Separation And Purification

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Notes

Paper Chromatography

Chromatography separates mixtures of **soluble coloured substances** based on differences in **solubility** in a solvent.
A **pencil line** is drawn on chromatography paper; spots of sample are placed on it. Pencil is used because ink would run.
The paper is lowered into solvent so the pencil line is **above the solvent level** to prevent samples washing off.
Solvent travels up by **capillary action**, carrying substances at different rates depending on solubility.
More soluble substances travel further up the paper.
A **chromatogram** shows separated spots; pure substances give one spot, mixtures give multiple spots.
Substances are identified by comparing spot positions with known reference samples.

Retention Factor (Rf) Values (Extended Tier Only)

**Rf value** = distance moved by substance ÷ distance moved by solvent.
Rf is a **ratio** with no units and is always less than 1.
Rf values are constant for a given compound under the same solvent conditions.
Rf values help identify unknown substances by comparison with known values.

Filtration

Separates an **insoluble solid** from a liquid (e.g., sand from sand–water mixture).
Mixture is poured through **filter paper** in a funnel; liquid passes as **filtrate**, solid remains as **residue**.
Relies on difference in **particle size** – solid particles are too large to pass through filter paper.

Crystallisation

Separates a **dissolved solid** from a solution by evaporating the solvent.
Solution is heated to evaporate solvent until **saturated** (test with a cold glass rod – crystals form on it).
Saturated solution is cooled slowly; crystals form as solubility decreases.
Crystals are collected by filtration, washed with distilled water, and dried (e.g., between filter paper or in a drying oven).

Simple Distillation

Separates a **liquid from a solution** (e.g., water from saltwater) or a pure liquid from a mixture of liquids.
Solution is heated; liquid with lower boiling point evaporates, passes through a **condenser** where it cools and condenses.
The condensed liquid (distillate) is collected; the solid solute remains in the flask.

Fractional Distillation

Separates **two or more miscible liquids** with different boiling points (e.g., ethanol and water).
Mixture is heated to the boiling point of the substance with the **lowest boiling point**; it evaporates first.
Vapours pass through a condenser and are collected; the other component(s) remain.
For ethanol/water: ethanol boils at 78°C, water at 100°C; heating stops when temperature rises above 78°C.

Assessing Purity

**Pure substances** melt and boil at sharp, specific temperatures (e.g., water: 0°C melting, 100°C boiling).
**Mixtures** have a range of melting and boiling points.
Melting/boiling point data can identify a pure substance by comparison with literature values.
Impurities **lower the melting point** and may **raise the boiling point** of a sample.

Particle arrangement in solid, liquid, and gas states – differences in spacing and order explain separation techniques like filtration and distillation.

Practice questions

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1.In paper chromatography, where should the sample spots be placed?
Easy
- AOn the pencil line
- BBelow the pencil line in the solvent
- CExactly 1 cm above the pencil line
- DExactly 1 cm below the pencil line
2.The diagram below illustrates the experimental set-up to carry out distillation. What are the correct labels for points A and B on the condenser?
Easy
- AA: water in, B: water in
- BA: water in, B: water out
- CA: water out, B: water in
- DA: water out, B: water out
3.Rocksalt contains sodium chloride and sand. What is the correct order of physical changes and separation techniques to separate sodium chloride and sand?
Easy
- AMelting, filtering, crystallising
- BMelting, filtering
- CDissolving, filtering, crystallising
- DDissolving, crystallising
4.A substance was analysed and found to have a boiling point of 450 °C. At room temperature it is a soft grey solid. Which temperature could be the melting point of a pure sample of the substance?
Medium
- A-12 °C
- B56 °C to 72 °C
- C250 °C to 275 °C
- D142 °C
5.Solid P melts at exactly 60 °C and boils at exactly 310 °C. Solid Q dissolves in ethanol and shows a green colour and a yellow colour when analysed using paper chromatography. Which row is correct?
Medium
- AP contains only one substance, Q contains more than one substance
- BP contains more than one substance, Q contains only one substance
- CBoth P and Q contain only one substance
- DBoth P and Q contain more than one substance
6.Pure water boils at 100 °C and freezes at 0 °C. What is the melting point and boiling point of water after the addition of potassium chloride?
Medium
- ABoiling point 103 °C, freezing point -3 °C
- BBoiling point 103 °C, freezing point 3 °C
- CBoiling point 97 °C, freezing point -3 °C
- DBoiling point 97 °C, freezing point no change
7.A student has performed an experiment which produced a precipitate. She correctly chooses to separate the precipitate using filtration. She prepares the filter paper by folding it as shown. Which diagram correctly illustrates the filter paper at stage 3?
Medium
- AA
- BB
- CC
- DD
8.The diagram below shows the apparatus a student used to separate a product from a reaction mixture. What did the mixture contain?
Medium
- AAqueous sodium chloride and unreacted solid catalyst
- BWater and ethanol
- CWater and oil
- DAqueous sodium chloride and oil

Slides

Why Separate Mixtures?

Most substances in nature are mixtures, not pure.
Separation techniques rely on differences in physical properties (e.g., solubility, boiling point).
The choice of technique depends on the substances being separated.

Paper Chromatography

Used to separate mixtures of soluble coloured substances.
Components have different solubilities in a solvent; more soluble substances travel further.
A pencil line is drawn on chromatography paper and spots of sample are placed on it.
The paper is lowered into solvent so that the pencil line is above the solvent level.

Why is a pencil line used in paper chromatography instead of ink?

Pencil is cheaper than ink.
Ink would run into the chromatogram and interfere with the results.
Pencil line is more visible.
Ink is insoluble in the solvent.

Interpreting Chromatograms

Pure substances produce only one spot on a chromatogram.
Impure substances (mixtures) produce more than one spot.
If two substances are identical, their spots will be at the same height (same Rf value).

Which of these are true about chromatography?

A pure substance gives one spot on a chromatogram.
A mixture always gives two spots.
Identical substances produce spots at the same height.

Rf Values (Extended Tier)

Rf (retention factor) is used to identify components of a mixture.
$Rf =$ distance moved by substance ÷ distance moved by solvent.
Rf is a ratio with no units, always less than 1.
The same substance has the same Rf under the same conditions (solvent, temperature).

The Rf value is calculated by dividing the distance moved by $the ____ by$ the distance moved by $the ____$ .

Filtration

Separates an insoluble solid from a liquid (e.g., sand from water).
Filter paper traps solid particles (residue); liquid passes through (filtrate).
Centrifugation can also be used for similar mixtures.

Which mixture can be separated by filtration?

Salt dissolved in water
Sand and water
Ethanol and water
Oil and water

Crystallisation

Separates a dissolved solid from a solution.
Heat the solution to evaporate some solvent, making it saturated.
Allow the saturated solution to cool slowly; crystals form as solubility decreases.
Collect crystals by filtration, wash with distilled water, and dry.

To obtain crystals of copper(II) sulfate from its solution, you first heat the solution to make $it ____$ , 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 first.
Vapour passes through a condenser where it cools and condenses back to liquid.
The condensed liquid (distillate) is collected in a separate container.

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

To heat the mixture
To cool and condense the vapour back to liquid
To increase the boiling point
To filter the solid

Fractional Distillation

Separates two or more miscible liquids with different boiling points (e.g., ethanol and water).
The mixture is heated to the boiling point of the lowest-boiling component.
That component evaporates, passes through a fractionating column, then condenses.
The process is repeated (vapour condenses and re-evaporates) to achieve better separation.

Which statements about fractional distillation are true?

It can separate ethanol (bp 78°C) and water (bp 100°C).
It is used to separate immiscible liquids like oil and water.
The fractionating column allows multiple condensation-evaporation cycles.

Assessing Purity

Pure substances have sharp, specific melting and boiling points (e.g., water: 0°C and 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 and other substances.

A sample of water boils at 103°C. What does this suggest?

The water is pure.
The water contains dissolved impurities.
The boiling point is measured incorrectly.
The water is at a higher altitude.

Summary of Separation Techniques

Filtration: insoluble solid from liquid.
Crystallisation: dissolved solid from solution.
Simple distillation: liquid from solution or two liquids with large boiling point difference.
Fractional distillation: two or more miscible liquids with close boiling points.
Chromatography: soluble coloured substances (and to identify them).

To separate a mixture of sand and salt, first add water to dissolve $the ____$ , then $____ to$ remove the sand, and finally $____ to$ obtain salt crystals.

Flashcards

What is the purpose of paper chromatography?

To separate mixtures of soluble coloured substances and help identify them based on different solubilities in a solvent.

Why is a pencil line used in paper chromatography instead of ink?

Ink would run into the chromatogram along with the samples, so a pencil line is used to avoid interference.

Where should the pencil line be placed relative to the solvent in paper chromatography?

The pencil line must be above the level of the solvent so the samples do not wash into the solvent.

How does a pure substance appear on a chromatogram?

A pure substance produces only one spot on the chromatogram.

How does an impure substance appear on a chromatogram?

An impure substance produces more than one spot on the chromatogram.

What is the formula for calculating Rf value?

$Rf =$ distance moved by substance / distance moved by solvent.

What are the properties of Rf values?

Rf is a ratio with no units and is always less than 1.

Which separation technique is used to separate an undissolved solid from a liquid?

Filtration (or centrifugation) is used to separate an undissolved solid from a liquid.

What is the filtrate and residue in filtration?

The filtrate is the liquid that passes through the filter paper; the residue is the solid that remains on the filter paper.

How can you obtain a dissolved solid from a solution?

By crystallisation: heat the solution to evaporate solvent, then allow to cool so crystals form, then filter and dry.

What is simple distillation used for?

To separate a liquid from a soluble solid (e.g., water from salt solution) or a pure liquid from a mixture of liquids.

What is fractional distillation used for?

To separate two or more miscible liquids with different boiling points (e.g., ethanol and water).

In fractional distillation, which liquid distils first?

The liquid with the lower boiling point distils first.

How can you test if a solution is saturated during crystallisation?

Dip a clean, dry, cold glass rod into the solution; if crystals form on the rod when removed and cooled, the solution is saturated.

What is the effect of impurities on melting point?

Impurities lower the melting point and broaden the melting range.

What is the effect of impurities on boiling point?

Impurities raise the boiling point and broaden the boiling range.

How can you separate a mixture of sand and salt?

Add water to dissolve salt, filter to remove sand, then crystallise to obtain salt crystals.

Why is a locating agent sometimes used in chromatography?

To form coloured spots with colourless substances so they become visible.

What does it mean if two substances have the same Rf value under the same conditions?

They are likely the same substance.

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

To cool and condense the vapour back into liquid.

How should the water flow in a condenser during distillation?

Water enters at the bottom (point B) and exits at the top (point A) to ensure efficient cooling.

What is the correct order to separate a mixture containing an insoluble solid and a soluble solid?

Dissolving, filtration, crystallisation.

What is the Rf value of a substance that moves 3 cm when the solvent moves 6 cm?

0.5

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