Solubility of a Gas in a Liquid

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🔵 Solubility of a Gas in a Liquid

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📘 Class 12 CBSE Chemistry Study Material Chapter: Solutions Topic: Solubility of a Gas in a Liquid

Many gases dissolve in liquids to form homogeneous solutions. Gas solubility is mainly affected by pressure and temperature.

Examples:

• O₂ in water – Essential for aquatic life.
• HCl in water – Forms hydrochloric acid.

Unlike solids, gas solubility is highly sensitive to pressure changes.

Dynamic Equilibrium in Gas–Liquid Solutions

When a gas is in contact with a liquid at constant pressure (p) and temperature (T):

1. Gas molecules dissolve into the liquid.
2. Dissolved gas molecules escape back to the gaseous state.

At equilibrium: Rate of gas entering solution = Rate of gas escaping from solution

Gas (g) ⇌ Gas (in solution)

This is called dynamic equilibrium. Any change in pressure or temperature shifts equilibrium (Le Chatelier’s Principle).

Effect of Pressure on Gas Solubility

Increasing pressure increases the solubility of a gas in a liquid.

➤ Henry’s Law

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

Mathematical Expression: p = K_H · x

• p = partial pressure of the gas
• x = mole fraction of gas in solution
• K_H = Henry’s Law constant

Graphical Representation:

A graph of p vs x gives a straight line. Slope = K_H

High K_H → Low solubility Low K_H → High solubility

Effect of Temperature on Gas Solubility

Dissolution of gases is generally an exothermic process.

Gas (g) → Gas (in solution) + Heat

Increase in temperature shifts equilibrium backward (Le Chatelier’s Principle), thus decreasing solubility.

• Cold water contains more dissolved oxygen.
• Aquatic animals survive better in cold water.

Higher Temperature → Lower Gas Solubility

Applications of Henry’s Law

1️⃣ Carbonated Beverages

• CO₂ dissolved under high pressure.
• On opening bottle, pressure decreases → CO₂ escapes as bubbles.

2️⃣ Scuba Diving and Bends

• At high pressure, more N₂ dissolves in blood.
• Sudden ascent → pressure decreases → N₂ forms bubbles.
• Causes decompression sickness (“bends”).
• Helium (less soluble) is used with oxygen in diving tanks.

3️⃣ High Altitude Anoxia

• Low partial pressure of oxygen (pO₂).
• Less oxygen dissolves in blood.
• Leads to weakness and confusion (anoxia).

Key Formula

p = K_H · x

• p = partial pressure of gas
• x = mole fraction of gas
• K_H = Henry’s Law constant

Important Terms

Term	Definition
Solubility	Maximum amount of gas dissolved at given temperature and pressure
Dynamic Equilibrium	Equal rate of dissolution and escape of gas
Henry’s Law	Solubility ∝ partial pressure of gas
KH	Henry’s law constant; depends on gas and temperature

📌 Final Revision: • Higher Pressure → Higher Solubility • Higher Temperature → Lower Solubility • Always remember the formula: p = K_H · x

📘 Class 12 CBSE Chemistry – Exam Oriented Notes
Chapter: Solutions
Topic: Solubility of a Gas in a Liquid

Board Focus Areas: ✔ Henry’s Law Statement
✔ Mathematical Expression
✔ Effect of Pressure & Temperature
✔ Applications (Very Important for 2–3 Marks)

1️⃣ Solubility of a Gas in a Liquid – Definition (1 Mark)

Solubility of a gas in a liquid is the amount of gas that dissolves in a given quantity of liquid at a specified temperature and pressure.

Always mention: “at given temperature and pressure”.

2️⃣ Dynamic Equilibrium in Gas–Liquid System (2–3 Marks)

When a gas is in contact with a liquid:

1. Gas molecules dissolve into the liquid.
2. Dissolved gas molecules escape back into gaseous phase.

At equilibrium: Rate of dissolution = Rate of escape of gas

Gas (g) ⇌ Gas (in solution)

This is called dynamic equilibrium. It is maintained at constant temperature and pressure.

3️⃣ Effect of Pressure – Henry’s Law (Very Important – 3 to 5 Marks)

Statement of Henry’s Law:

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

Mathematical Expression: p = K_H · x

• p = partial pressure of gas
• x = mole fraction of gas in solution
• K_H = Henry’s Law constant

Graph: Plot of p vs x is a straight line. Slope = K_H

High K_H → Low solubility Low K_H → High solubility

4️⃣ Effect of Temperature on Gas Solubility (3 Marks)

Dissolution of gases in liquids is generally an exothermic process.

Gas (g) → Gas (in solution) + Heat

According to Le Chatelier’s Principle:

• Increase in temperature → equilibrium shifts backward
• Gas escapes → solubility decreases

Higher Temperature → Lower Gas Solubility Cold liquids dissolve more gas.

5️⃣ Applications of Henry’s Law (Frequently Asked – 3 Marks)

1️⃣ Carbonated Drinks

• CO₂ dissolved under high pressure.
• On opening bottle → pressure decreases → CO₂ escapes as bubbles.

2️⃣ Scuba Diving (Bends)

• High pressure underwater → more N₂ dissolves in blood.
• Sudden ascent → N₂ forms bubbles → decompression sickness.
• Helium (less soluble gas) is used with oxygen.

3️⃣ High Altitude Anoxia

• Low partial pressure of O₂ at high altitude.
• Less O₂ dissolves in blood.
• Causes breathlessness and weakness.

6️⃣ Important Points for Board Examination

1. Pressure ↑ → Gas solubility ↑
2. Temperature ↑ → Gas solubility ↓
3. Henry’s Law valid for dilute solutions and low pressure.
4. K_H depends on nature of gas and temperature.

If a numerical question appears, use: p = K_H · x and solve for required quantity carefully.

📘 CBSE 2026 Chemistry Worksheet
Class: XII | Chapter: Solutions
Topic: Solubility of a Gas in a Liquid

Pattern Based on CBSE 2026:
Section A – MCQs
Section B – Assertion–Reason
Section C – Case Study
Section D – Short Answer
Section E – Long Answer / Numerical

SECTION A – MCQs (1 Mark Each)

1. The solubility of a gas in a liquid increases with:
   (a) Decrease in pressure
   (b) Increase in pressure
   (c) Increase in temperature
   (d) Decrease in volume of liquid
2. Henry’s Law is represented by:
   (a) p = K_H / x
   (b) p = x / K_H
   (c) p = K_H · x
   (d) x = p²
3. The slope of the graph between p and x is equal to:
   (a) 1 / K_H
   (b) K_H
   (c) x
   (d) p
4. Gas solubility decreases with rise in temperature because dissolution is generally:
   (a) Endothermic
   (b) Isothermal
   (c) Exothermic
   (d) Adiabatic
5. A high value of K_H indicates:
   (a) High solubility
   (b) Low solubility
   (c) No solubility
   (d) Infinite solubility

SECTION B – ASSERTION–REASON (1 Mark Each)

1. Assertion (A): Solubility of gases decreases with increase in temperature.
   Reason (R): Dissolution of gases in liquids is exothermic.
   
   (a) Both A and R are true and R is correct explanation
   (b) Both A and R are true but R is not correct explanation
   (c) A true, R false
   (d) A false, R true
   
2. Assertion (A): Pressure has no effect on solubility of gases.
   Reason (R): Solids and liquids are incompressible.
   
   (a) Both true
   (b) Both false
   (c) A false, R true
   (d) A true, R false
   

SECTION C – CASE STUDY (4 Marks)

A scuba diver dives deep into the ocean where pressure is very high. After spending time underwater, he suddenly ascends rapidly to the surface.

1. Why does more nitrogen dissolve in blood at greater depths? (1 Mark)
2. What happens when the diver ascends rapidly? (1 Mark)
3. Name the condition caused due to bubble formation. (1 Mark)
4. Which gas is mixed with oxygen in diving cylinders and why? (1 Mark)

Concept Tested: Henry’s Law + Real-life Application

SECTION D – SHORT ANSWER QUESTIONS (2–3 Marks Each)

1. State Henry’s Law and write its mathematical expression.
2. Explain dynamic equilibrium in gas–liquid system with equation.
3. Why does cold water contain more dissolved oxygen than warm water?

SECTION E – LONG ANSWER / NUMERICAL (5 Marks)

1️⃣ Explain the effect of pressure and temperature on solubility of gases with suitable equations and reasoning.

2️⃣ Numerical Problem: The value of K_H for a gas at 298 K is 1.25 × 10⁵ atm.
Calculate the mole fraction of the gas in solution if its partial pressure is 2 atm.

Use Henry’s Law:
p = K_H · x
Solve for x.

📌 Exam Strategy:
• Always write Henry’s Law statement exactly.
• Mention “at constant temperature”.
• For applications, relate directly to pressure change.
• In numericals, check units carefully.

📘 ANSWER KEY – CBSE 2026 WORKSHEET
Class: XII | Chapter: Solutions
Topic: Solubility of a Gas in a Liquid

Answers are structured strictly as per CBSE marking scheme.

SECTION A – MCQs (1 Mark Each)

1. (b) Increase in pressure
2. (c) p = K_H · x
3. (b) K_H
4. (c) Exothermic
5. (b) Low solubility

SECTION B – ASSERTION–REASON (1 Mark Each)

1. (a) Both A and R are true and R is correct explanation
2. (c) A false, R true

SECTION C – CASE STUDY (4 Marks)

1️⃣ Why does more nitrogen dissolve in blood at greater depths? (1 Mark)

At greater depths, pressure is high. According to Henry’s Law, solubility of a gas increases with increase in pressure. Hence, more nitrogen dissolves in blood.

2️⃣ What happens when the diver ascends rapidly? (1 Mark)

When the diver ascends rapidly, pressure decreases suddenly. Dissolved nitrogen comes out of solution and forms bubbles in the blood.

3️⃣ Name the condition caused. (1 Mark)

Decompression sickness (also called “bends”).

4️⃣ Which gas is mixed with oxygen and why? (1 Mark)

Helium is mixed with oxygen because helium is less soluble in blood compared to nitrogen, reducing risk of bubble formation.

SECTION D – SHORT ANSWER QUESTIONS (2–3 Marks Each)

1️⃣ State Henry’s Law and write its expression. (3 Marks)

Henry’s Law: At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

Mathematical expression: p = K_H · x

• p = partial pressure of gas
• x = mole fraction of gas
• K_H = Henry’s Law constant

2️⃣ Explain dynamic equilibrium in gas–liquid system. (2–3 Marks)

When a gas is in contact with a liquid, gas molecules dissolve into the liquid while some dissolved molecules escape back to the gaseous phase.

Gas (g) ⇌ Gas (in solution)

At equilibrium: Rate of dissolution = Rate of escape. This state is called dynamic equilibrium.

3️⃣ Why does cold water contain more dissolved oxygen? (2 Marks)

Dissolution of gases is exothermic. Increase in temperature shifts equilibrium backward, reducing solubility. Therefore, cold water contains more dissolved oxygen than warm water.

SECTION E – LONG ANSWER / NUMERICAL (5 Marks)

1️⃣ Effect of Pressure and Temperature (5 Marks)

Effect of Pressure: According to Henry’s Law, solubility of a gas is directly proportional to its partial pressure.

p = K_H · x

Thus, increase in pressure increases solubility.

Effect of Temperature: Dissolution of gases is generally exothermic.

Gas (g) → Gas (in solution) + Heat

Increase in temperature shifts equilibrium backward (Le Chatelier’s Principle), decreasing solubility.

2️⃣ Numerical Problem (5 Marks)

Given: K_H = 1.25 × 10⁵ atm
p = 2 atm
Formula: p = K_H · x

x = p / K_H

x = \( \dfrac{2}{1.25 × 10^5} \)

x = 1.6 × 10⁻⁵

Mole fraction of gas = 1.6 × 10⁻⁵

📌 Final Board Reminder: • Always write Henry’s Law statement completely. • Mention “at constant temperature”. • Use proper units in numericals. • Structure long answers with headings for full marks.

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