Master Q&A on Distillation and Liquid-Liquid Extraction

Distillation

A separation process that relies on differences in boiling points of components in a mixture.

Boiling Point

The temperature at which a liquid's vapor pressure equals the external pressure.

Vapor Pressure

The pressure exerted by a vapor in equilibrium with its liquid or solid form at a given temperature.

Simple Distillation

A distillation method that uses one vaporization-condensation cycle.

Fractional Distillation

A distillation method that uses a fractionating column for multiple vaporization-condensation cycles.

Fractionating Column

A column that increases surface area to allow more condensation and vaporization, improving separation.

Theoretical Plates

Represent individual vaporization-condensation cycles; more plates mean better separation.

Distillation Rate

The speed at which distillation occurs; must be slow and controlled for proper separation.

Vapor-Liquid Equilibrium

A state where liquid and vapor compositions remain constant at a given temperature.

Azeotrope

A mixture that has a constant boiling point and composition, making it unseparable by standard distillation.

Distillate

The purified liquid collected from the distillation process.

Boiling Chips

Small pieces added to a liquid to promote smooth boiling and prevent bumping.

Insulation of Fractionating Column

Maintains a temperature gradient and avoids heat loss during distillation.

Vapor Composition

The vapor is richer in the more volatile component during distillation.

Thermometer Placement

Placed just below the sidearm to measure the temperature of the vapor entering the condenser.

Cooling of Distillate Receiver

Keeps the collected liquid from evaporating.

Condenser Water Flow

Water should enter at the bottom and exit at the top to ensure efficient cooling.

Dry Distillation System

A system with no water contamination or leaks.

Switching Receiving Flasks

Occurs when the boiling temperature changes significantly.

Safety Precautions in Distillation

Includes wearing goggles, not distilling to dryness, and ensuring proper airflow and heating rate.

Temperature-Volume Graph

A sloped graph indicates poor separation or fast heating.

Flooding in Column

Caused by excessive heating or overpacking.

Distillate Smell

A distillate smelling like both components suggests incomplete separation.

Co-distillation

Components co-distilled due to insufficient separation.

Condenser water flow failure

Vapors may escape; poor condensation occurs.

Bumping causing liquid to reach condenser

It contaminates the distillate.

Simple distillation unsuitability for acetone-ethanol

Their boiling points are too close; use fractional distillation.

Improving separation of hexane and toluene

Use a longer, better-packed column and slow heating.

Testing distillate purity

Use boiling point measurement, gas chromatography, or refractive index.

Plateau in distillation graph

A component with a constant boiling point is distilling.

Estimating original mixture composition from graph

Based on the volume collected at each boiling point plateau.

Sloped curve in distillation graph

Incomplete separation or azeotrope formation.

Multiple plateaus in distillation graph

Good separation of multiple components.

Total vapor pressure calculation

P_total = (0.4)(100) + (0.6)(40) = 64 torr.

Vapor-phase mole fraction of benzene

Y_benzene = 40 / 64 = 0.625.

Effect of increasing mole fraction of volatile component

Total vapor pressure increases.

Effect of reduced pressure on boiling point

It lowers the boiling point.

Vacuum distillation for high boiling point compounds

It reduces the boiling point by lowering external pressure.

Finding boiling point from vapor pressure data

Look for the temperature at which vapor pressure equals atmospheric pressure.

Calculating mole fractions and partial pressures

Use Raoult's Law: X_A = n_A / total, then P_A = X_A × P°_A.

Low mole fraction compound dominating vapor phase

Because of its high vapor pressure (volatility).

Constant boiling but mixed distillate

Presence of an azeotrope or incomplete separation.

Liquid boiling condition

When its vapor pressure equals the external pressure.

Ethanol-water in lab distillations

It forms an azeotrope and is a common test of column efficiency.

First component to distill in ethanol-water

Ethanol.

Successful separation indicators

Sharp boiling plateaus, consistent temperature, pure fractions.

Testing column efficiency

Compare temperature-volume curves for different columns or rates.

Composition from collected volumes

Approximately 30% ethanol, 70% water.

Separation of acetone and ether

No; their boiling points are too close.

Recording temperature before and after each fraction

To identify which compound is distilling.

Intermolecular forces and distillation

Stronger forces = higher boiling points = later in distillation.

Energy use in fractional distillation

To maintain multiple equilibrium steps in the column.

Reason for using simple distillation

Effective for mixtures with large boiling point differences.

Temperature plateau absence reasons

Heating too fast, bad thermometer placement, poor column.

Consequences of not using boiling chips

Superheating or bumping.

Sloped distillation curve meaning

Mixture is distilling; poor separation.

Improperly packed column effect

Less surface area = inefficient separation.

Incorrect thermometer placement effect

Misleading boiling point readings.

Overfilled flask consequences

Bumping, contamination of distillate.

Low percent recovery reasons

Loss to transfer, evaporation, or incomplete distillation.

Distillate appearance before heating

Leaky or improperly sealed system.

Liquid-liquid extraction

A separation technique where a solute is transferred from one immiscible liquid phase to another based on solubility differences.

Immiscible solvents

So that they form separate layers and can be easily separated after extraction.

Layer positioning

The relative densities of the two solvents.

Partition coefficient (K)

The ratio of solute concentrations in the two layers: K = [solute]_organic / [solute]_aqueous.

Multiple small extractions

Multiple extractions remove more solute due to better equilibrium distribution.

Properties of extracting solvent

Immiscible with original solvent, selective for target compound, non-reactive, easily removable (volatile).

Acidifying or basifying a solution

To change the solubility of acids or bases by converting them to their ionic form (soluble in aqueous phase) or neutral form (soluble in organic phase).

Drying the organic layer

To remove trace water before evaporation or further analysis.

Emulsion

A suspension of droplets between layers. Break with salt, centrifugation, or gentle swirling.

Improper venting of a separatory funnel

Pressure builds up, potentially causing leaks or explosions.

Identifying aqueous vs organic layer

Use density: water is denser than most organics; add water drop to see which layer it joins.

Using a separatory funnel safely

Add liquids, stopper, invert and swirl, vent often, allow layers to separate, drain bottom layer first.

Venting the funnel during extraction

To release built-up pressure from volatile solvents or acid/base reactions.

Common drying agents

Anhydrous sodium sulfate or magnesium sulfate.

Knowing when drying is complete

The drying agent remains free-flowing and no longer clumps.

Back extraction

Re-extracting a solute from the second solvent into the first (e.g., removing impurities from organic into aqueous).

Precautions when shaking acidic or basic solutions

Vent frequently due to CO2 or gas buildup; use protective gear.

Dry glassware for organic solvents

To prevent water from dissolving in and contaminating the organic layer.

Removing stopper before draining

To avoid creating a vacuum that prevents liquid flow.

Labeling layers during multi-step extraction

To avoid confusion and potential loss of product.

Recovering discarded organic layer

Recover by re-extracting the aqueous layer if the solute hasn't been lost.

Fixing an emulsion

Add saturated salt solution, wait, or centrifuge.

Identifying organic layer

Add a drop of water; see which layer it joins.

NaOH and benzoic acid extraction

It forms sodium benzoate, an ionic, water-soluble salt.

HCl and amine solution extraction

Converts amine to water-soluble ammonium salt.

Extracting a neutral compound

Use a suitable solvent that does not react with the compound.

Acid/base extraction

A method to remove acids and bases, leaving neutral compounds in the organic layer.

Drying organic layer

Failure to dry results in water remaining, leading to an impure product and reduced yield.

Evaporating solvent with no residue

This occurs when the solute remained in the aqueous layer or was lost during transfer.

Methanol extraction problem

Methanol is miscible with water, preventing layer separation.

Salting out

A process where salt decreases the solubility of organic compounds in water, pushing them into the organic layer.

Extraction calculation

Given K = 5 and 100 mg of compound in 100 mL water, approximately 83.3 mg is extracted into 100 mL ether.

Multiple extractions

Two 50 mL extractions recover more than a single 100 mL extraction, calculated using the partition coefficient.

Formula for compound remaining

C_final = C_initial × (V_aq / (K × V_org + V_aq))^n.

Increasing organic volume

Increases recovery, especially in single-step extraction.

Percent recovery calculation

% recovery = (amount extracted / initial amount) × 100.

Ether as extraction solvent

Ether is commonly used because it is immiscible with water, has a low boiling point, and good solvating ability.

Acetone in aqueous extraction

Acetone is not used because it is miscible with water, preventing layer formation.

Isolating final compound

The process involves drying the organic layer, filtering, evaporating the solvent, and collecting the residue.