Carbon dioxide (CO2) dissolves in water because water molecules are attracted to the polar areas of the CO2 molecule.
While the bond between carbon and oxygen isn't as polar as the bond between hydrogen and oxygen in water (H2O), it's polar enough that CO2 can dissolve. This solubility arises from the intermolecular forces of attraction between the polar regions of CO2 and the partial charges on water molecules.
Here's a more detailed breakdown:
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Polarity of CO2: Although the CO2 molecule is linear, the electronegativity difference between carbon and oxygen creates polar bonds. Oxygen is more electronegative, so it pulls electron density away from the carbon atom, creating partial negative charges (δ-) on the oxygen atoms and a partial positive charge (δ+) on the carbon atom.
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Attraction to Water: Water is a polar molecule (H2O). The oxygen atom has a partial negative charge, and the hydrogen atoms have partial positive charges. The partial positive charge on the carbon of CO2 is attracted to the partial negative charge on the oxygen of water, and the partial negative charges on the oxygen of CO2 are attracted to the partial positive charges on the hydrogen of water. These attractions, based on electrostatic forces, facilitate the dissolution of CO2 in water.
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Chemical Reaction: While some CO2 molecules simply dissolve, existing as dissolved CO2, a portion also reacts with water to form carbonic acid (H2CO3):
CO2 (aq) + H2O (l) ⇌ H2CO3 (aq)
This reaction is reversible, and carbonic acid can further dissociate into bicarbonate (HCO3-) and hydrogen ions (H+). This process is crucial in understanding the chemistry of natural waters and oceans, including ocean acidification.
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Solubility Factors: The solubility of CO2 in water is influenced by several factors:
- Temperature: As temperature increases, the solubility of CO2 decreases. Warmer water holds less dissolved CO2.
- Pressure: As pressure increases, the solubility of CO2 increases (Henry's Law). Higher partial pressures of CO2 force more of the gas into the liquid phase.
- Salinity: Higher salinity generally decreases the solubility of CO2.
