As temperature goes up, which goes down?

• A. Dissolved Oxygen
• B. Biochemical Oxygen Demand
• C. Total Suspended Solids
• D. pH

Answer: (A)

Dissolved oxygen decreases as temperature increases because the solubility of gases in water falls as the water gets warmer. Warmer water can’t hold as much oxygen, so the amount of dissolved oxygen drops with higher temperatures. This has practical implications for wastewater treatment and aquatic life, since less oxygen in the water can limit aerobic processes and stress organisms that rely on oxygen.

Biochemical oxygen demand, on the other hand, can actually rise with temperature because microbial activity speeds up, increasing the rate at which oxygen is consumed. Total suspended solids aren’t directly governed by temperature, and pH doesn’t follow a simple downward trend with temperature; it can shift in various ways depending on buffering and gas exchange. So the only factor that reliably decreases as temperature goes up is dissolved oxygen.