Gas solubility decreases with increasing temperature primarily because adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, making it easier for dissolved gas to escape the liquid.
The Fundamental Reason: Overcoming Attractive Forces
The dissolution of gas in a liquid is typically an exothermic process, meaning it releases heat. When a gas dissolves, its molecules interact with solvent molecules, forming weak attractive forces. These forces help hold the gas within the liquid.
As the temperature of a solution increases, the molecules within both the gas and the solvent gain more kinetic energy. According to the provided reference, this increased thermal energy directly counters the attractive forces: "adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas." This surge in energy allows the gas molecules to move more vigorously, break free from the solvent's grasp, and return to the gaseous phase.
Le Chatelier's Principle in Action
This phenomenon can also be understood through Le Chatelier's Principle, which states that if a dynamic equilibrium is disturbed by changing the conditions, the position of the equilibrium shifts to counteract the change. For gas dissolution, the equilibrium can be represented generally as:
$$ \text{Gas}{\text{(g)}} + \text{Solvent}{\text{(l)}} \rightleftharpoons \text{Dissolved Gas}_{\text{(aq)}} + \text{Heat} $$
Since heat is produced during dissolution (an exothermic process), increasing the temperature (adding more heat) acts as a stress on the system. To relieve this stress, the equilibrium shifts to the left, favoring the reactants – the undissolved gas. This "pushes the reaction...to the left," as mentioned in the reference, leading to a reduction in the amount of gas dissolved in the liquid.
Impact of Increased Kinetic Energy
The elevated temperature translates to faster-moving molecules:
- Gas Molecules: Dissolved gas molecules possess higher kinetic energy, making it easier for them to overcome the intermolecular forces that bind them to the solvent molecules. They gain sufficient energy to break free and escape the liquid phase into the atmosphere above.
- Solvent Molecules: Solvent molecules also move more rapidly, which can disrupt the transient "cages" they form around the gas molecules, further facilitating the gas's escape.
Practical Implications and Examples
This principle has several everyday and environmental consequences:
Everyday Observations
- Carbonated Beverages: A cold soda retains its fizz (dissolved carbon dioxide) much longer than a warm one. When warm, the CO$_2$ escapes rapidly, making the drink go "flat."
- Boiling Water: As water is heated towards its boiling point, small bubbles often form on the sides of the pot before boiling actually begins. These are not steam; they are dissolved air (nitrogen and oxygen) escaping the water as its solubility decreases with rising temperature.
- Aquatic Life: Warmer bodies of water contain less dissolved oxygen than colder ones. This can be detrimental to aquatic organisms like fish, which rely on dissolved oxygen for survival, potentially leading to fish kills during heat waves.
Solubility Trend Overview
The relationship between temperature and gas solubility can be simply illustrated:
| Temperature Level | Gas Solubility |
|---|---|
| Low | High |
| Medium | Medium |
| High | Low |
In summary, the decrease in gas solubility with increasing temperature is a direct consequence of increased thermal energy disrupting the attractive forces between gas and solvent molecules and shifting the dissolution equilibrium towards the undissolved gas state.
