Carbon dioxide (CO2) dissolves in water because of the attraction between water molecules and the slightly polar areas of the CO2 molecule.
The Process of Dissolution
Here's a breakdown of how this process occurs:
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Polarity of CO2: While carbon dioxide is a linear molecule with two identical carbon-oxygen bonds, it isn't completely nonpolar. The oxygen atoms are more electronegative than the carbon atom, meaning they attract electrons more strongly. This creates a slight negative charge (δ-) on the oxygen atoms and a slight positive charge (δ+) on the carbon atom.
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Polarity of Water: Water (H2O) is a polar molecule. The oxygen atom is more electronegative than the hydrogen atoms, resulting in a partial negative charge on the oxygen and partial positive charges on the hydrogens.
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Attraction and Dissolution: The slightly positive carbon atom in CO2 is attracted to the slightly negative oxygen atom in water molecules. Conversely, the slightly negative oxygen atoms in CO2 are attracted to the slightly positive hydrogen atoms in water molecules. These attractive forces, though weaker than strong chemical bonds, are strong enough to allow CO2 molecules to disperse among water molecules, effectively dissolving the gas.
Chemical Reactions
It's important to note that a small portion of dissolved CO2 also reacts chemically with water to form carbonic acid (H2CO3). This is a reversible reaction:
CO2 (aq) + H2O (l) ⇌ H2CO3 (aq)
Carbonic acid then can dissociate into bicarbonate (HCO3-) and hydrogen ions (H+), further impacting the water's pH. This is a crucial process in natural waters and biological systems.
H2CO3 (aq) ⇌ HCO3- (aq) + H+ (aq)
Factors Affecting Solubility
Several factors can influence how much CO2 dissolves in water:
- Temperature: Lower temperatures increase CO2 solubility. As temperature increases, the kinetic energy of the molecules increases, making it easier for CO2 to escape from the water.
- Pressure: Higher pressure increases CO2 solubility (Henry's Law). Increasing the partial pressure of CO2 above the water forces more CO2 into the solution.
In summary, carbon dioxide dissolves in water primarily due to the intermolecular attractions between the slightly polar CO2 molecule and the polar water molecules, and to a lesser extent due to chemical reactions which create carbonic acid, bicarbonate and hydronium ions.
