Which statement about temperature and dissolved oxygen is correct?

• A. Dissolved Oxygen decreases as temperature increases
• B. Dissolved Oxygen increases as temperature increases
• C. Dissolved Oxygen remains unchanged with temperature
• D. Dissolved Oxygen first increases then decreases with temperature

Answer: (A)

Temperature controls how much oxygen can stay dissolved in water. As water heats up, the solubility of oxygen decreases, so the amount of dissolved oxygen at equilibrium with the air becomes lower. This happens because gas molecules have more energy at higher temperatures and are more likely to escape from the water, and the solubility constant in Henry’s law declines with increasing temperature. In practical terms, warm water tends to have less dissolved oxygen than cold water, which can stress aquatic life and affect aerobic processes in wastewater treatment. The other statements don’t fit this basic physical behavior, since solubility does not increase with temperature, remain unchanged, or show a nonmonotonic increase-then-decrease pattern for this property.