of zero because the geometry forces the bond vectors to cancel. That cancellation is why CO2 is a gas at room temperature while the bent water molecule, with the same number of heavy atoms, is a liquid. CO2 has no normal boiling point at 1 atm — instead it sublimes from solid (dry ice) directly to gas at −78.5 °C, and the only way to handle it as a liquid is above its triple point at −56.6 °C and 5.18 atm. The supercritical phase boundary at 31.1 °C and 73.8 atm is one of the most useful in process chemistry: above that, scCO2 has gas-like diffusivity and liquid-like density, which is why it is the green solvent of choice for decaffeinating coffee (the standard Swiss Water alternative is run at about 250 atm and 60 °C) and for extracting hops, essential oils, and pharmaceutical APIs without leaving a solvent residue. In aqueous solution, CO2 is in equilibrium with H2CO3 and the bicarbonate buffer that holds blood pH between 7.35 and 7.45, mediated by the zinc enzyme carbonic anhydrase, which catalyzes the hydration step at one of the highest known turnover numbers — about 10⁶ s⁻¹. Atmospheric CO2 has climbed from about 280 ppm pre-industrial to over 420 ppm now, and the corresponding 0.1 unit drop in surface ocean pH is the chemistry behind ocean acidification.
Where you'll encounter it
Every fermentation lab knows the sound of CO2 bubbling out of a fermenter through a water trap. Every welding rig running shielded MIG uses pure or argon-blended CO2 at 15 to 25 cubic feet per hour to keep oxygen off the molten weld pool. Every soda-stream cartridge is liquid CO2 at about 60 atm. Dry ice ships frozen pharmaceuticals and biological samples at −78.5 °C, sublimes cleanly without leaving residue, and is the lab's go-to cooling bath when used in acetone (gives −78 °C) for organolithium and trapping experiments. In a teaching lab, the classic limewater test — bubble exhaled breath through Ca(OH)2 solution and watch CaCO3 precipitate — is the simplest qualitative CO2 detection that exists.
Common Uses
- Carbonation of soft drinks, beer, and sparkling water at 2 to 4 volumes of CO2 per volume of liquid
- Supercritical CO2 extraction of caffeine, hops, and pharmaceutical APIs at 73 to 300 atm
- Shielding gas for MIG welding of mild steel at 15 to 25 cfh, often blended with argon
- Dry ice (solid CO2) for shipping pharmaceuticals and biological samples at −78.5 °C
- Bath solvent for −78 °C cooling in dry-ice/acetone for organolithium and Swern-oxidation chemistry
Safety Information
CO2 is non-toxic at ambient atmospheric levels but acts as a simple asphyxiant by displacing oxygen — the OSHA PEL is 5,000 ppm (0.5 percent) as an 8-hour TWA, and the IDLH is 40,000 ppm (4 percent). Above 5 percent, headache, tachypnea, and confusion start within minutes; above 10 percent, unconsciousness and death follow rapidly. The hazard is geometry-dependent: CO2 is denser than air and pools in pits, fermentation cellars, dry-ice storage rooms, and walk-in fridges, where lethal incidents occur regularly when ventilation fails. Dry ice contact causes frostbite within seconds — handle with insulated gloves. Pressurized CO2 cylinders and supercritical extractors operate at 60 to 300 atm and require trained operation.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.