Solubility of a Gas in a Liquid

 📘 Class 12 CBSE Chemistry Study Material

Chapter: Solutions
Topic: Solubility of a Gas in a Liquid

Many gases dissolve in liquids to form homogeneous mixtures. This is an important phenomenon not just in chemistry but also in environmental science, biology, and industry.

Some common examples include:

• Oxygen dissolving in water, which is vital for aquatic life.

• Hydrogen chloride (HCl), which is highly soluble in water and forms hydrochloric acid.

However, the solubility of gases is highly affected by pressure and temperature, unlike solids, which are mostly unaffected by pressure.

Dynamic Equilibrium in Gas-Liquid Solutions

Consider a container partly filled with a liquid and some gas above it, both at constant pressure (p) and temperature (T). Initially, gas molecules start dissolving in the liquid. Eventually, an equilibrium is reached when:

Rate of gas molecules entering the solution = Rate of gas molecules escaping from the solution

This is called dynamic equilibrium, represented as:

                         Gas(g) ⇌ Gas (in solution)

When this equilibrium is disturbed (e.g., by changing pressure or temperature), the system shifts to restore balance, as predicted by Le Chatelier’s Principle.

Effect of Pressure on Gas Solubility

As pressure increases, more gas molecules are forced into the solution.

➤ Henry’s Law

Henry’s Law gives a quantitative relationship between pressure and gas solubility.

Statement: At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the surface of the liquid.

Mathematically:

p = K_H. x     …… (1.11)

Where,

• p = partial pressure of the gas above the liquid

• x = mole fraction of the gas in the solution

• K_H = Henry’s Law constant (depends on nature of the gas and temperature)

Graphical Representation:

If we plot p vs x, we get a straight line with slope K_H 

Interpretation:

• A high K_H value indicates low solubility of the gas.

• A low K_H value means high solubility of the gas.

K_H  increases with temperature, so gas solubility decreases as temperature rises.

Effect of Temperature on Gas Solubility

The dissolution of gases in liquids is typically an exothermic process:

Gas (g) → Gas (in solution) + Heat

Hence, by Le Chatelier’s Principle:

When temperature increases, the equilibrium shifts backwards (i.e., towards the gaseous state), reducing gas solubility.

This is why:

• Cold water contains more dissolved oxygen than warm water.

• Aquatic animals thrive better in cold water due to higher oxygen content.

Applications of Henry’s Law

🔹 Carbonated Beverages:

• CO₂ is dissolved under high pressure in soda bottles.

• When opened, pressure is released, solubility drops, and CO₂ escapes as bubbles.

🔹 Scuba Diving and Bends:

• At high underwater pressures, more N₂ dissolves in blood.

• Upon sudden ascent, pressure drops quickly, and nitrogen comes out as bubbles in blood.

• This causes "bends" (decompression sickness), which can be fatal.

• Helium, a less soluble gas, is used in diving tanks along with oxygen.

🔹 High Altitude Anoxia:

• At high altitudes, pO₂ (partial pressure of oxygen) is low.

• Less oxygen dissolves in blood, leading to anoxia – a condition marked by weakness, confusion, and difficulty in thinking clearly.

Conclusion

• Solubility of gases is governed primarily by Henry’s Law, pressure, and temperature.

• Higher pressure = higher gas solubility

• Higher temperature = lower gas solubility

• Understanding this is crucial for real-world applications like scuba diving, soda bottling, and managing oxygen supply at high altitudes.

Key Formula:

p = K_H . x

Where:

• p = partial pressure of the gas

• x = mole fraction of the gas in the solution

• K_H = Henry's Law constant

Important Terms:

Term	Definition
Solubility	Maximum amount of a substance that can dissolve in a solvent at given T and p
Dynamic Equilibrium	Equal rate of dissolution and escape of gas
Henry’s Law	Solubility ∝ partial pressure of the gas
KH	Henry’s law constant; varies with gas and temperature