Mixture: Physical combination of two or more substances

• Components retain their chemical identities
• Can be separated by physical methods
• No chemical bonds formed between components

Homogeneous Mixtures (Solutions)

Definition: Uniform composition throughout

• Same properties in all parts
• Components not visibly distinguishable
• Molecularly mixed

Examples:

• Air (N₂, O₂, Ar mixed)
• Salt water (NaCl dissolved in H₂O)
• Brass (Cu and Zn alloy)
• Vinegar (acetic acid in water)
• Gasoline (mixture of hydrocarbons)

Particle size: Molecular level (< 1 nm)

Properties:

• Clear (may be colored)
• Does not settle
• Cannot be separated by filtration
• Passes through filter paper

Heterogeneous Mixtures

Definition: Non-uniform composition

• Different properties in different parts
• Components visibly distinguishable
• Not molecularly mixed

Types:

1. Suspensions

• Large particles (> 1000 nm)
• Particles settle over time
• Cloudy/opaque appearance
• Can be separated by filtration

Examples:

• Muddy water
• Orange juice with pulp
• Sand in water
• Blood (cells in plasma)

2. Colloids

• Intermediate particle size (1-1000 nm)
• Particles do NOT settle
• Cloudy but stable
• Cannot be separated by filtration
• Show Tyndall effect (scatter light)

Examples:

• Milk (fat droplets in water)
• Fog (water droplets in air)
• Smoke (solid particles in air)
• Gelatin
• Mayonnaise

Comparison Table

Tyndall Effect

Definition: Scattering of light by colloidal particles

Observation:

• Beam of light passes through solution: invisible path
• Beam of light passes through colloid: visible path (scattering)

Example:

• Flashlight beam in fog (colloid) → visible
• Flashlight beam in clear air (solution of gases) → invisible

Why it occurs:

• Colloidal particles large enough to scatter light
• Solution particles too small to scatter light

Test for colloid:

• Shine light through mixture
• If beam visible → colloid
• If beam invisible → solution

Separation Techniques

General principle: Use differences in physical properties to separate components

1. Filtration

Separates: Solid from liquid in suspension

Based on: Particle size

Method:

• Pour mixture through filter paper
• Solid particles trapped (residue)
• Liquid passes through (filtrate)

Examples:

• Separate sand from water
• Coffee filter (grounds vs. liquid)
• Laboratory: precipitate from solution

Limitations:

• Only works for large particles (suspensions)
• Cannot separate solutions or colloids

2. Distillation

Separates: Liquids with different boiling points

Based on: Boiling point differences

Simple Distillation:

Setup:

1. Heat mixture in flask
2. Vapor rises and enters condenser
3. Condenser cools vapor back to liquid
4. Collect purified liquid (distillate)

Process:

• Component with lower BP evaporates first
• Vapor travels to condenser
• Condenses and is collected
• Component with higher BP remains in flask

Best for: Large BP differences (> 25°C)

Examples:

• Purify water (remove salt)
• Separate ethanol from water
• Crude oil refining

Fractional Distillation:

For: Liquids with similar boiling points

Difference from simple:

• Uses fractionating column
• Multiple vaporization-condensation cycles
• Better separation

Examples:

• Separate crude oil into fractions (gasoline, kerosene, diesel)
• Purify ethanol-water mixtures

3. Chromatography

Separates: Components based on different affinities for mobile and stationary phases

Based on:

• Polarity differences
• Solubility differences
• Adsorption to stationary phase

Two phases:

Stationary phase: Fixed material (paper, silica gel, column packing)

Mobile phase: Moving solvent that carries mixture

Types:

Paper Chromatography

Setup:

• Spot of mixture placed on paper (stationary phase)
• Paper placed in solvent (mobile phase)
• Solvent travels up paper by capillary action
• Components separate as solvent moves

How it works:

• More polar components: Stick to polar paper, move slowly
• Less polar components: Dissolve better in solvent, move faster

Result: Components separate into spots at different heights

Rf value (Retention factor):

$R_f = \frac{\text{distance traveled by component}}{\text{distance traveled by solvent}}$

Range: 0 < Rf < 1

Uses:

• Each component has characteristic Rf
• Identify unknown substances
• Check purity

Example:

• Separate pigments in black ink
• Separate amino acids
• Analyze dyes

Thin Layer Chromatography (TLC)

Similar to paper chromatography:

• Stationary phase: Silica gel or alumina on glass/plastic plate
• Mobile phase: Organic solvent

Advantages over paper:

• Faster
• Better resolution
• More reproducible Rf values

Column Chromatography

Setup:

• Column packed with stationary phase (silica, alumina)
• Pour mixture at top
• Add mobile phase (solvent)
• Collect fractions as they elute

How it works:

• Components with weak affinity for stationary phase → elute first (faster)
• Components with strong affinity → elute last (slower)

Uses:

• Purify compounds
• Separate mixtures
• Isolate products from reactions

Large scale: Can purify grams to kilograms

Gas Chromatography (GC)

For: Volatile compounds

Mobile phase: Inert gas (He, N₂) - carrier gas

Stationary phase: Liquid coating inside column

Process:

1. Inject sample (vaporizes)
2. Carrier gas pushes through column
3. Components separate based on BP and polarity
4. Detector measures components as they exit

Output: Chromatogram (peaks at different retention times)

Uses:

• Analyze volatile organic compounds
• Forensics (blood alcohol, drugs)
• Environmental testing (pollutants)

Advantages:

• Very sensitive
• Can separate complex mixtures
• Quantitative (peak area ∝ amount)

4. Crystallization

Separates: Solid from impurities

Based on: Solubility differences with temperature

Process:

1. Dissolve impure solid in hot solvent
2. Cool solution slowly
3. Pure crystals form (impurities stay dissolved)
4. Filter to collect pure crystals

Why it works:

• Desired compound: High solubility when hot, low solubility when cold
• Impurities: Remain dissolved at all temperatures (if low concentration)

Example:

• Purify sugar
• Purify aspirin
• Make rock candy

5. Extraction

Separates: Components based on solubility in different solvents

Based on: "Like dissolves like" principle

Liquid-Liquid Extraction:

Setup:

• Two immiscible solvents (don't mix)
• Example: Water and diethyl ether

Process:

1. Mixture dissolved in solvent 1
2. Add immiscible solvent 2
3. Shake (components partition between solvents)
4. Separate layers in separatory funnel
5. Component soluble in solvent 2 extracted

Example:

• Extract caffeine from tea (polar) into CH₂Cl₂ (nonpolar)
• Nonpolar caffeine prefers organic layer
• Polar tea components stay in water

Distribution coefficient (Kd):

$K_d = \frac{[\text{compound in organic}]}{[\text{compound in water}]}$

Large Kd: Compound prefers organic layer

6. Evaporation

Separates: Dissolved solid from liquid

Based on: Volatility difference (liquid evaporates, solid doesn't)

Process:

• Heat solution
• Liquid evaporates
• Solid remains

Example:

• Recover salt from seawater
• Concentrate solution

Difference from distillation:

• Evaporation: Discard liquid, keep solid
• Distillation: Keep liquid (purify it)

7. Decanting

Separates: Liquid from settled solid

Process:

• Allow solid to settle
• Carefully pour off liquid
• Solid remains in container

Simple but limited:

• Cannot achieve complete separation
• Some solid particles may remain suspended

Example:

• Pour wine from bottle (leave sediment)
• Separate sand from water after settling

Summary of Separation Techniques

Chromatography in Detail

Interpreting Chromatograms

Paper/TLC chromatography:

Rf calculation:

• Measure distance from origin to spot center
• Measure distance from origin to solvent front
• Calculate ratio

Factors affecting Rf:

1. Polarity of compound:

   • More polar → lower Rf (sticks to polar paper)
   • Less polar → higher Rf (travels with solvent)

2. Polarity of solvent:

   • More polar solvent → all Rf values increase
   • Less polar solvent → all Rf values decrease

3. Temperature: Affects solvent properties

Using Rf for identification:

• Run known standards alongside unknown
• Compare Rf values
• Match to identify unknown

Limitations:

• Rf can vary with conditions
• Two compounds may have same Rf (need additional tests)

Column Chromatography

Elution order:

• Least polar compounds elute first
• Most polar compounds elute last

Why?

• Polar stationary phase (silica, alumina)
• Polar compounds stick more → retained longer
• Nonpolar compounds don't stick → elute quickly

Choosing solvent:

• Start with nonpolar solvent (elute nonpolar compounds)
• Gradually increase polarity (elute progressively more polar compounds)
• Common sequence: Hexane → CH₂Cl₂ → Ethyl acetate → Methanol

Practical Applications

Water Purification

Multiple techniques combined:

1. Filtration: Remove large particles (sand, debris)
2. Coagulation: Add chemicals to clump small particles
3. Sedimentation: Allow particles to settle
4. Filtration again: Remove coagulated particles
5. Disinfection: Add chlorine or UV to kill bacteria
6. Distillation (optional): Remove dissolved salts (desalination)

Petroleum Refining

Fractional distillation of crude oil:

Process:

• Heat crude oil in fractionating tower
• Components vaporize at different temperatures
• Collect fractions at different heights

Forensic Chemistry

Gas chromatography applications:

• Blood alcohol content (BAC)
• Drug testing
• Arson investigation (identify accelerants)
• Explosive residue analysis

Paper chromatography:

• Ink analysis (questioned documents)
• Dye identification

Key Concepts Summary

1. Mixtures vs. Pure Substances:

• Mixtures: Variable composition, can be separated physically
• Pure substances: Fixed composition, definite properties

2. Physical vs. Chemical Separation:

• Physical: Based on physical properties (BP, polarity, size)
  • No new substances formed
  • Examples: All techniques in this topic
• Chemical: Breaking/forming bonds
  • New substances formed
  • Example: Electrolysis of water

3. Choosing Separation Method:

• Consider physical property differences
• Solid/liquid → filtration, evaporation, crystallization
• Liquid/liquid → distillation, extraction
• Components in solution → chromatography
• Multiple methods often needed

4. Polarity is Key:

• Chromatography: Polar vs. nonpolar separation
• Extraction: "Like dissolves like"
• Solubility: Affects crystallization

5. Scale Matters:

• Lab scale: Paper chromatography, simple distillation
• Industrial scale: Fractional distillation, column chromatography
• Analytical: TLC, GC (small amounts for analysis)
• Preparative: Column chromatography (purify products)

Type: Two miscible liquids (form solution)

• Cannot use filtration (both dissolved)
• Cannot use chromatography easily (not good for large amounts)
• Cannot use crystallization (both liquids)

Key property difference: Boiling points

• BP difference = 100°C - 78°C = 22°C
• Moderate difference

Best technique: DISTILLATION

Why distillation?

1. Liquids have different BPs

   • Component with lower BP vaporizes first
   • Can be selectively evaporated and condensed

2. Components are miscible

   • Cannot separate by density/filtration
   • Need to use volatility difference

3. Want to purify/collect both components

   • Distillation allows collection of volatile component
   • Leaves less volatile component in flask

Answer (a):

$\boxed{\text{Distillation (specifically simple or fractional distillation)}}$

Reason: Separates miscible liquids based on boiling point differences. Ethanol (lower BP) vaporizes first, travels to condenser, and is collected. Water (higher BP) remains in flask.

Part (b): Which component distills first?

Principle: Component with lower boiling point vaporizes first

Comparison:

• Ethanol BP: 78°C (lower)
• Water BP: 100°C (higher)

During distillation:

Step 1: Heat mixture

• Temperature rises toward lowest BP
• At ~78°C, ethanol begins to boil

Step 2: Vaporization

• Ethanol vaporizes preferentially
• Vapor is enriched in ethanol (more ethanol than water in vapor)

Step 3: Condensation

• Ethanol vapor enters condenser
• Cools back to liquid
• Collected as distillate

Answer (b):

$\boxed{\text{Ethanol distills over first}}$

Reason: Lower boiling point (78°C < 100°C). Component with lower BP is more volatile and evaporates first.

Part (c): Why can't 100% pure ethanol be obtained?

Key concept: Ethanol-water forms an AZEOTROPE

Azeotrope: Mixture that boils at constant temperature with constant composition

• Vapor has same composition as liquid
• Cannot be separated further by simple distillation

Ethanol-water azeotrope:

• Composition: 95.6% ethanol, 4.4% water (by volume)
• Boiling point: 78.1°C
• This is as pure as you can get by normal distillation

Why azeotrope forms:

Intermolecular forces:

• Ethanol and water form hydrogen bonds with each other
• Ethanol: O-H can H-bond
• Water: O-H can H-bond

H-bonding between ethanol and water:

$C2H5OH ··· H-O-H$

These interactions:

• Stabilize the mixture
• Change vapor pressure behavior
• Create composition where liquid and vapor have same ratio

Distillation behavior:

Starting with dilute ethanol (e.g., 10% ethanol):

1. Heat to ~78°C
2. Vapor enriched in ethanol (more volatile)
3. Collect distillate (higher % ethanol)
4. Repeat: gradually increase ethanol concentration

But at 95.6% ethanol:

• Reach azeotrope
• Vapor composition = liquid composition
• Cannot increase ethanol % further
• No matter how many times you distill!

Answer (c):

\boxed{\text{Ethanol-water forms an azeotrope at 95.6% ethanol}}

Explanation: At this composition, liquid and vapor have identical composition. Further distillation cannot increase purity beyond 95.6%. Strong hydrogen bonding between ethanol and water creates this constant-boiling mixture.

Part (d): Techniques for better separation

To get > 95.6% ethanol (absolute ethanol), use:

Method 1: Add drying agent

• Add anhydrous calcium oxide (CaO) or molecular sieves
• Drying agent chemically reacts with or absorbs water
• Removes water from azeotrope
• Then distill to get pure ethanol

Chemical reaction:

$CaO + H2O \rightarrow Ca(OH)2$

Method 2: Azeotropic distillation

• Add third component (e.g., benzene, cyclohexane)
• Forms new azeotrope with water
• This azeotrope boils at different temperature
• Can distill off water with third component
• Leaves pure ethanol

Method 3: Fractional distillation

• Won't break azeotrope, but...
• Better separation before reaching azeotrope
• Uses fractionating column
• Multiple vaporization-condensation cycles
• Gets to 95.6% more efficiently than simple distillation

For initial question (a) - should specify:

If BP difference is moderate (20-25°C):

• Fractional distillation preferred over simple distillation
• Better separation
• Higher purity

Answer (d):

$\boxed{\text{Fractional distillation (for better separation to azeotrope)}}$ \boxed{\text{+ Drying agent (to break azeotrope and get 100% ethanol)}}

Fractional distillation advantages:

• Fractionating column provides multiple theoretical distillations
• Better separation than simple distillation
• More efficient path to 95.6% purity

To get absolute ethanol (100%):

• Must use chemical method (drying agent)
• Or azeotropic distillation with third component
• Physical distillation alone cannot break azeotrope

Summary comparison:

Key concepts:

1. Choose separation by property difference

   • Different BPs → distillation
   • Different polarities → chromatography/extraction
   • Different sizes → filtration

2. Lower BP → distills first

   • More volatile
   • Evaporates at lower temperature

3. Azeotropes limit distillation

   • Constant-boiling mixtures
   • Same composition in liquid and vapor
   • Cannot be separated further by distillation alone

4. Fractional > Simple distillation

   • Better separation
   • Multiple stages
   • Use when BP difference < ~25°C

5. Break azeotrope chemically

   • Add drying agent
   • Add third component
   • Change IMFs in mixture

SEPARATION SCHEME:

STEP 1: SUBLIMATION

(i) Technique: Sublimation

(ii) Property exploited: Naphthalene sublimes (solid → gas directly) at 80°C; sand and salt do not sublime

(iii) What is separated: Naphthalene (collected) from sand + salt (remain)

Procedure:

1. Place mixture in evaporating dish
2. Cover with inverted funnel with cold water flask on top
3. Heat gently to ~80°C
4. Naphthalene sublimes (becomes vapor)
5. Vapor contacts cold surface, deposits as solid crystals
6. Collect pure naphthalene from cold surface

Result after Step 1:

• ✓ Pure naphthalene (collected on cold surface)
• Sand + salt mixture (remains in dish)

STEP 2: DISSOLUTION

(i) Technique: Dissolution / Extraction with water

(ii) Property exploited: Salt is soluble in water; sand is not

(iii) What is separated: Salt (dissolves) from sand (does not dissolve)

Procedure:

1. Add distilled water to sand + salt mixture
2. Stir thoroughly to dissolve all salt
3. Result: Aqueous solution of NaCl + undissolved sand

Result after Step 2:

• Aqueous solution containing dissolved NaCl
• Solid sand (undissolved)

STEP 3: FILTRATION

(i) Technique: Filtration

(ii) Property exploited: Particle size - sand particles too large to pass through filter

(iii) What is separated: Sand (trapped) from salt solution (passes through)

Procedure:

1. Pour mixture through filter paper in funnel
2. Sand trapped on filter paper (residue)
3. Salt solution passes through (filtrate)

Result after Step 3:

• Wet sand on filter paper
• Salt solution collected as filtrate

STEP 4: DRYING SAND

(i) Technique: Drying / Evaporation

(ii) Property exploited: Water evaporates; sand does not

(iii) What is separated: Water (evaporates) from sand (remains)

Procedure:

1. Leave sand on filter paper in warm place
2. Or place in drying oven at low temperature (100-110°C)
3. Water evaporates

Result after Step 4:

• ✓ Pure dry sand

STEP 5: EVAPORATION / CRYSTALLIZATION

(i) Technique: Evaporation (or crystallization for better purity)

(ii) Property exploited: Water evaporates; salt does not (non-volatile)

(iii) What is separated: Water (evaporates) from salt (remains)

Procedure:

Method A - Evaporation (faster):

1. Heat salt solution in evaporating dish
2. Water evaporates
3. Salt crystals remain

Method B - Crystallization (purer):

1. Heat salt solution to near boiling
2. Evaporate some water (concentrated solution)
3. Cool slowly
4. Salt crystals form
5. Filter to collect crystals
6. Dry crystals

Result after Step 5:

• ✓ Pure dry salt (NaCl)

COMPLETE FLOW CHART:

Step 1: SUBLIMATION (80°C)

• Input: Sand + Salt + Naphthalene
• Property: Naphthalene sublimes
• Output: NAPHTHALENE (pure) + remaining (Sand + Salt)

Step 2: ADD WATER

• Input: Sand + Salt
• Property: Salt dissolves, sand doesn't
• Output: Sand + Salt solution

Step 3: FILTRATION

• Input: Sand + Salt solution
• Property: Particle size
• Output: SAND (wet, on filter) + Salt solution (filtrate)

Step 4: DRY SAND

• Input: Wet sand
• Property: Water evaporates
• Output: SAND (pure, dry)

Step 5: EVAPORATION

• Input: Salt solution
• Property: Water evaporates
• Output: SALT (pure, dry)

ALTERNATIVE SCHEME (Different order):

Could also start with dissolution:

1. MIXTURE + ADD WATER
2. FILTRATION gives: Salt solution + (Sand + Naphthalene)
3. EVAPORATE salt solution → SALT
4. SUBLIMATION of (Sand + Naphthalene) → NAPHTHALENE

But first scheme is better because:

• Sublimation first removes naphthalene without solvents
• Cleaner separation
• Less contamination risk

DETAILED SUMMARY TABLE:

KEY CONSIDERATIONS:

Order matters:

• Must do sublimation before adding water
• If you add water first, naphthalene might stick to sand (harder to separate)
• Sublimation is cleanest first step

Temperature control:

• Sublimation: ~80°C (not too high or salt might decompose)
• Drying sand: 100-110°C (above water BP)
• Evaporating salt solution: Can heat to boiling

Purity checks:

• Naphthalene: Check melting point (should be 80°C)
• Sand: Rinse with water again to ensure no salt
• Salt: Could recrystallize for higher purity

Common mistakes to avoid: ❌ Filtering before dissolving salt (would trap salt + sand together) ❌ Adding water before sublimation (naphthalene hard to remove when wet) ❌ Not drying sand properly (would have salt contamination from residual water)

ALTERNATIVE TECHNIQUES (if available):

Instead of sublimation, could use:

• Solvent extraction with nonpolar solvent
  • Add hexane or toluene
  • Naphthalene dissolves (nonpolar)
  • Sand and salt don't dissolve
  • Filter, evaporate hexane → naphthalene

Advantage of sublimation over extraction:

• No organic solvents needed
• Environmentally friendlier
• Simpler procedure
• Direct collection of pure solid

FINAL ANSWER:

Complete separation scheme:

1. Sublimation (80°C) → isolate naphthalene
2. Add water → dissolve salt
3. Filtration → separate sand
4. Dry sand (100°C) → pure sand
5. Evaporate water → pure salt

All three components isolated in pure form!

$\boxed{\text{Naphthalene (sublimation) → Sand (filtration + drying) → Salt (evaporation)}}$

Key concepts applied:

• Multiple techniques needed for complex mixture
• Order of operations critical
• Choose technique based on property differences
• Physical properties enable separation (sublimation, solubility, size)