When pressure is increased at constant temperature, the solubility of gas in liquid increases. This is a fundamental principle in chemistry that dictates how gases dissolve into liquid solvents.
This phenomenon is governed by Henry's Law, a key concept in physical chemistry that explains the direct relationship between the pressure of a gas and its ability to dissolve in a liquid. The effect of change of pressure on the solubility of gases is such that solubility is directly proportional to pressure.
Understanding Gas Solubility and Henry's Law
The solubility of a gas in a liquid refers to the maximum amount of that gas that can dissolve in a given amount of liquid at a specific temperature and pressure. When the external pressure exerted on a gas above a liquid is increased, more gas molecules are forced into the liquid phase, thereby increasing its concentration within the solvent.
Henry's Law: The Core Principle
Henry's Law precisely quantifies this relationship, stating that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid, provided the temperature remains constant.
Mathematically, this can be expressed as:
S = kP
Where:
Sis the solubility of the gas (e.g., in mol/L or g/L)kis Henry's Law constant (a unique value for each gas-liquid pair at a specific temperature)Pis the partial pressure of the gas above the liquid
This direct proportionality means that if you double the partial pressure of a gas above a liquid, you effectively double the amount of that gas that will dissolve into the liquid, assuming the temperature remains unchanged.
Why Does Pressure Increase Solubility?
From a molecular perspective, increasing the pressure of a gas above a liquid means there are more gas molecules per unit volume in the gas phase. This leads to:
- Increased Collision Rate: More gas molecules collide with the liquid surface at a higher frequency.
- Enhanced Penetration: The greater number of collisions pushes more gas molecules into the liquid phase, where they become solvated (surrounded by solvent molecules).
- New Equilibrium: A new equilibrium is established where the rate of gas molecules dissolving into the liquid equals the rate of dissolved gas molecules escaping back into the gas phase, but this new equilibrium is achieved at a higher concentration of dissolved gas.
Factors Affecting Gas Solubility
While pressure is a primary determinant, other factors also influence how readily a gas dissolves in a liquid.
- Temperature: In general, an increase in temperature decreases the solubility of gases in liquids. This is because higher temperatures provide gas molecules with more kinetic energy, making it easier for them to escape from the dissolved state back into the gas phase. This is why carbonated drinks go flat faster when warm.
- Nature of the Gas and Solvent: The chemical properties and intermolecular forces between the gas and the liquid play a crucial role. Gases that react with the solvent (e.g., ammonia in water) or have similar intermolecular forces (e.g., nonpolar gas in a nonpolar solvent) tend to be more soluble.
Real-World Applications of Pressure's Effect on Gas Solubility
The principle that increasing pressure enhances gas solubility is not just a theoretical concept; it is vital for numerous everyday applications and industrial processes.
1. Carbonated Beverages
This is perhaps the most familiar example, encompassing sodas, sparkling water, beer, and other fizzy drinks.
- Process: During bottling, carbon dioxide (CO₂) gas is dissolved into the beverage under high pressure. This maximizes the amount of CO₂ that remains dissolved in the liquid.
- Observation: When you open a bottle or can, the pressure above the liquid is released (returning to atmospheric pressure). This sudden drop in pressure reduces the solubility of CO₂, causing the dissolved gas to rapidly escape from the solution, creating bubbles and the characteristic "fizz."
2. Deep-Sea Diving
The solubility of gases is critical for divers exploring underwater environments.
- Risk: As divers descend into the water, the ambient pressure increases significantly. This increased pressure causes more of the gases they breathe (primarily nitrogen from air) to dissolve into their blood and tissues. If a diver ascends too quickly, the rapid decrease in external pressure causes these dissolved gases to come out of solution too rapidly, forming bubbles in the blood and tissues.
- Solution: This dangerous condition, known as decompression sickness or "the bends," is prevented by controlled, slow ascents or by using decompression chambers, allowing the dissolved gases to safely diffuse out of the body at a gradual rate.
3. Industrial Gas Handling and Storage
Industries leverage this principle for various applications:
- Gas Storage: Gases like oxygen, nitrogen, and carbon dioxide are often stored in liquefied form under high pressure to maximize the amount of gas contained in a given volume.
- Chemical Synthesis: In certain chemical reactions, gases are bubbled through liquids under elevated pressure to increase their concentration in the reaction mixture, thereby enhancing reaction rates or yields.
Summary of Gas Solubility Trends
Understanding how different factors influence gas solubility is key to predicting and controlling chemical and physical processes.
| Factor | Effect on Gas Solubility (at Constant Temperature/Pressure) | Explanation |
|---|---|---|
| Pressure | Increases | More gas molecules are forced into the liquid phase; direct proportionality as per Henry's Law. |
| Temperature | Decreases | Higher kinetic energy of gas molecules allows easier escape from the liquid phase. |
Key Takeaways
- When pressure is increased at a constant temperature, the solubility of a gas in a liquid increases.
- This relationship is precisely described by Henry's Law, which states that the solubility is directly proportional to the partial pressure of the gas.
- The phenomenon is due to more gas molecules being forced into the liquid phase under higher pressure, establishing a new equilibrium with a greater concentration of dissolved gas.
- Practical applications include the fizz in carbonated beverages and preventing decompression sickness in divers.
