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School Chemistry Notes: Describing and explaining simple and fractional distillation

Part 2a: GCSE level chemistry exam notes on Mixture separation using SIMPLE DISTILLATION and FRACTIONAL DISTILLATION

[Author © Dr WP Brown PhD: Doc Brown's chemistry exam revision notes suitable for chemistry students studying AQA, Edexcel and OCR GCSE level chemistry courses & ~ US grades 9-10 chemistry, page updated Feb 6th 2026 *]

Methods of separating miscible liquids using simple and fractional distillation

PART 2a Methods of separating mixtures are described e.g. simple distillation, fractional distillation, purifying liquids by distillation Re-edit

Main sub-index for sections 1, 2a-2b and 3

Part 1 Definitions in Chemistry, Elements, Compounds & Mixture pictures & Physical & Chemical Changes

Part 2 Methods of Separating Mixtures of substances

Part 3 How to write equations, work out formula and name compounds

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Alphabetical list of KEYWORDS for Elements, compounds and mixtures Parts 1-3

atom  *  balancing equations *  centrifuges/centrifuging  *  chemical reaction/change  *  chromatography (paper/thin layer)  *  compound  *  covalency *  crystallisation  *  decanting/decantation  * displayed formula  *  distillation (simple or fractional)  *  element  *  equations  *  evaporation  *  filtration  *  formula  * gas chromatography  *  impure/pure  *  insoluble  *  ionic equations  *  ionic valence  *  iron-sulphur separation and heating experiment  *  magnet  *  mixture  *  molecule  *  naming compounds and ions  *  particle pictures of elements/compounds/mixtures  *  physical change  *  precipitation  *  products  *  pure substance  *  purification  *  reactants  *  sand/salt separation  *  separating funnel  *  separating mixtures  *  soluble/solution/solvent/solute  *  solvent extraction  *  symbols (for elements, formula, in equations)  *   state symbols  * working out formulae  *

METHODS of SEPARATING MIXTURES and purifying substances by distillation

Simple distillation and fractional distillation are two techniques used in the isolation and purification of liquid product from a chemical reaction.

Theses are techniques for separating a mixture to obtain a specific liquid product from a liquid mixture or a solution.

Remember, in the physical separation processes of simple distillation and fractional distillation, no chemical reaction changes are involved, so no new substances are made.

After the detailed notes below, is a section of key revision points on  simple and fractional distillation

2.1 Simple Distillation

simple distillation diagram

Distillation involves 2 stages and both are physical state changes.

(1) In simple distillation the liquid or solution mixture is heated to boil and vaporise the most volatile component in the mixture (liquid ==> gas). The ant-bumping granules give a smoother boiling action for the distillation process.

(2) The vapour passes up from the flask and down into the condenser, where it is cooled by cold water and condenses (gas ==> liquid) back to a liquid (the distillate) which is collected in the flask.

(3) Any dissolved solids are left in solution because they have to high a boiling point to be distilled over e.g. the salts in sea water.

Simple distillation can be used to separate a liquid from a solution (separating the solvent that dissolves substances, from a solute - the substance that had dissolved). If the dissolved soluble substance is a solid at room temperature, it will NOT distil over - much to high a boiling point. The blue solution could represent copper sulfate solution.

This can be used to purify water because the dissolved solids have a much higher boiling point and will not evaporate with the steam. You can purify seawater, at great cost, by distilling it, pure water is distilled over and condensed out and you left with a mass of salt crystals.

If you distil blue ink (illustrated) then you can collect pure colourless water in the distillate flask. The higher boiling ink (a solid) is left as a residue.

If you don't boil it dry and collect the residual concentrate in solution, you could then investigate whether the ink is made up of one or more colours by paper chromatography.

You see the same thing if you distil copper sulfate solution, pure colourless liquid water distils over, the copper sulfate remains in the solution which gets progressively a deeper blue as the mixture becomes more concentrated.

Simple distillation works fine if the substances to be separated have very different boiling points like salt and water or water (boiling point 100oC) and propanone (boiling point 60oC), BUT it is too simple a method to separate a more complex mixture of liquids especially if the boiling points of the components are relatively close e.g. (i) separating the fractions in crude oil and (ii) separating water (boiling point 100oC) and ethanol (boiling point 80oC) obtained from fermenting sugar to alcohol with yeast.

For these sort of mixtures you need fractional distillation - read on!

 
click on word (c) doc b click on word

A particle picture for distillation: liquid == boiling ==> gas == condensation ==> liquid

key revision points on  simple and fractional distillation

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fractional distillation diagram and theory2.2 Fractional distillation and theory

How do you separate two liquids that have similar boiling points and are miscible? Miscible means they completely mix (e.g. alcohol and water) and do not form two layers (e.g. oil and water).

How can you separate a complex mixture of liquids by a method of distillation? Simple distillation isn't good enough to do an efficient job of separating liquids with boiling points that may be relatively close together. The liquids must all dissolve in each other i.e. they are all fully miscible.

Fractional distillation involves 2 main stages and both are physical state changes. It can only work with liquids with different boiling points but the boiling points can be quite close together. However, this method only works if all the liquids in the mixture are miscible (e.g. alcohol & water, hydrocarbons in crude oil etc.) and do NOT separate out into layers like oil & water (immiscible).

(1) For a laboratory demonstration of fractional distillation, the liquid or solution mixture is poured into the round-bottomed flask, heated and boiled to vaporise the most volatile component in the mixture (liquid ==> gas).

Anti-bumping granules give a smoother boiling action by providing a surface for small bubbles of vapour to form on.

(2) The vapour passes up through a fractionating column, where the separation takes place (theory at the end).

The fractionating column is packed with glass beads or short rods/tubes to give a large surface area for both evaporation and condensation to take place on.

This column is not used in the simple distillation described above, but is essential to the procedure of fractional distillation. When the temperature at the top of the column reaches the boiling point of the lowest boiling component, that component will then distil over into the condenser.

The higher boiling liquids (with greater intermolecular bonding forces between the molecules), are the most easily condensed liquids, condense and separate out, running back into the flask mixture, whereas the lower boiling liquid's vapour passes through into the Liebig condenser.

This is because the temperature falls as you rise up the special fractionating column.

In fractional distillation condensers you must think of the boiling point as equal to the condensation point as it decreases up the condenser column.

(3) The (e.g. ethanol) vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected in the flask.

The 1st liquid, with the lowest boiling point, evaporates-boils off first, and is called the 1st fraction.

Then, each liquid distils, with increasing boiling point, over the top of the column, when it reaches its particular boiling point to give the 2nd, 3rd fraction etc.

The boiling point of each fraction can be read off from the thermometer.

In the diagram, think of the yellow liquid as water and the blue liquid as ethanol ('alcohol') - just to help visually to understand what is happening, I do know they are both colourless! 

The mixture might that of ethanol ('alcohol') and water from the fermentation of sugar. Imagine the green liquid is a mixture of a blue liquid (boiling point 80oC) and a yellow liquid (boiling point 100oC), so we have a coloured diagram simulation of the fractional distillation colourless alcohol and water mixture!

As the vapour from the boiling mixture enters the fractionating column it begins to cool and condense.

The highest boiling or least volatile liquid tends to condense more i.e. the yellow liquid (water).

The lower boiling more volatile blue liquid gets further up the column.

Gradually up the column the blue and yellow separate from each other so that yellow condenses back into the flask and pure blue distils over to be collected.

This ensures that only reasonably pure ethanol distils over.

The continuous evaporation and condensation over a large surface area is crucial to the success of fractional distillation and that large surface area is provided by using lots of little pieces of glass tubing or glass beads

The same arguments apply to the fractional distillation of complex multi-component mixtures like crude oil. The diagram above illustrates the school laboratory demonstration of a distilling simulated crude oil.

As the vapour ascends the fractionating column the highest boiling liquids condense out and the lowest boiling liquid's vapour exits the top of the fractionating column and enters the condenser and runs into the collection tube.

In this way you can progressively distil over higher and higher boiling fractions, which are themselves narrow boiling ranges of hydrocarbons.

doc b oil notes

Fractional distillation is used to separate oxygen and nitrogen from liquefied air, though the distillation takes place at very low temperatures, below -160oC !

Fractional distillation is used on a large scale to separate the components of crude oil, because the different hydrocarbons have different boiling and condensation points (see oil). Very tall fractionating columns are used to ensure the separation of an extremely complex mixture of hydrocarbons.

However, in the school/college laboratory, to increase the separation efficiency of the glass fractionating column, it is usually packed with glass beads, short glass tubes or glass rings etc. which greatly increase the surface area for evaporation and condensation in the fractional distillation process. 

In the fractional distillation of crude oil, the system works slightly differently, the different fractions are condensed out at different points in a huge fractionating column.

At the top are the very low boiling fuel gases like butane and at the bottom are the high boiling big molecules of waxes and tar.

A fraction can be a pure purified compound OR several similar molecules with very similar boiling points e.g. the fractions from distilling crude oil.

More details on the Fractional distillation of crude oil & uses of fractions

AND in these separation procedures there always loss of product so at a higher level (GCSE/IGCSE/A Level) you need to know about

 % reaction yield & reasons for loss of product

atom economy  *  % purity of a product

I'm afraid there is a bit more to it than just a distillation apparatus !!!

key revision points on  simple and fractional distillation

Key revision points on simple distillation and fractional distillation,

aligned with the expectations of AQA, Edexcel, OCR Gateway, OCR 21st Century, WJEC, CCEA, and CIE IGCSE Chemistry.

Everything is pitched at the level needed for secure Grade 7–9 understanding, with exam tips and typical misconceptions that exam boards repeatedly highlight.

Simple Distillation

What it is

A separation technique used to separate a solvent from a solution or separate liquids with very different boiling points (large boiling‑point difference, typically >70 °C for GCSE level).

When it is used

  • Purifying drinking water from seawater (all boards).

  • Separating ethanol from a fermentation mixture only if the boiling points are far apart (but fractional distillation is usually preferred).

  • Recovering a solvent from a dissolved solid (e.g., water from copper sulfate solution).

How it works

  • The solution is heated.

  • The solvent (lower boiling point) evaporates.

  • Vapour enters the condenser, where cold water circulates around the outside.

  • Vapour condenses back to liquid and is collected as the distillate.

  • The solute remains in the flask.

Key apparatus

  • Round‑bottom or pear‑shaped flask

  • Heat source

  • Thermometer

  • Condenser (Liebig condenser)

  • Receiver

Key scientific ideas

  • Boiling point: temperature at which vapour pressure equals atmospheric pressure.

  • Condensation: gas → liquid by cooling.

  • Purity check: pure substances have sharp, fixed boiling points.

Fractional Distillation

What it is

A separation technique used to separate a mixture of miscible liquids with different boiling points (smaller differences, e.g., 20–70 °C).

When it is used

  • Separating ethanol and water.

  • Separating liquid air into nitrogen, oxygen, argon (CIE, Edexcel, OCR).

  • Separating crude oil into fractions (all boards).

  • Laboratory separation of two or more liquids with close boiling points.

How it works

  • Mixture is heated.

  • Vapours rise up the fractionating column.

  • The column has glass beads or plates providing a temperature gradient.

  • Liquids with higher boiling points condense lower down the column.

  • Liquids with lower boiling points rise further and reach the condenser.

  • The thermometer shows the boiling point of the substance being collected.

Why the fractionating column matters

  • Repeated evaporation–condensation cycles occur.

  • Ensures better separation when boiling points are close.

  • Creates a temperature gradient: hottest at the bottom, coolest at the top.

Comparison Table for simple and fractional distillation

Feature

Simple Distillation

Fractional Distillation

Purpose

Separate solvent from solute OR liquids with large boiling‑point differences

Separate miscible liquids with close boiling points

Column?

No

Yes – fractionating column

Efficiency

Lower

Higher due to repeated condensation cycles

Examples

Water from seawater

Ethanol from water, crude oil fractions

Temperature control

Less critical

Very important – thermometer must show the boiling point of the liquid being collected

Typical Exam Board Requirements for simple and fractional distillation

  • Describe apparatus and method clearly.

  • Explain separation using boiling points and changes of state.

  • Interpret temperature versus time graphs.

  • Identify which liquid is collected first (the one with the lowest boiling point).

  • Explain why cold water enters the condenser at the bottom (ensures efficient cooling).

  • Recognise that impurities raise or lower boiling points.

  • Purity tests (sharp melting/boiling point).

  • Industrial fractional distillation (crude oil, air).

  • Energy changes during phase changes.

  • Particle‑level explanations of evaporation and condensation.

  • Why mixtures behave differently from pure substances.

  • Practical method recall.

  • Safety (anti‑bumping granules, heat mats, secure clamps).

Typical GCSE Misconceptions (All Boards) about simple and fractional distillation

X “The thermometer goes in the water.”

Correct: The thermometer bulb must be at the top of the flask / entrance to the condenser to measure the vapour temperature, not the liquid temperature.

X “The highest boiling point evaporates first.”

Correct: The lowest boiling point substance evaporates first.

X “Cold water enters the condenser at the top.”

Correct: It enters at the bottom to ensure the condenser fills completely and cools efficiently.

X “Fractional distillation works because substances have different densities.”

Correct: It works because substances have different boiling points.

X “The fractionating column cools the vapour.”

Correct: It creates a temperature gradient; vapours condense only if their boiling point is higher than the temperature at that point in the column.

X “Distillation creates new substances.”

Correct: It is a physical separation, not a chemical change.

Exam Tips for simple and fractional distillation

 Method‑writing tips

  • Use keywords: boiling point, evaporation, condensation, fractionating column, temperature gradient.

  • State why each step is done, not just what is done.

  • Mention anti‑bumping granules to prevent sudden boiling.

 Diagram‑based questions

  • Label: thermometer, condenser, cold water in, cold water out, fractionating column.

  • Examiners often award marks for correct placement of the thermometer.

 Data‑interpretation questions

  • Identify the first fraction by the lowest boiling point.

  • If a temperature plateau appears on a graph, it corresponds to a boiling point.

 Practical‑based questions

  • Explain why the distillate is pure (constant boiling point).

  • Explain why fractional distillation gives better separation (repeated condensation cycles).

 Crude oil questions

  • Fractions are separated by boiling point, not by size or density.

  • The column is hot at the bottom, cool at the top.

Very concise summary of main points for simple and fractional distillation

  • Simple distillation = solvent recovery.

  • Fractional distillation = separating liquids.

  • Lowest boiling point leaves first.

  • Thermometer at top of flask.

  • Cold water in at bottom.

  • Fractionating column = repeated condensation cycles.

Learning objectives for separating mixtures by distillation

Know that distillation is way of separating a liquid mixture due to the components having different boiling points.

Know that distillation involves the process of vaporisation by boiling the liquid mixture and condensing the vapour using a condenser.

Know, describe and be able to explain how to use simple distillation apparatus (e.g. flasks and condenser) to separate a liquid from a solution of liquids with different boiling points or a dissolved solid of very high boiling point.

Know that the condensed liquid is called the distillate

Be able to draw and explain distillation using particle model pictures.

Know that when the mixture consists of liquids of a similar boiling point range, you need to add an extra piece of apparatus called a fractionating column to more efficiently separate substance of different, but similar boiling points.

Know this process is called fractional distillation and can separate a complex mixture e.g. hydrocarbon molecules in oil, even though their boiling points might be similar.

Know that narrow range of liquids with very similar boiling points are collected and called fractions.

A fraction is a pure compound or a mixture of several similar molecules with very similar boiling points.

Know that distillation is a way of purifying compounds to obtain a liquid from a mixture of liquids or a solution..

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Based on the syllabus-specifications for students taking the IGCSE/GCSE level chemistry examinations summary revision notes and key points on explaining fractional distillation of a miscible liquid mixture for students taking the AQA igcse/gcse chemistry notes on explaining fractional distillation of a miscible liquid mixture, Edexcel gcse chemistry notes on explaining fractional distillation of a miscible liquid mixture,  OCR 21st century GCSE chemistry notes on explaining fractional distillation of a miscible liquid mixture, OCR gateway GCSE chemistry notes on explaining fractional distillation of a miscible liquid mixture, WJEC gcse chemistry notes on explaining fractional distillation of a miscible liquid mixture, CCEA gcse chemistry notes on explaining fractional distillation of a miscible liquid mixture for students taking CIE Cambridge igcse chemistry, or any other GCSE or IGCSE level chemistry exams notes on explaining fractional distillation of a miscible liquid mixture, useful for US grade 9-10 chemistry courses, Explaining the importance of diagram to explain simple distillation to separate a liquid from a solution of a solid in GCSE level chemistry, What you need to know about diagram to explain simple distillation to separate a liquid from a solution of a solid for GCSE level chemistry, Explaining the use of diagram to explain simple distillation to separate a liquid from a solution of a solid knowledge in GCSE level chemistry, Examples of diagram to explain simple distillation to separate a liquid from a solution of a solid explained when studying GCSE level chemistry, What is the significance of diagram to explain simple distillation to separate a liquid from a solution of a solid in GCSE level chemistry, describing and explaining the theory of diagram to explain simple distillation to separate a liquid from a solution of a solid when studying GCSE level chemistry, revision notes for diagram to explain simple distillation to separate a liquid from a solution of a solid in exams, online help for understanding diagram to explain simple distillation to separate a liquid from a solution of a solid, exam revision notes for diagram to explain simple distillation to separate a liquid from a solution of a solid, what do I need to learn about diagram to explain simple distillation to separate a liquid from a solution of a solid? revision summary for diagram to explain simple distillation to separate a liquid from a solution of a solid, learning notes for diagram to explain simple distillation to separate a liquid from a solution of a solid, help to pass the diagram to explain simple distillation to separate a liquid from a solution of a solid topic in an exam question, how to prepare for questions on diagram to explain simple distillation to separate a liquid from a solution of a solid in a GCSE chemistry examination?

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