Question

Why do aquatic species, such as fish, generally feel more comfortable in cold water than in warm water?

• A. The solubility of oxygen gas increases as the temperature of the water decreases.
• B. The solubility of oxygen gas decreases as the temperature of the water decreases.
• C. The Henry's law constant ($K_H$) for oxygen decreases as temperature increases.
• D. Warm water contains more dissolved toxic gases that harm aquatic metabolism.

Hint:

Gas dissolution is exothermic ($\Delta H & Lt; 0$). Therefore, gas solubility decreases with an increase in temperature.

Answer 1

The Correct Option is A

Concept: The process of dissolving gaseous solutes into liquid solvents represents a phase transition that achieves dynamic chemical equilibrium. When a gas dissolves, its individual molecules contract from a high-volume gas state into a restricted dissolved state, releasing kinetic energy. As a result, gas dissolution is universally classified as an exothermic physical process: \[ \text{Gas(g)} + \text{Liquid Solvent(l)} \rightleftharpoons \text{Dissolved Solution(aq)} + \text{Heat} \quad (\Delta H & Lt; 0) \] We use Le Chatelier’s principle to evaluate how thermal energetic changes force this equilibrium to shift.

Step 1: Applying Le Chatelier’s principle to the exothermic dissolution equation under cold conditions.
Le Chatelier's principle states that if a dynamic system at equilibrium experiences a temperature change, the position of the equilibrium shifts to counteract that change. When the ambient temperature of the aqueous system decreases (as seen in cold water), the system responds by attempting to generate thermal energy. To accomplish this, the equilibrium shifts toward the exothermic forward direction (to the right).

Step 2: Connecting the equilibrium shift to the breathing comfort of aquatic species.
The forward shift drives a larger mass of gaseous oxygen molecules to dissolve into the liquid phase. Consequently, cold water retains a substantially higher concentration of dissolved oxygen ($\text{DO}$) than warm water. Because fish and other aquatic species rely on extracting dissolved oxygen from water via their gills for respiration, the higher concentration of available oxygen makes cold water significantly more favorable and comfortable for their survival.