Q1.11:
Why do gases always tend to be less soluble in liquids as the temperature is raised?

Solution :

1. The Thermodynamic Basis of Gas Solubility

The dissolution of a gas in a liquid is typically an exothermic process. When gas molecules are incorporated into a liquid solvent, the process involves two distinct stages:

Cavity Formation: Solvent molecules must move apart to create space for gas molecules [Endothermic; requires energy to overcome intermolecular forces of the solvent].
Solvation/Interaction: Gas molecules interact with solvent molecules, releasing energy as bonds/intermolecular forces (e.g., dipole-dipole, London dispersion) are established [Exothermic].

The net enthalpy of solution ($\Delta_{sol}H$) for most gas-liquid systems is negative ($\Delta_{sol}H < 0$), representing a release of heat into the surroundings.

2. Le Chatelier’s Principle Application

According to Le Chatelier’s Principle, if a system at equilibrium is subjected to a change in temperature, the equilibrium will shift to counteract that change. The dissolution equilibrium can be represented as:

$\text{Gas} + \text{Solvent} \rightleftharpoons \text{Solution} + \text{Heat}$

Because the reaction is exothermic, heat acts as a product. Increasing the temperature ($T$) adds energy to the system. To counteract this stress, the equilibrium shifts in the direction that consumes the excess heat—the backward direction (reverse reaction).

[Justification: By the principle of chemical equilibrium, increasing the temperature of an exothermic system favors the endothermic reverse reaction, thereby decreasing the solubility of the gas].

3. Molecular Kinetic Energy Analysis

From a kinetic perspective, solubility is determined by the balance between the rate of dissolution and the rate of escape (evaporation) of gas molecules from the solution:

As temperature increases, the average kinetic energy ($KE = \frac{3}{2}RT$) of the dissolved gas molecules increases.
Increased kinetic energy allows gas molecules to overcome the intermolecular attractive forces between the solvent and the solute.
Consequently, the rate of gas escape increases significantly, resulting in a net decrease in the concentration of the dissolved gas.

4. Visualizing the Solubility Equilibrium

5. Summary Table of Effects

Variable	Effect on Exothermic Dissolution
Temperature Increase ($T \uparrow$)	Equilibrium shifts left (Reverse reaction)
Kinetic Energy of Gas	Increases, promoting solute escape
Net Solubility	Decreases

Final Solution: The dissolution of gases in liquids is an exothermic process ($\Delta_{sol}H < 0$). Upon increasing the temperature, the system acts to dissipate the added heat by favoring the endothermic reverse process, causing the dissolved gas to escape the solution, thereby reducing its solubility.

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