Separation & Purification

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Notes

Paper Chromatography

Chromatography separates substances based on **different solubilities** in a solvent.
Draw a **pencil line** on chromatography paper (ink would run) and place sample spots on it.
Lower paper into solvent so the pencil line is **above the solvent** to avoid washing samples off.
Solvent travels up by **capillary action**, carrying substances at different rates.
More soluble substances travel **further** up the paper.
A **pure substance** produces one spot; an **impure mixture** produces multiple spots.
Identical substances produce **identical chromatograms** (spots at same height).

Locating Agents & Rf Values (Extended)

**Locating agents** (e.g., ninhydrin) react with colourless substances like amino acids to form visible coloured spots.
Apply locating agent **after** the chromatogram has run.
**Retention factor (Rf)** = distance moved by substance ÷ distance moved by solvent.
Rf is a **ratio** with no units, always **less than 1**.
Rf value is **constant** for a given compound under the same solvent conditions.
Compare Rf values to known substances to **identify unknowns**.

Filtration

Separates an **insoluble solid** from a liquid (e.g., sand from water).
Use **filter paper** in a funnel; liquid passes through as **filtrate**, solid remains as **residue**.
Works because solid particles are **too large** to pass through filter paper pores.
Also called **centrifugation** when using a centrifuge.

Crystallisation

Separates a **dissolved solid** from a solution (e.g., copper(II) sulfate from water).
Heat solution to evaporate solvent until **saturated** (test with cold glass rod – crystals form).
Allow saturated solution to **cool slowly**; crystals form as solubility decreases.
Collect crystals by **filtration**, wash with distilled water, and dry (e.g., between filter paper or in oven).

Simple Distillation

Separates a **liquid from a dissolved solid** (e.g., water from salt solution) or a pure liquid from a mixture.
Heat solution; liquid with lower boiling point **evaporates** first.
Vapour passes through a **condenser** where it cools and **condenses** back to liquid.
Collect **distillate** (pure liquid); solid remains in the flask.

Fractional Distillation

Separates **two or more miscible liquids** with different boiling points (e.g., ethanol and water).
Heat to the **lowest boiling point**; that substance evaporates first.
Vapour passes through a **fractionating column** (or condenser) and is collected.
Ethanol (b.p. 78°C) distils first; water (b.p. 100°C) remains.
Used industrially for **crude oil** separation.

Assessing Purity

**Pure substances** melt and boil at **sharp, specific temperatures** (e.g., water: 0°C, 100°C).
**Mixtures** melt/boil over a **range** of temperatures.
Impurities **lower** the melting point and **raise** the boiling point.
Compare experimental melting/boiling points to **literature values** to check purity.

Paper chromatography setup showing pencil line, sample spots, and separated components.

Filtration apparatus: sand remains as residue on filter paper; salt water passes through as filtrate.

Simple distillation: salt water is heated; water vapour condenses and is collected as pure water.

Practice questions

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1.In paper chromatography, why is a pencil line used to mark the starting position instead of a pen?
Easy
- APencil is insoluble in the solvent and will not run up the paper.
- BPencil marks are more visible than pen marks.
- CPen ink would react with the sample.
- DPencil is cheaper than pen.
2.What is the correct order of steps to separate a mixture of sand and salt to obtain pure salt crystals?
Easy
- AFiltration, then crystallisation
- BCrystallisation, then filtration
- CDistillation, then filtration
- DFiltration, then distillation
3.Which separation technique would be most suitable to obtain pure water from a solution of salt water?
Easy
- ASimple distillation
- BFractional distillation
- CCrystallisation
- DFiltration
4.In filtration, what is the name of the solid that remains on the filter paper?
Easy
- AResidue
- BFiltrate
- CDistillate
- DPrecipitate
5.A student performs paper chromatography on a sample of black ink. The chromatogram shows three spots. What does this indicate about the black ink?
Easy
- AIt is a mixture of at least three substances.
- BIt is a pure substance.
- CIt is a single compound.
- DIt contains only one pigment.
6.Which of the following is a physical property that can be used to separate mixtures?
Easy
- ABoiling point
- BReactivity with acid
- CColour
- DTaste
7.The Rf value of a substance in paper chromatography is 0.6. The solvent front moved 10 cm. How far did the substance travel?
Medium
- A6.0 cm
- B0.6 cm
- C16.7 cm
- D6.0 mm
8.A mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C) is separated by fractional distillation. Which liquid distils over first?
Medium
- AEthanol
- BWater
- CBoth together
- DNeither

Slides

Separation & Purification Techniques

Separation techniques rely on differences in physical properties like solubility or boiling point.
Common techniques include filtration, crystallisation, simple distillation, fractional distillation, and chromatography.
The choice of technique depends on the mixture's composition.

Filtration

Used to separate an insoluble solid from a liquid (e.g., sand from water).
The mixture is poured through filter paper; liquid passes through (filtrate), solid remains (residue).
Centrifugation can also be used for this purpose.

Which separation technique would you use to separate sand from a mixture of sand and water?

Filtration
Crystallisation
Distillation
Chromatography

Crystallisation

Used to obtain a dissolved solid from a solution (e.g., copper(II) sulfate from aqueous solution).
Heat the solution to evaporate solvent, leaving a saturated solution.
Cool slowly to allow crystals to form; collect by filtration, wash with distilled water, and dry.

To obtain crystals from a solution, you first ____ the solution to evaporate some solvent, then allow it to ____ slowly.

Simple Distillation

Separates a liquid from a solution (e.g., water from salt water) or a pure liquid from a mixture.
The solution is heated; the liquid with lower boiling point evaporates, then condenses in a condenser.
The condensed liquid is collected; the solid solute remains in the flask.

In simple distillation, what is the purpose of the condenser?

To heat the mixture
To cool and condense vapour into liquid
To collect the solid residue
To stir the solution

Fractional Distillation

Separates two or more miscible liquids with different boiling points (e.g., ethanol and water).
The mixture is heated; the liquid with the lowest boiling point evaporates first.
Vapour passes through a fractionating column (or directly to condenser), condenses, and is collected.
Used when boiling points are close or higher purity is needed (e.g., crude oil).

A mixture of ethanol (boiling point 78°C) and water (boiling point 100°C) is separated by fractional distillation. Which liquid distils first?

Ethanol
Water
Both together
Neither

Paper Chromatography

Used to separate substances based on their solubility in a solvent.
A pencil line is drawn on chromatography paper; spots of sample are placed on it.
The paper is placed in solvent so the pencil line is above the solvent level.
Solvent travels up by capillary action; different substances travel at different rates.

Which of these are correct about paper chromatography?

A pencil line is used because ink would run into the chromatogram.
The pencil line should be below the solvent level.
Substances with higher solubility travel further.
Pure substances produce multiple spots.

Interpreting Chromatograms

Pure substances give one spot; impure mixtures give multiple spots.
Identical substances produce spots at the same height (same Rf value).
A reference spot of a known compound helps identify unknown spots.

A chromatogram shows three spots for a sample. What does this indicate?

The sample is pure
The sample is a mixture of at least three substances
The solvent is impure
The paper is faulty

Locating Agents & Rf Values (Extended)

Colourless substances (e.g., amino acids) need a locating agent to be seen after chromatography.
The locating agent reacts with the sample to produce a coloured spot.
Rf (retention factor) = distance moved by substance / distance moved by solvent.
Rf is a ratio with no units, always less than 1, and depends on the solvent used.

The Rf value is calculated as distance moved by ____ divided by distance moved by ____.

A substance travels 4 cm and the solvent front travels 8 cm. What is the Rf value?

0.5
2.0
4.0
0.25

Assessing Purity

Pure substances have sharp melting and boiling points (e.g., water melts at 0°C, boils at 100°C).
Mixtures melt/boil over a range of temperatures.
Impurities lower the melting point and raise the boiling point.
Melting point analysis is used to check purity of drugs.

Which statements about purity are true?

Pure substances have a range of melting points.
Impurities lower the melting point.
Boiling point can be used to identify a pure substance.
A mixture has a sharp boiling point.

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