Distillation - Simple, Fractional, and Steam Distillation

Real-Life Hook: From Crude Oil to Your Car

Every time you fill petrol in your vehicle, you’re using products of fractional distillation! Crude oil - a black, thick liquid - is separated into petrol, diesel, kerosene, and lubricants through massive distillation columns that work 24/7 in refineries.

But distillation isn’t just about petroleum:

• Whiskey and perfumes: Steam distillation extracts essential oils from flowers
• Drinking water: Desalination plants use distillation to convert seawater to freshwater
• Pharmaceutical labs: Purifying solvents and drug molecules
• Forensic science: Extracting and analyzing volatile compounds from evidence
• Your chemistry lab: Purifying organic liquids for experiments

From ancient alchemists trying to create gold to modern pharmaceutical companies creating life-saving drugs, distillation remains one of chemistry’s most powerful separation techniques!

What is Distillation?

Distillation is a purification technique for separating liquids based on differences in their boiling points. The mixture is heated to vaporize the component with lower boiling point, and the vapors are condensed back to liquid in a separate container.

Fundamental Principle

Distillation exploits the fact that:

1. Different liquids have different boiling points
2. When a liquid boils, it converts to vapor
3. These vapors can be condensed back to pure liquid

Key Requirement: The boiling points of components should differ by at least 25-30°C for simple distillation.

Types of Distillation

Comparison Table

Type	B.P. Difference Required	Best For	Example
Simple Distillation	>30°C	Liquid + solid impurities	Purifying tap water
Fractional Distillation	<30°C (even 5-10°C)	Miscible liquids with close b.p.	Separating crude oil
Steam Distillation	Not applicable	Heat-sensitive, water-immiscible compounds	Extracting essential oils
Vacuum Distillation	For high b.p. compounds	Heat-sensitive materials	Glycerol (b.p. 290°C)

Simple Distillation

Principle

When a liquid mixture is heated:

1. The component with lower boiling point vaporizes first
2. Vapors rise up and enter condenser
3. Cold water in condenser jacket cools vapors
4. Vapors condense to pure liquid (distillate)
5. Higher b.p. component remains in distillation flask

Apparatus Components

Essential Components:

1. Round-bottom flask (distillation flask)

   • Contains the mixture
   • Round shape for uniform heating
   • Never fill more than 2/3rd capacity

2. Thermometer

   • Bulb at level of side arm (T-junction)
   • Measures vapor temperature (NOT liquid!)
   • Indicates when desired component is distilling

3. Condenser (Liebig condenser)

   • Inner tube: vapors pass through
   • Outer jacket: cooling water flows
   • Water enters from bottom, exits from top (countercurrent)

4. Receiver flask

   • Collects distillate
   • Keep cool (ice bath sometimes)

5. Heat source

   • Bunsen burner, heating mantle, or water bath
   • For flammable liquids: water bath or heating mantle (NO direct flame!)

6. Boiling chips (porous pot pieces)

   • Prevent superheating and bumping
   • Provide surface for bubble formation
   • NEVER add to hot liquid (can cause violent boiling!)

Detailed Procedure

Step 1: Setup Assembly

1. Clamp distillation flask securely
2. Add mixture (50-60% of flask volume)
3. Add 2-3 boiling chips (porous pot pieces)
4. Insert thermometer through cork - bulb at side-arm level
5. Connect condenser with rubber tubing
6. Attach water inlet at bottom, outlet at top
7. Place receiver flask at condenser outlet
8. Check all joints for air-tightness

Step 2: Start Heating

1. Turn on cooling water (gentle flow)
2. Start heating gradually
3. Never heat too rapidly (causes superheating)
4. Watch thermometer reading

Step 3: Collect Distillate

1. When temperature reaches b.p. of desired compound, vapors enter condenser
2. Condensed liquid drips into receiver
3. Temperature remains constant during distillation of one component
4. Collect distillate in receiver

Step 4: Monitor Temperature

Temperature pattern:

• Rises gradually
• Becomes constant at b.p. of first component (plateau)
• Rises again when first component is exhausted
• Becomes constant at b.p. of second component

Step 5: Stop Distillation

1. When desired component is collected, stop heating
2. Remove receiver flask
3. Turn off water supply
4. Let apparatus cool before dismantling

Memory Trick: “WATCHDOG”

Water inlet at bottom (Water Always Travels Counter-currently, Height Downward, Outflowग-goes up) Add boiling chips Thermometer at side-arm level Condenser water flow Heat gradually Distillate collection Observe temperature Gentle heating

Distillation Apparatus Diagram

Study the complete setup with all components labeled:

Interactive Demo: Visualize Distillation Process

See how liquid mixtures are separated based on boiling point differences.

Common Mistakes in Simple Distillation

Mistake 1: Thermometer Bulb in Liquid

Problem: Measures liquid temperature (always boiling point of mixture), not vapor temperature

Correct: Bulb should be at the level of side-arm opening to measure vapor temperature

Why it matters: Vapor temperature tells you WHICH component is distilling

---

Mistake 2: Reverse Water Flow in Condenser

Problem: Water enters from top, exits from bottom

• Poor cooling efficiency
• Condenser may not be fully filled with water
• Vapors may escape uncondensed

Correct: Water IN from bottom, OUT from top

• Ensures complete filling of jacket
• Countercurrent flow maximizes heat transfer
• Cold water meets hottest vapors

---

Mistake 3: Adding Boiling Chips to Hot Liquid

Problem: Causes violent, explosive boiling (bumping)

Correct: Add boiling chips BEFORE heating

If forgotten: Cool liquid completely, then add boiling chips

---

Mistake 4: Rapid Heating

Problem:

• Liquid superheats and bumps violently
• Vapor rushes too fast → condenser can’t cool properly
• Impure distillate (liquid droplets carry over)

Correct: Heat gently and steadily

• 1-2 drops of distillate per second is ideal rate

---

Mistake 5: Not Checking for Leaks

Problem: Vapors escape, reducing yield and creating safety hazard

Correct: Check all joints before heating

• Apply grease to ground glass joints
• Ensure tight connections

---

Mistake 6: Overfilling Distillation Flask

Problem: Liquid can splash into condenser during boiling

Correct: Fill maximum 50-60% of flask volume

Applications of Simple Distillation

1. Purifying water: Removing dissolved salts and impurities
2. Purifying organic solvents: Acetone, ethanol from water
3. Separating alcohol from water: In spirit industry (if b.p. difference >30°C)
4. Removing solid impurities: From liquids
5. Desalination: Converting seawater to drinking water

When NOT to Use Simple Distillation

❌ Components have close boiling points (<30°C difference) ❌ Heat-sensitive compounds (will decompose) ❌ Water-immiscible organic compounds (use steam distillation) ❌ Very high boiling point compounds (use vacuum distillation)

Alternative: Use fractional distillation for close boiling points

Fractional Distillation

Principle

Fractional distillation achieves repeated vaporization-condensation cycles to separate liquids with close boiling points (even 5-10°C difference).

Key Addition: Fractionating column between distillation flask and condenser

How is it Different from Simple Distillation?

Aspect	Simple Distillation	Fractional Distillation
Apparatus	No fractionating column	Has fractionating column
Separation	One vaporization-condensation	Multiple V-C cycles
B.P. Difference	>30°C	Can be 5-10°C
Efficiency	Low	High
Example	Water from salt solution	Petrol from crude oil

The Fractionating Column

Structure:

• Long vertical tube with packing material
• Packed with glass beads, metal pieces, or specially designed packing
• Provides large surface area

Packing Materials:

1. Glass beads: Provide surface for condensation
2. Broken porcelain pieces: Increase surface area
3. Metal helices (Raschig rings): Industrial use
4. Wire mesh: Laboratory use

Function:

When vapor rises through the column:

1. Lower part (hot): Vapor of both components rises
2. Middle part: Higher b.p. component condenses, drips back
3. Upper part (cool): Only lower b.p. component reaches top
4. Multiple cycles: Each drip-back is a purification cycle

Raoult’s Law and Ideal Solutions

For ideal liquid mixtures:

Raoult’s Law: Partial pressure of component A = (Mole fraction of A) × (Vapor pressure of pure A)

$$P_A = X_A \times P_A^0$$

Total Pressure: $P_{total} = P_A + P_B$

Composition of Vapor: Different from liquid composition!

• Vapor is enriched in more volatile (lower b.p.) component
• Each vaporization-condensation enriches further
• Fractionating column provides multiple such cycles

Theoretical Plates

Theoretical Plate: One complete vaporization-condensation cycle

Higher efficiency = More theoretical plates

A good fractionating column has 10-20 theoretical plates

Example: Separating benzene (b.p. 80°C) and toluene (b.p. 111°C)

• Simple distillation: Partial separation
• Fractional distillation (10 plates): Pure benzene, then pure toluene

Procedure for Fractional Distillation

Setup:

1. Assemble like simple distillation
2. Insert fractionating column between flask and condenser
3. Pack column with glass beads or wire mesh
4. Insulate column with asbestos or glass wool (prevents heat loss)

Operation:

1. Heat mixture slowly
2. First distillate: Component with lowest b.p. (first fraction)
3. Temperature rises when first component exhausted
4. Change receiver flask
5. Collect second component (second fraction)
6. Repeat for all components

Temperature Monitoring:

• Plateau 1: First component distills (constant temp = b.p. of component 1)
• Rise: Transition zone (discard this fraction - mixed)
• Plateau 2: Second component distills (constant temp = b.p. of component 2)
• Rise: Next transition

Fractional Distillation of Crude Oil

Industrial Application: Petroleum refining

Crude oil contains hundreds of hydrocarbons with different boiling points.

Fractionating Tower (distillation column):

• Huge tower (50-60 meters tall)
• Crude oil heated to ~400°C at bottom
• Temperature decreases with height

Fractions (bottom to top):

Fraction	B.P. Range	Carbon Atoms	Use
Residue	>350°C	>20	Asphalt, bitumen
Lubricating oil	300-350°C	20-30	Motor oil
Heavy gas oil	250-300°C	15-20	Diesel, heating oil
Light gas oil	200-250°C	12-15	Diesel fuel
Kerosene	150-200°C	10-12	Jet fuel, heating
Naphtha	70-150°C	6-10	Petrol (gasoline)
Petroleum gases	<40°C	1-4	LPG (butane, propane)

Process:

1. Crude oil heated in furnace
2. Vapors enter at bottom of tower
3. As vapors rise, temperature decreases
4. Each fraction condenses at its b.p. range
5. Collected at different levels

Memory Trick: Petroleum Fractions “PINK LANDS”

Petroleum gases (lightest, top) I - (skip) Naphtha (petrol) Kerosene (jet fuel) Light gas oil (diesel) A - (skip) No… wait: heavy gas oil Deep down: lubricating oil Solid at bottom: residue (asphalt)

Better Mnemonic - “People Never Keep Lighting Diesel Lamps Regularly”

• Petroleum gases
• Naphtha
• Kerosene
• Light gas oil
• Diesel (heavy gas oil)
• Lubricating oil
• Residue

Applications of Fractional Distillation

1. Petroleum refining: Separating crude oil into fractions
2. Air separation: Liquefied air → N₂, O₂, Ar (cryogenic distillation)
3. Alcohol industry: Purifying ethanol from fermented liquor
4. Chemical industry: Separating organic mixtures
5. Laboratory: Purifying solvents with close b.p.

Steam Distillation

Principle

Steam distillation is used for heat-sensitive, water-immiscible organic compounds that decompose at their normal boiling points.

Key Concept: When two immiscible liquids are heated together:

• They boil when sum of their vapor pressures = atmospheric pressure
• This happens at temperature lower than the b.p. of either pure liquid!

Mathematical Expression:

$$P_{total} = P_{water} + P_{organic}$$

When $P_{total} = P_{atmospheric}$, mixture boils

Since both contribute to total pressure, boiling occurs at temperature lower than 100°C (and much lower than b.p. of organic compound)!

Why Steam Distillation Works

Example: Aniline (b.p. 184°C) decomposes on heating

Using steam distillation:

• Mixture of aniline + water boils at ~98°C
• Below decomposition temperature of aniline
• Aniline distills over with steam
• Collected in receiver and separated

Dalton’s Law Application:

For water: $P_{H_2O} = P^0_{H_2O}$ (at distillation temp) For organic compound: $P_{org} = P^0_{org}$ (at distillation temp)

At boiling: $P_{H_2O} + P_{org} = 760$ mm Hg (1 atm)

Ratio of Components in Vapor:

$$\frac{n_{org}}{n_{H_2O}} = \frac{P_{org}}{P_{H_2O}}$$

Mass Ratio:

$$\frac{m_{org}}{m_{H_2O}} = \frac{P_{org} \times M_{org}}{P_{H_2O} \times M_{H_2O}}$$

Where M = molar mass

Apparatus for Steam Distillation

Setup 1: External Steam Method (preferred)

Components:

1. Steam generator: Produces steam continuously
2. Distillation flask: Contains organic compound + some water
3. Delivery tube: Brings steam into flask
4. Condenser: Cools vapor mixture
5. Receiver: Collects distillate (two layers)

Setup 2: Internal Steam Method

• Organic compound + excess water in flask
• Heat directly
• Steam generated in same flask
• Less efficient than external steam

Detailed Procedure

Step 1: Preparation

1. Take organic compound in distillation flask
2. Add some water (1/3rd volume)
3. Add boiling chips
4. Set up steam generator (if using external steam)

Step 2: Generate Steam

1. Heat water in steam generator
2. Pass steam through delivery tube into distillation flask
3. Steam bubbles through organic compound

Step 3: Distillation

1. Heat distillation flask gently (if needed)
2. Mixture boils at <100°C
3. Organic compound + water vapor rise together
4. Enter condenser and condense

Step 4: Collection

1. Collect distillate in receiver
2. Distillate has TWO layers:
   • Upper layer: Organic compound (if less dense than water)
   • Lower layer: Water
3. Continue until no more oily drops in distillate

Step 5: Separation

1. Transfer to separating funnel
2. Allow layers to separate
3. Drain lower layer (usually water)
4. Collect upper layer (organic compound)
5. Dry organic compound with anhydrous Na₂SO₄

Applications of Steam Distillation

1. Essential oil extraction:

   • Rose oil from rose petals
   • Eucalyptus oil from leaves
   • Clove oil from cloves
   • Perfume industry

2. Purifying organic compounds:

   • Aniline (b.p. 184°C, decomposes)
   • Nitrobenzene (b.p. 211°C)
   • Phenols
   • Aromatic carboxylic acids

3. Natural product chemistry:

   • Isolating compounds from plants
   • Turpentine from pine trees
   • Camphor from camphor tree

4. Industrial processes:

   • Purifying heat-sensitive materials
   • Fatty acid separation
   • Removing residual solvents

When to Use Steam Distillation

✓ Compound decomposes at its normal b.p. ✓ Compound is immiscible with water ✓ Compound has appreciable vapor pressure at 100°C ✓ Extracting essential oils from plant materials

Advantages of Steam Distillation

1. Low temperature operation: Prevents decomposition
2. No solvent needed: Uses water (cheap and safe)
3. Gentle process: Suitable for natural products
4. Effective for heat-sensitive compounds

Limitations

1. Only for water-immiscible compounds
2. Compound must have some volatility at steam temperature
3. Two-phase distillate requires separation
4. Not suitable for water-soluble compounds

Vacuum Distillation (Distillation Under Reduced Pressure)

Principle

Boiling point decreases with decrease in pressure.

Reason: At lower pressure, lower vapor pressure is needed to equal external pressure, so boiling occurs at lower temperature.

Application: Distilling compounds that:

• Have very high boiling points (>200°C)
• Decompose before reaching their b.p. at normal pressure
• Are heat-sensitive

Example: Glycerol

• B.P. at 760 mm Hg = 290°C (decomposes)
• B.P. at 12 mm Hg = 150°C (distills safely)

Apparatus

Similar to simple distillation but:

1. Closed system: All joints must be airtight
2. Vacuum pump: Connected to receiver
3. Claisen flask: Special flask with two necks (one for thermometer, one for distillation)
4. Capillary tube: Provides air bubbles to prevent bumping

Relationship: Pressure vs Boiling Point

Approximate Rule: For every halving of pressure, b.p. decreases by 10-15°C

Clausius-Clapeyron Equation:

$$\ln\left(\frac{P_2}{P_1}\right) = \frac{\Delta H_{vap}}{R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)$$

This allows calculating b.p. at different pressures.

Applications

1. Concentrating fruit juices: Without destroying vitamins
2. Purifying vitamins and antibiotics: Heat-sensitive pharmaceuticals
3. Petroleum industry: Vacuum distillation of crude oil residues
4. Laboratory: Distilling high b.p. organic compounds

Azeotropes and Their Distillation

What is an Azeotrope?

Azeotrope: A mixture of two or more liquids that has a constant boiling point and composition throughout distillation.

Problem: Cannot be separated by normal fractional distillation!

Types:

1. Minimum-boiling azeotrope (more common)

   • Boils at temperature lower than either pure component
   • Example: Ethanol-Water (95.6% ethanol, 4.4% water, b.p. 78.2°C)
   • Pure ethanol b.p. = 78.5°C, pure water b.p. = 100°C

2. Maximum-boiling azeotrope

   • Boils at temperature higher than either pure component
   • Example: HCl-Water (20.2% HCl, 79.8% water, b.p. 108.6°C)

Ethanol-Water Azeotrope

Problem: Cannot get 100% pure ethanol by simple distillation of water-ethanol mixture

Solution Methods:

1. Adding third component (Benzene - old method, now banned due to toxicity)
2. Using drying agents: CaO, molecular sieves
3. Azeotropic distillation: Add substance that forms different azeotrope
4. Pressure-swing distillation: Changing pressure changes azeotropic composition

Breaking Azeotropes

Method 1: Chemical Method

• Add CaO (quicklime) to 95% ethanol
• CaO + H₂O → Ca(OH)₂
• Water removed chemically
• Distill to get absolute ethanol (99.5%+)

Method 2: Using Molecular Sieves

• Porous materials that selectively adsorb water
• 3Å molecular sieves commonly used
• Physical process, no chemical reaction

Practice Problems

Level 1 - JEE Main (Basics)

Problem 1: Why should water enter the condenser from the bottom and leave from the top?

Solution:

Reason 1 - Complete Filling:

• Water entering from bottom ensures condenser jacket is completely filled with water
• If water enters from top, air pockets may form at bottom
• Incomplete filling → poor cooling

Reason 2 - Countercurrent Flow:

• Hottest vapors enter condenser at top
• If coldest water also enters from top, temperature difference is small
• Countercurrent: Hot vapor meets coldest water at top → maximum heat transfer
• As water flows up, it warms up and meets cooler vapors → still effective cooling

Reason 3 - Gravity:

• Water entering from bottom flows against gravity with pressure
• Ensures positive flow and no drainage issues

Answer: Water enters from bottom for complete filling and efficient countercurrent cooling.

---

Problem 2: Why are boiling chips added before distillation?

Solution:

Purpose of Boiling Chips: Provide nucleation sites for bubble formation during boiling

Without boiling chips:

• Liquid can superheat (heat above b.p. without boiling)
• Sudden formation of bubbles → violent bumping
• Dangerous! Can break apparatus

With boiling chips:

• Porous surface provides sites for gradual bubble formation
• Smooth, controlled boiling
• No superheating or bumping

Why add BEFORE heating: Adding to hot liquid causes instant vaporization → violent bumping!

Safety Rule: Always add boiling chips to cold liquid, never to hot liquid.

---

Problem 3: Differentiate between simple and fractional distillation.

Solution:

Feature	Simple Distillation	Fractional Distillation
Apparatus	Flask + condenser	Flask + fractionating column + condenser
Separation	One vaporization-condensation	Multiple V-C cycles
B.P. Difference	>30°C required	Even 5-10°C difference works
Efficiency	Low (partial separation)	High (complete separation)
Use	Liquid + solid OR large b.p. difference	Miscible liquids, close b.p.
Example	Water from salt solution	Benzene-toluene mixture
Principle	Simple volatility difference	Repeated partial distillation
Product purity	Moderate	High
Time required	Less	More (slower process)

Level 2 - JEE Main (Application)

Problem 4: During distillation of ethanol-water mixture, the thermometer reading remains constant at 78.2°C even after prolonged heating. Explain.

Solution:

This is an azeotrope formation!

Ethanol-Water Azeotrope:

• Composition: 95.6% ethanol + 4.4% water
• Boiling point: 78.2°C
• Constant b.p. and constant composition

Why constant temperature:

1. Azeotropic mixture has same composition in liquid and vapor phases
2. No preferential evaporation of one component
3. Behaves like a pure substance with fixed b.p.
4. Cannot be separated further by ordinary distillation

What happens:

• Initially, if mixture has <95.6% ethanol → water distills preferentially
• If mixture has >95.6% ethanol → ethanol distills preferentially
• Eventually, azeotropic composition reached → constant temperature

To get pure ethanol:

• Add CaO to remove water chemically
• Use molecular sieves
• Cannot use simple distillation

---

Problem 5: Calculate the ratio of aniline to water in distillate during steam distillation at 98°C. Given: Vapor pressure of aniline at 98°C = 42 mm Hg, vapor pressure of water at 98°C = 718 mm Hg. Molar mass: aniline = 93 g/mol, water = 18 g/mol.

Solution:

Given:

• $P_{aniline} = 42$ mm Hg
• $P_{water} = 718$ mm Hg
• $M_{aniline} = 93$ g/mol
• $M_{water} = 18$ g/mol

Step 1: Verify boiling condition

$$P_{total} = P_{aniline} + P_{water} = 42 + 718 = 760 \text{ mm Hg}$$

✓ Equals atmospheric pressure, so mixture boils at 98°C

Step 2: Mole ratio in vapor

$$\frac{n_{aniline}}{n_{water}} = \frac{P_{aniline}}{P_{water}} = \frac{42}{718} = \frac{1}{17.1}$$

Step 3: Mass ratio in distillate

$$\frac{m_{aniline}}{m_{water}} = \frac{n_{aniline} \times M_{aniline}}{n_{water} \times M_{water}}$$ $$= \frac{P_{aniline} \times M_{aniline}}{P_{water} \times M_{water}} = \frac{42 \times 93}{718 \times 18}$$ $$= \frac{3906}{12924} = 0.302$$

Answer:

• Mole ratio (aniline : water) = 1 : 17.1
• Mass ratio (aniline : water) = 0.302 : 1 or approximately 3 : 10

Interpretation: For every 10 g water, approximately 3 g aniline distills over.

---

Problem 6: Why is the thermometer bulb kept at the side-arm level and not inside the liquid during distillation?

Solution:

If bulb is in liquid:

• Measures liquid temperature
• Liquid temperature = boiling point of the mixture (not individual components)
• Remains approximately constant throughout (can’t distinguish components)
• No indication of which component is distilling

If bulb is at side-arm level (correct position):

• Measures vapor temperature
• When component A distills → vapor temp = b.p. of A
• When component A exhausted → temp rises
• When component B distills → vapor temp = b.p. of B
• Clear indication of which component is being collected

Example: Separating acetone (b.p. 56°C) and ethanol (b.p. 78°C)

• Correct position: Temp shows 56°C (acetone distilling) → rises → 78°C (ethanol distilling)
• Wrong position (in liquid): Temp shows ~65-70°C throughout (useless information)

Purpose of thermometer: Monitor and identify which fraction is distilling

Answer: Thermometer at side-arm level measures vapor temperature, which indicates which component is currently distilling. Bulb in liquid would only show mixture b.p., giving no useful information.

Level 3 - JEE Advanced (Conceptual & Numerical)

Problem 7: A mixture of two liquids A (b.p. 350 K) and B (b.p. 370 K) forms an ideal solution. At 360 K, vapor pressure of pure A is 400 mm Hg and pure B is 200 mm Hg. If the mixture contains equal moles of A and B, calculate: (a) The composition of vapor in equilibrium with liquid, (b) Whether simple or fractional distillation would be more effective.

Solution:

Given:

• $T = 360$ K
• $X_A = X_B = 0.5$ (equal moles)
• $P_A^0 = 400$ mm Hg
• $P_B^0 = 200$ mm Hg

(a) Composition of vapor:

Step 1: Calculate partial pressures (Raoult’s Law)

$$P_A = X_A \times P_A^0 = 0.5 \times 400 = 200 \text{ mm Hg}$$ $$P_B = X_B \times P_B^0 = 0.5 \times 200 = 100 \text{ mm Hg}$$

Step 2: Total pressure

$$P_{total} = P_A + P_B = 200 + 100 = 300 \text{ mm Hg}$$

Step 3: Mole fraction in vapor

$$Y_A = \frac{P_A}{P_{total}} = \frac{200}{300} = 0.667$$ $$Y_B = \frac{P_B}{P_{total}} = \frac{100}{300} = 0.333$$

Answer (a): Vapor contains 66.7% A and 33.3% B

Enrichment: Vapor is enriched in more volatile component (A)

(b) Simple vs Fractional Distillation:

B.P. difference = 370 - 350 = 20 K (or 20°C)

Analysis:

• Difference is <30°C → close boiling points
• Vapor enrichment factor = 0.667/0.5 = 1.33 (modest enrichment)
• Single distillation gives only partial separation

Answer (b): Fractional distillation would be more effective

Reasoning:

• Simple distillation: Partial separation (first fraction enriched in A, but not pure)
• Fractional distillation: Multiple stages → can achieve nearly complete separation
• Each theoretical plate enriches A further
• 5-10 plates would give pure A, then pure B

---

Problem 8: During vacuum distillation, a compound that normally boils at 250°C at 760 mm Hg is distilled at 100 mm Hg pressure. Estimate the new boiling point. (Given: ΔHvap = 40 kJ/mol)

Solution:

Given:

• $T_1 = 250°C = 523$ K at $P_1 = 760$ mm Hg
• $P_2 = 100$ mm Hg
• $\Delta H_{vap} = 40$ kJ/mol $= 40000$ J/mol
• $R = 8.314$ J/(mol·K)

Using Clausius-Clapeyron Equation:

$$\ln\left(\frac{P_2}{P_1}\right) = \frac{\Delta H_{vap}}{R}\left(\frac{1}{T_1} - \frac{1}{T_2}\right)$$

Step 1: Calculate left side

$$\ln\left(\frac{100}{760}\right) = \ln(0.1316) = -2.028$$

Step 2: Calculate coefficient

$$\frac{\Delta H_{vap}}{R} = \frac{40000}{8.314} = 4811 \text{ K}$$

Step 3: Substitute in equation

$$-2.028 = 4811\left(\frac{1}{523} - \frac{1}{T_2}\right)$$ $$-2.028 = 4811 \times \frac{1}{523} - \frac{4811}{T_2}$$ $$-2.028 = 9.199 - \frac{4811}{T_2}$$ $$\frac{4811}{T_2} = 9.199 + 2.028 = 11.227$$ $$T_2 = \frac{4811}{11.227} = 428.5 \text{ K} = 155.5°C$$

Answer: The compound boils at approximately 156°C at 100 mm Hg pressure.

Verification: B.P. decreased by (250 - 156) = 94°C when pressure reduced by factor of 7.6

Practical significance:

• Can distill at 156°C instead of 250°C
• Prevents thermal decomposition
• Safer and more efficient

---

Problem 9: Explain why steam distillation of aniline works even though aniline is only slightly soluble in water. Wouldn’t this low solubility prevent effective distillation?

Solution:

This is a common misconception! Let’s clarify:

Key Principle: Steam distillation does NOT depend on solubility in water!

What matters: Vapor pressure, not solubility

Explanation:

1. Immiscibility is actually REQUIRED:

• Aniline and water are immiscible (don’t mix)
• They form two separate liquid phases
• Each phase contributes its own vapor pressure independently
• Total vapor pressure = sum of individual vapor pressures (Dalton’s Law)

2. Vapor pressure is independent of quantity:

• Whether 1 drop or 100 mL of aniline, its vapor pressure at given temperature is same
• Vapor pressure depends only on temperature and nature of liquid
• NOT on amount present

3. Why it works:

• Both liquids evaporate independently
• Vapor pressures add up
• When total = atmospheric pressure → mixture boils

4. Mathematical proof:

At 98°C:

• $P_{water}$ ≈ 718 mm Hg (high, because close to 100°C)
• $P_{aniline}$ ≈ 42 mm Hg (significant, even though b.p. is 184°C)
• $P_{total} = 760$ mm Hg → boiling occurs

5. Solubility vs Miscibility:

• Solubility: Amount that dissolves (doesn’t matter here)
• Miscibility: Mixing at molecular level (should be low for steam distillation)
• Volatility: Tendency to evaporate (this is what matters!)

Answer: Steam distillation works because of vapor pressure, not solubility. Immiscible liquids (low solubility) each exert their own vapor pressure independently. The mixture boils when sum of vapor pressures equals atmospheric pressure. Low solubility is actually a requirement, not a hindrance!

Analogy: Think of two separate pots of water boiling side by side - each produces steam independently. In steam distillation, two immiscible liquids in same flask act like this!

---

Problem 10: A fractionating column has 8 theoretical plates. If the vapor from a mixture of A (more volatile) and B has ratio A:B = 55:45 and each theoretical plate provides enrichment factor of 1.2 for component A, what will be the composition after passing through the column?

Solution:

Given:

• Initial composition: A:B = 55:45 (or XA = 0.55, XB = 0.45)
• Number of plates = 8
• Enrichment factor (α) = 1.2 per plate

Enrichment factor: Ratio of (A/B in vapor) to (A/B in liquid) after one stage

Step 1: Initial ratio

$$\left(\frac{A}{B}\right)_0 = \frac{55}{45} = 1.222$$

Step 2: After each plate, ratio multiplies by enrichment factor

After 1 plate:

$$\left(\frac{A}{B}\right)_1 = 1.222 \times 1.2 = 1.467$$

After 2 plates:

$$\left(\frac{A}{B}\right)_2 = 1.467 \times 1.2 = 1.760$$

General formula after n plates:

$$\left(\frac{A}{B}\right)_n = \left(\frac{A}{B}\right)_0 \times \alpha^n$$

Step 3: After 8 plates

$$\left(\frac{A}{B}\right)_8 = 1.222 \times (1.2)^8$$

Calculate $(1.2)^8$:

$$1.2^8 = 4.300$$ $$\left(\frac{A}{B}\right)_8 = 1.222 \times 4.300 = 5.255$$

Step 4: Convert to percentages

If A/B = 5.255, then A = 5.255B

Total = A + B = 5.255B + B = 6.255B

$$X_A = \frac{5.255B}{6.255B} = 0.840 = 84.0\%$$ $$X_B = \frac{B}{6.255B} = 0.160 = 16.0\%$$

Answer: After 8 theoretical plates:

• Component A: 84.0%
• Component B: 16.0%

Interpretation:

• Started with 55% A
• After 8 plates: 84% A
• Significant enrichment in more volatile component
• More plates → higher purity (approaching 100% A)

Practical application: This shows why tall fractionating columns (more theoretical plates) give better separation!

Cross-Links to Related Topics

Organic Chemistry Connections

1. Crystallization: Compare solid purification (crystallization) vs liquid purification (distillation)

2. Chromatography: Modern analytical technique; compare separation principles

3. Detection of Elements: After distillation, analyze pure compound for elements

4. Organic Preparations: Many synthesis procedures end with distillation to purify product

5. Hydrocarbons: Petroleum distillation separates hydrocarbon mixtures

Physical Chemistry Connections

1. Solutions: Raoult’s Law, vapor pressure, ideal and non-ideal solutions

2. Chemical Thermodynamics: Enthalpy of vaporization, phase transitions, Clausius-Clapeyron equation

3. Equilibrium: Liquid-vapor equilibrium, phase diagrams

4. States of Matter: Phase transitions, vapor pressure

Inorganic Chemistry Connections

1. Practical Chemistry: Lab techniques for salt purification

2. Qualitative Analysis: Preparing pure reagents for analysis

Industrial Applications

1. Hydrocarbons - Petroleum Refining: Fractional distillation of crude oil

2. Oxygen Compounds: Alcohol purification, ether preparation

3. Biomolecules: Essential oil extraction, vitamin isolation

Key Takeaways

1. Simple Distillation: For b.p. difference >30°C; liquid + solid impurities; one vaporization-condensation cycle

2. Fractional Distillation: For close b.p. (even 5-10°C); uses fractionating column; multiple V-C cycles; petroleum refining

3. Steam Distillation: For heat-sensitive, water-immiscible compounds; works on vapor pressure principle; essential oils

4. Vacuum Distillation: For high b.p. compounds; reduces b.p. by reducing pressure; prevents decomposition

5. Azeotropes: Constant boiling mixtures; cannot be separated by normal distillation; ethanol-water (95.6% azeotrope)

6. Thermometer position: At side-arm level to measure vapor temperature (NOT in liquid)

7. Condenser water: Enter from bottom, exit from top (countercurrent flow)

8. Boiling chips: Add before heating; prevent superheating and bumping

9. Safety: Water bath for flammable solvents; never direct flame for organic liquids

10. Raoult’s Law: Governs ideal solution behavior in fractional distillation

Quick Revision Points

✓ Simple: Large b.p. difference, single stage, liquid-solid separation ✓ Fractional: Close b.p., fractionating column, petroleum refining ✓ Steam: Heat-sensitive + water-immiscible, vapor pressure principle ✓ Vacuum: High b.p. compounds, reduced pressure → lower b.p. ✓ Thermometer: Side-arm level (vapor temp, not liquid) ✓ Water flow: Bottom → Top (countercurrent) ✓ Boiling chips: Before heating (prevent bumping) ✓ Azeotrope: Constant b.p. mixture (e.g., 95.6% ethanol-water) ✓ Safety: Water bath for organics, no direct flame ✓ Applications: Petroleum (fractional), perfumes (steam), water purification (simple)

Master these three distillation types, and you’ve mastered liquid purification for JEE and beyond!

---

Interactive Demo: Visualize Distillation Process

Explore the thermodynamic principles behind distillation with interactive temperature-composition diagrams.