Oxygen is only sparingly soluble in water. At 25°C, the solubility of oxygen in water is approximately 264 µM (micromolar).
This limited solubility is due to several factors:
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Weak Intermolecular Forces: Oxygen (O₂) is a nonpolar molecule. Water (H₂O) is a polar molecule and forms hydrogen bonds with itself. The attraction between nonpolar oxygen molecules and polar water molecules is relatively weak compared to the strong hydrogen bonding between water molecules. This makes it energetically unfavorable for oxygen to dissolve in water.
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Entropy: Dissolving a gas in a liquid generally reduces the entropy (disorder) of the system. This is because the gas molecules are more ordered in the liquid phase than in the gaseous phase. This entropic effect further disfavors the dissolution of oxygen in water.
Because of the low solubility of oxygen in water, aquatic organisms and animals require specialized systems for oxygen transport, such as hemoglobin in blood. Hemoglobin is a protein that binds to oxygen and increases the oxygen-carrying capacity of the blood far beyond what would be possible if oxygen were simply dissolved in the plasma.
