Increasing Order of Boiling Point of Aqueous Solutions – Solutions JEE Main 2026 PYQ

Quick Summary

Question Type: Colligative properties and boiling point elevation comparison
Chapter: Solutions – Elevation in Boiling Point
Difficulty: ⭐⭐ Easy
Time to Solve: 2-3 minutes
Key Formula: ΔT_b = iK_bm
Correct Answer: (D) I < II < III < IV
Why: Boiling point elevation depends on i × m. Comparing effective particle concentration for all four solutions gives the increasing order I < II < III < IV.

The Question

JEE Main 2026 (28 January Evening Shift) – Solutions

Consider the following aqueous solutions.

I. 2.2 g Glucose in 125 mL of solution.
II. 1.9 g Calcium chloride in 250 mL of solution.
III. 9.0 g Urea in 500 mL of solution.
IV. 20.5 g Aluminium sulphate in 750 mL of solution.

The correct increasing order of boiling point of these solutions will be:

Given: Molar mass in g mol^-1 : H = 1, C = 12, N = 14, O = 16, Cl = 35.5, Ca = 40, Al = 27 and S = 32

(A) III < I < II < IV
(B) II < III < IV < I
(C) II < III < I < IV
(D) I < II < III < IV

Quick Answer

Correct Option: (D) I < II < III < IV

Reasoning: For dilute aqueous solutions, boiling point elevation depends on:

• ΔT_b ∝ i × m
• Glucose and urea are non-electrolytes, so i = 1
• CaCl₂ dissociates into 3 ions, so i = 3
• Al₂(SO₄)₃ dissociates into 5 ions, so i = 5

Comparing effective particle concentration shows:
I < II < III < IV

Video Solution

If you want the full explanation in a clear step-by-step teaching format, watch the video solution below:

Watch Full Video Solution on YouTube

Understanding the Concept

Why Boiling Point Depends on Number of Solute Particles

Elevation in boiling point is a colligative property. It depends not on the chemical nature of the solute, but on the number of solute particles present in the solution.

ΔT_b = iK_bm

Where:

• ΔT_b = elevation in boiling point
• i = van’t Hoff factor
• K_b = ebullioscopic constant of solvent
• m = molality of solution

The Key Principle

To compare boiling points:

1. Calculate moles of solute
2. Estimate concentration from the given solution volume
3. Multiply by van’t Hoff factor i for electrolytes
4. Higher effective particle concentration means higher boiling point

Detailed Step-by-Step Solution

Step 1: Use the Boiling Point Relation

• Boiling point elevation follows ΔT_b = iK_bm
• Since the solvent is the same, K_b is common for all
• So we only need to compare i × concentration

Step 2: Calculate for Each Solution

Download Handwritten Solution

Solution I: Glucose

• Molar mass of glucose = 180 g mol^-1
• Moles = 2.2 / 180
• Non-electrolyte, so i = 1
• Effective particle concentration is the lowest

Solution II: Calcium chloride

• Molar mass of CaCl₂ = 40 + 2 × 35.5 = 111 g mol^-1
• Moles = 1.9 / 111
• CaCl₂ dissociates into 3 ions, so i = 3

Solution III: Urea

• Molar mass of urea = 60 g mol^-1
• Moles = 9.0 / 60 = 0.15
• Non-electrolyte, so i = 1

Solution IV: Aluminium sulphate

• Molar mass of Al₂(SO₄)₃ = 2 × 27 + 3 × (32 + 4 × 16) = 342 g mol^-1
• Moles = 20.5 / 342
• Al₂(SO₄)₃ dissociates into 5 ions, so i = 5

Step 3: Compare Effective Particle Concentration

On comparing i × concentration for all four:

• I has the least value
• II is greater than I
• III is greater than II
• IV is the highest

Step 4: Write the Increasing Order of Boiling Point

Since boiling point increases with ΔT_b:

• I < II < III < IV

So the correct option is:

• (D)

Final Answer

Option (D): I < II < III < IV ✓

The increasing order of boiling point is Glucose solution < Calcium chloride solution < Urea solution < Aluminium sulphate solution.

Essential Formulas for This Topic

Primary Equations

1. Elevation in Boiling Point:
   • ΔT_b = iK_bm
   • Boiling point increases with number of solute particles
   • For the same solvent, compare i × m
2. Moles of Solute:
   • Moles = mass / molar mass
   • Needed to compare concentration
   • Used for both electrolytes and non-electrolytes
3. van’t Hoff Factor:
   • Glucose: i = 1
   • Urea: i = 1
   • CaCl₂: i = 3
   • Al₂(SO₄)₃: i = 5

Important Constants

• Glucose and urea are non-electrolytes
• Electrolytes increase particle count after dissociation
• Boiling point elevation is a colligative property
• Higher effective particle concentration means higher boiling point

Common Mistakes to Avoid

❌ Mistake 1: Comparing Only Mass of Solute

Wrong Thinking: “Higher mass means higher boiling point.”

Correct Approach: Boiling point depends on number of particles, not directly on mass. Always convert to moles and include dissociation.

❌ Mistake 2: Ignoring van’t Hoff Factor

Wrong Approach: Treating CaCl₂ and Al₂(SO₄)₃ like non-electrolytes

Correct Approach: Electrolytes dissociate in solution, so multiply by i while comparing boiling point elevation.

❌ Mistake 3: Missing the Effect of Volume

Common Error:

• Comparing only moles of solute
• Ignoring that solution volumes are different

Correct Approach: Concentration comparison requires attention to the amount of solvent or solution volume given.

❌ Mistake 4: Confusing Increasing and Decreasing Order

Wrong Thinking: “IV should come first, so it must be the answer order directly.”

Correct Understanding:

• The question asks for increasing order
• So start with the lowest boiling point
• Write the sequence from smallest to greatest
• That gives I < II < III < IV

Key Concept Summary

What You Must Remember

1. Boiling point elevation is colligative: It depends on the number of particles
2. Use ΔT_b = iK_bm: Compare i × m for the same solvent
3. Electrolytes matter more: Dissociation increases particle count
4. Always calculate moles correctly: Mass alone is not enough
5. Read the order carefully: Increasing order means lowest to highest

The Golden Rule for Boiling Point Order Questions

“For the same solvent, the solution with larger i × concentration has higher boiling point.”

Frequently Asked Questions

Q1: Why does boiling point elevation depend on i × m?

A: Because boiling point elevation is a colligative property, so it depends on the number of dissolved particles present in the solution.

Q2: Why is glucose lower than calcium chloride here?

A: Glucose does not dissociate, but calcium chloride produces three ions in solution, increasing the effective particle concentration.

Q3: Why is aluminium sulphate highest in the order?

A: Aluminium sulphate dissociates into five ions and gives the greatest effective particle concentration among the given solutions.

Q4: Do we always need exact molality in such questions?

A: Not always. For comparison questions, a proportional comparison of i × concentration is often enough.

Q5: What is the final increasing order of boiling point here?

A: I < II < III < IV.

Prerequisites to Solve This Question

Before attempting this problem, you should understand:

1. Colligative properties: Especially boiling point elevation
2. Mole concept: Converting mass into moles
3. Dissociation of electrolytes: van’t Hoff factor idea
4. Basic concentration comparison: Relating moles to volume
5. Reading order questions carefully: Increasing vs decreasing order

After Solving This, You Can:

✅ Compare boiling points of different solutions confidently
✅ Use van’t Hoff factor in colligative property questions
✅ Convert given mass data into useful mole comparison
✅ Solve JEE Main Solutions chapter questions faster
✅ Avoid common mistakes in boiling point order problems
✅ Apply the same logic to freezing point and osmotic pressure questions

Study Tips for This Topic

For JEE Main:

1. Memorize the formula: ΔT_b = iK_bm
2. Focus on particle count: That is the heart of colligative properties
3. Practice electrolyte vs non-electrolyte comparison: It saves time in MCQs
4. Watch units and order carefully: Many mistakes happen in comparison questions

Common JEE Variants:

• Increasing order of boiling point
• Increasing order of freezing point depression
• Comparison of osmotic pressure
• Electrolyte vs non-electrolyte colligative property questions

Difficulty Rating & Exam Frequency

Difficulty Level: ⭐⭐ (2/5) – Easy
JEE Main Frequency: High – Colligative property comparisons are common
JEE Advanced Frequency: Medium – Often combined with dissociation or association concepts
Topic Importance: Very High – Solutions chapter frequently tests colligative properties

Written by Nishant Kumar
Chemistry Educator with 10+ Years of Experience Teaching JEE Aspirants
Founder – PadhoLikhoJEE

Last Updated: March 2026
Question Source: JEE Main 2026 PYQ
Topic: Solutions – Elevation in Boiling Point