Quick Summary
Concept Tested: Purification techniques for thermally unstable compounds.
Chapter: Basic Organic Chemistry.
Difficulty: ★☆☆☆☆
Time to Solve: 1 minute.
Key Formula: $$\ln \left( \frac{P_2}{P_1} \right) = \frac{\Delta H_{vap}}{R} \left( \frac{1}{T_1} – \frac{1}{T_2} \right)$$
Answer: (D)
One-line Reason: Reduced pressure distillation lowers the boiling point, allowing the liquid to vaporize below its decomposition temperature.
The Question
Which purification technique is used for high boiling organic liquids which decomposes at or below their boiling point?
(A) Simple distillation
(B) Steam distillation
(C) Fractional distillation
(D) Reduced pressure distillation
Quick Answer
The correct answer is (D) Reduced pressure distillation.
When an organic liquid has a high boiling point and is unstable (decomposes near its boiling point), heating it at atmospheric pressure causes thermal decomposition. By reducing the external pressure, the boiling point of the compound decreases significantly, allowing it to vaporize and be purified at a temperature below its decomposition point.
Why Other Options Are Incorrect
Option (A) Simple Distillation
Simple distillation operates at atmospheric pressure (1 atm). It requires the liquid to reach its normal boiling point to vaporize. Since the compound decomposes near this temperature, heating it to this point would destroy the molecule before it could be collected as vapor.
Option (B) Steam Distillation
Steam distillation is effective only for immiscible liquids (e.g., separating essential oils from water). It relies on co-distillation with steam but does not lower the boiling point of the pure organic compound itself sufficiently to prevent thermal decomposition of high-boiling, unstable liquids.
Option (C) Fractional Distillation
Fractional distillation is used to separate mixtures of liquids with close boiling points (e.g., petroleum fractions). It does not lower the boiling point of a single compound to avoid decomposition; it merely improves separation efficiency between components.
Video Solution
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Understanding the Concept
The key to this question lies in understanding the relationship between pressure, boiling point, and thermal stability.
When a liquid is heated, its vapor pressure increases until it equals the external pressure surrounding it. At this point, the liquid boils. For high-boiling liquids, this requires high temperatures. If the compound is thermally unstable, it will break down (decompose) before it can boil and vaporize at atmospheric pressure.
Reduced pressure distillation (also known as vacuum distillation) solves this by lowering the external pressure. According to the Clausius-Clapeyron equation, lowering the pressure decreases the boiling point of the liquid.
$$\ln \left( \frac{P_2}{P_1} \right) = \frac{\Delta H_{vap}}{R} \left( \frac{1}{T_1} – \frac{1}{T_2} \right)$$
By reducing \(P_2\), the new boiling point \(T_2\) becomes much lower than the normal boiling point \(T_1\), allowing the compound to be distilled safely.
Detailed Step-by-Step Solution
Step 1: Analyze the Nature of the Compound
The problem states the organic liquid is “high boiling” and “decomposes at or below its boiling point.” This implies that heating the compound to its standard atmospheric boiling point will cause it to degrade. Therefore, any purification method relying on reaching this temperature is unsuitable.
Step 2: Evaluate Simple Distillation (Option A)
Simple distillation is the standard method for purifying liquids that are stable at their boiling points. However, since this compound decomposes near its boiling point, simple distillation is not viable. The compound would break down in the distillation flask before vaporizing.
Step 3: Evaluate Steam Distillation (Option B)
Steam distillation is used for separating compounds that are immiscible with water and have low solubility in it. While it allows distillation at temperatures below 100°C, it is primarily a separation technique for mixtures, not a general method for preventing the thermal decomposition of a single, pure, high-boiling organic liquid.
Step 4: Evaluate Fractional Distillation (Option C)
Fractional distillation is an enhancement of simple distillation for separating mixtures of liquids with similar boiling points. Like simple distillation, it requires the liquid to reach its boiling point. It does not modify the boiling point to prevent thermal decomposition.
Step 5: Identify the Correct Technique (Option D)
Reduced pressure distillation works by applying a vacuum to the system. This lowers the external pressure, which in turn lowers the boiling point of the liquid. This allows the liquid to vaporize at a significantly lower temperature where it is stable and does not decompose.
Final Answer
Correct Option: (D) Reduced pressure distillation
Essential Formulas for This Topic
1. Clausius-Clapeyron Equation: Relates pressure and boiling point.
$$\ln \left( \frac{P_2}{P_1} \right) = \frac{\Delta H_{vap}}{R} \left( \frac{1}{T_1} – \frac{1}{T_2} \right)$$
Where:
- \(P_1\) = Normal atmospheric pressure (1 atm)
- \(P_2\) = Reduced pressure (in atm)
- \(T_1\) = Normal boiling point (in Kelvin)
- \(T_2\) = Boiling point at reduced pressure (in Kelvin)
2. Raoult’s Law (for mixtures): $$P_{total} = X_A P_A^* + X_B P_B^*$$
Used to understand steam distillation partial pressures.
Common Mistakes to Avoid
Mistake 1: Confusing Fractional Distillation with Vacuum Distillation
Wrong Thinking: Fractional distillation is better than simple distillation, so it must work here.
Correct Approach: Fractional distillation improves separation between components, but it does not lower the boiling point. It cannot prevent decomposition of a thermally unstable compound.
Mistake 2: Misunderstanding Steam Distillation
Wrong Thinking: Steam distillation lowers the boiling point, so it must be the answer.
Correct Approach: Steam distillation lowers the boiling point of the *mixture*, not the pure compound. It is used for immiscible liquids, not generally for high-boiling unstable pure liquids.
Mistake 3: Ignoring the Decomposition Condition
Wrong Thinking: Any distillation method can purify organic liquids.
Correct Approach: The question specifies the liquid decomposes near its boiling point. The method must lower the boiling point to avoid the decomposition temperature. Only reduced pressure distillation does this.
Key Concept Summary
- High boiling point liquids often require high temperatures to vaporize.
- Thermal decomposition occurs when heat energy exceeds the bond strength of the molecule.
- Reducing external pressure lowers the boiling point, allowing vaporization at lower temperatures.
- Vacuum distillation is the standard industrial method for distilling heat-sensitive materials.
- Glycerol and Vitamin A are classic examples of compounds purified via vacuum distillation.
Golden Rule: If a compound decomposes at its boiling point, you cannot distill it at atmospheric pressure. You must lower the boiling point by reducing the pressure.
Frequently Asked Questions
Q: What is the difference between “Reduced Pressure Distillation” and “Vacuum Distillation”?
A: The terms are often used interchangeably. “Reduced Pressure Distillation” is the general term for the process of lowering the pressure. “Vacuum Distillation” specifically refers to the equipment or method used to create that vacuum (often using a vacuum pump or water aspirator).
Q: Can steam distillation be used for high boiling organic compounds?
A: Steam distillation can be used to distill high-boiling compounds if they are immiscible with water and do not decompose under the lower temperature (usually <100°C) of the steam distillation process. However, if the compound decomposes even at 100°C, vacuum distillation is still required.
Q: Why does reducing pressure lower the boiling point?
A> According to the Clausius-Clapeyron equation, boiling occurs when the vapor pressure of the liquid equals the external pressure. By reducing the external pressure, the liquid’s vapor pressure requires less energy (temperature) to reach equilibrium, thus boiling at a lower temperature.
Q: Is reduced pressure distillation used in the laboratory or industry?
A> It is used in both. In the lab, it is used for heat-sensitive pharmaceuticals and natural products. In industry, it is used for refining heavy oils and separating components like lubricating oil fractions from crude oil.
Prerequisites to Solve This Question
- Understanding the definition of boiling point and vapor pressure.
- Knowledge of the different types of distillation (Simple, Fractional, Steam, Vacuum).
- Understanding the concept of thermal decomposition (breaking down due to heat).
- Familiarity with the Clausius-Clapeyron equation relationship.
After Solving This, You Can:
- ✔ Identify which distillation method is suitable for heat-sensitive compounds.
- ✔ Apply the Clausius-Clapeyron equation concept to qualitative problems.
- ✔ Distinguish between separation techniques for mixtures vs. purification techniques for unstable compounds.
Study Tips for This Topic
In JEE exams, questions on this topic are usually conceptual. To score well:
- Memorize the boiling point behavior of common organic compounds like Glycerol (bp 290°C, decomposes), Vitamin A (bp 257°C, decomposes), and Nitrobenzene (bp 211°C).
- Focus on the keyword “decomposes” or “unstable” in the question stem; this is the trigger to switch from Simple/Fractional distillation to Vacuum distillation.
- Practice the Clausius-Clapeyron equation to understand the inverse relationship between Pressure and Boiling Point.
Difficulty Rating & Exam Frequency
Difficulty: ★☆☆☆☆ (Very Easy)
JEE Main Frequency: Moderate. Concepts appear frequently in Physical Chemistry and Organic Chemistry sections.
JEE Advanced Frequency: Low. Usually appears as a part of a multi-concept problem rather than a standalone question.
Importance: High. Essential for understanding industrial processes and laboratory techniques.
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Written by Nishant Kumar Gupta
Founder of Padho Likho JEE & Senior Chemistry Educator — 12+ Years Experience, Ex-Faculty Allen/Aakash/Narayana.
Last Updated: July 2026
Question Source: JEE Main 2021 PYQ
Topic: Basic Organic Chemistry