8.1
Chromatography
Understand the history, principle and uses of chromatography for separating mixtures.
Mixtures and Separation of Mixtures
Learn how differences in movement, adsorption and boiling point allow components of mixtures to be separated using chromatography and fractional distillation.
8.1.1
Column Chromatography
Learn stationary phase, mobile phase, adsorbent, eluent, procedure and applications.
8.2.1
Fractional Distillation
Separate miscible liquids with close boiling points through repeated vaporization and condensation.
8.2.2
Fractional vs Simple Distillation
Compare the apparatus, process and suitability of the two methods.
8.2.3
Fractional Distillation of Crude Oil
See how hydrocarbons are separated into useful fractions in a refinery.
Learning Sequence
1
Understand separation.
Different components of a mixture can be separated when they behave differently under suitable conditions.
Different components of a mixture can be separated when they behave differently under suitable conditions.
2
Study chromatography.
Components move through a stationary medium at different rates.
Components move through a stationary medium at different rates.
3
Study column chromatography.
Different adsorption tendencies cause components to travel different distances through the column.
Different adsorption tendencies cause components to travel different distances through the column.
4
Study fractional distillation.
Repeated condensation and vaporization improve separation of liquids with close boiling points.
Repeated condensation and vaporization improve separation of liquids with close boiling points.
5
Apply the idea to crude oil.
Different hydrocarbon fractions are collected at different levels according to their boiling points.
Different hydrocarbon fractions are collected at different levels according to their boiling points.
8.1 Chromatography
Chromatography is a separation technique used to separate the components of a mixture. It is also used for purification of compounds and for testing the purity of compounds.
Origin of chromatography
Chromatography was first developed by the Russian botanist Mikhail Tswett in 1906 while studying plant pigments. He used the technique to separate the different coloured constituents of chlorophyll.
Name: The word chromatography comes from the Greek words
chroma, meaning colour, and graphein, meaning to write.
Principle of chromatography
The technique is based on the difference in the rates at which the components move through a stationary medium under the influence of a moving phase.
Core idea: If the components of a mixture interact differently
with the stationary and moving phases, they travel at different rates and can therefore be
separated.
Applications
- Separation of components of mixtures
- Purification of compounds
- Testing the purity of compounds
- Identification and separation of substances in chemistry, biology and medicine
Quick Check
- What is chromatography?
- Who developed chromatography and in which year?
- What are the two phases involved in chromatography?
- Why do components move at different rates?
8.1.1 Column Chromatography
Column chromatography is a method used for separating mixtures into their components and for purification. The stationary phase is held inside a cylindrical column, while a suitable liquid moves through it as the mobile phase.
Principle: Different compounds are adsorbed on an adsorbent to
different degrees. Consequently, they move through the column at different rates.
Stationary phase and mobile phase
| Part | Meaning in column chromatography |
|---|---|
| Stationary phase | The fixed solid material packed into the column, such as silica gel or alumina. |
| Mobile phase | The liquid solvent that moves through the column under gravity. |
| Eluent | The suitable solvent poured from above and allowed to flow through the column. |
| Fractions | The portions of liquid collected separately as the solvent comes out of the column. |
Apparatus and procedure
Figure 8.1: Principle of Column Chromatography: Differential adsorption rates cause distinct colored bands to separate and elute at different times.
Applications
- Separation of blue and red dyes
- Separation and purification of plant pigments
- Purification of compounds in chemistry laboratories
Quick Check
- Name two common adsorbents used in column chromatography.
- What is an eluent?
- Why do different substances move at different speeds in a column?
- If A comes out before B, what does that suggest about A's interaction with the stationary phase?
8.2.1 Fractional Distillation
Fractional distillation is used to separate a mixture of miscible liquids whose boiling points are close to each other. The process uses a fractionating column between the distillation flask and condenser.
Principle: Fractional distillation separates liquids on the
basis of differences in their boiling points. Repeated condensation and vaporization improve the
separation.
Fractionating column
A fractionating column is a long tube provided with obstructions to the passage of vapours moving upwards and liquid moving downwards. These obstructions increase the cooling surface area and allow repeated condensation and vaporization.
How separation occurs
1
The liquid mixture is heated in the distillation flask.
2
The vapour of the more volatile, lower-boiling liquid rises more readily.
3
Some vapour condenses on surfaces inside the fractionating column and flows downward.
4
The descending liquid gains heat from ascending vapour and revaporizes.
5
This repeated condensation and vaporization makes the rising vapour richer in the lower-boiling
component.
6
The vapour passes through the condenser, becomes liquid and is collected in the receiver.
Why repeated cycles help: each cycle preferentially enriches the
vapour in the more volatile component, giving better separation.
Quick Check
- What is fractional distillation?
- What is the role of the fractionating column?
- Why does repeated condensation and vaporization improve separation?
- On what property is fractional distillation based?
8.2.2 Fractional Distillation vs Simple Distillation
| Feature | Simple distillation | Fractional distillation |
|---|---|---|
| Fractionating column | Not used. | Placed between the distillation flask and condenser. |
| Repeated condensation and vaporization | Limited. | Occurs repeatedly in the fractionating column. |
| Separation of close boiling points | Not suitable when boiling points are very close. | Provides better separation when boiling points are close. |
| Principle | Separation by difference in boiling points. | Separation by difference in boiling points with repeated enrichment. |
The supplied chapter states that simple distillation is used for miscible liquids differing in boiling point by at least \(25^\circ\text{C}\), while fractional distillation is used when the difference is less than \(25^\circ\text{C}\).
Quick Check
- Why is simple distillation not suitable for liquids with close boiling points?
- What extra apparatus is used in fractional distillation?
- How does the fractionating column improve separation?
8.2.3 Application: Fractional Distillation of Crude Oil
Crude oil is a complex mixture of hydrocarbons having different boiling points. In a refinery, crude oil is heated and the vapours enter a tall fractionating column. As the vapours rise, they cool and condense at different levels according to their boiling points.
Important fractions named in the chapter
- Petroleum gas — LPG, containing butane and propane
- Petrol — gasoline
- Kerosene
- Diesel
- Fuel oils
- Lubricating oil and heavy oils
Quick Check
- Why can crude oil be separated by fractional distillation?
- Why do different fractions condense at different heights?
- Name any four useful fractions obtained from crude oil.
Exercise 8.1 — Mixtures and Separation of Mixtures
Attempt each question first. Click Show Solution only when you are ready to check your answer.
1. What is chromatography? Mention its two main phases.
Chromatography is a technique used to separate the components of a mixture,
and it can also be used for purification and testing purity. Its two main phases are the
stationary phase and the mobile phase.
2. Who discovered chromatography and in which year?
The supplied chapter states that chromatography was first developed by the
Russian botanist Mikhail Tswett in 1906.
3. What is meant by stationary phase and mobile phase?
The stationary phase is the medium that remains fixed while
separation occurs. The mobile phase is the phase that moves through or over the
stationary phase and carries the mixture components.
4. Name two common adsorbents used in column chromatography.
Silica gel and alumina.
5. What is an eluent in column chromatography?
An eluent is the suitable liquid solvent poured from above the column and
allowed to flow through the stationary phase. It acts as the mobile phase.
6. Why do different substances move at different speeds in
column chromatography?
Different substances have different adsorption tendencies toward the
stationary phase. Therefore, they are retained to different extents and move through the column
at different rates.
7. What is fractional distillation?
Fractional distillation is a technique used to separate a mixture of miscible
liquids having different boiling points, particularly when their boiling points are close. It
uses a fractionating column to provide repeated condensation and vaporization.
8. When is fractional distillation preferred over simple
distillation?
It is preferred when miscible liquids have close boiling points. The supplied
chapter specifies fractional distillation for a boiling-point difference of less than 25°C.
9. What is the role of the fractionating column?
It provides surfaces that allow repeated condensation and vaporization. This
enriches the rising vapour in the more volatile component and gives better separation.
10. In column chromatography, a mixture of two compounds A and
B is separated. A comes out first. What can you say about its interaction with the stationary phase?
A is less strongly adsorbed by the stationary phase than B, so it travels
through the column faster and comes out first.
11. A mixture of ethanol (b.p. 78°C) and water (b.p. 100°C) is
to be separated. Which method will you use and why?
Use fractional distillation because ethanol and water are
miscible liquids with relatively close boiling points. The fractionating column improves their
separation through repeated condensation and vaporization.
12. Explain why repeated condensation and vaporization improve
separation in fractional distillation.
During repeated cycles, the vapour becomes progressively richer in the
lower-boiling, more volatile component. This repeated enrichment produces better separation than
a single vaporization-condensation step.
13. In a fractional distillation column, why does temperature
decrease from bottom to top?
The lower part of the column is closer to the heated distillation flask, so it
is hotter. As vapours rise through the column, they encounter cooler regions; therefore
temperature decreases toward the top.
14. Why is simple distillation not suitable for separating
liquids with close boiling points?
When boiling points are close, the vapours produced in simple distillation
contain significant amounts of both liquids. Without repeated condensation and vaporization in a
fractionating column, the separation is not sufficiently effective.
15. Assertion: In chromatography, separation occurs due to
difference in boiling points. Reason: Components move at different speeds in the column. Choose the
correct option.
Answer: D. The assertion is wrong because chromatography is
based on differences in movement through the stationary/mobile phases, not boiling points. The
reason is correct.
16. Assertion: Fractional distillation gives better separation
than simple distillation. Reason: It involves repeated condensation and vaporization. Choose the
correct option.
Answer: A. Both statements are correct, and repeated
condensation and vaporization explain the improved separation.
17. Difference in which property forms the basis for separating
components in fractional distillation? A. Solubility B. Boiling points C. Particle size D. Chemical
reactivity
Answer: B. Boiling points.
18. What is the main purpose of the fractionating column in
fractional distillation? A. To heat the mixture faster. B. To cool the vapours at fast rate. C. To
provide more surface area for vapours. D. To let the vapours of two liquids mix properly
Answer: C. The column provides increased surface area and
obstructions that promote repeated condensation and vaporization.
19. In column chromatography, the solid substance that is
filled in the column is called the: A. Mobile phase B. Solvent C. Stationary phase D. Mixture
Answer: C. Stationary phase.
20. That component of a mixture moves down the column at a
faster rate which is A. most attracted to the stationary phase. B. having the highest boiling point.
C. the one most soluble in the mobile phase (solvent). D. the one with the largest particle size
Answer: C. A component that is more soluble in the mobile
phase and less strongly retained by the stationary phase moves down the column faster.
Worksheets
Practice worksheets for chromatography, column chromatography, fractional distillation and crude-oil separation will be added here.
Coming SoonQuick Revision
| Topic | Key point |
|---|---|
| Chromatography | Separation technique based on different movement rates through a stationary medium under the influence of a moving phase. |
| Mikhail Tswett | Developed chromatography in 1906 while studying plant pigments. |
| Stationary phase | The phase or medium that remains fixed during separation. |
| Mobile phase | The phase that moves through or over the stationary phase. |
| Column chromatography | Uses a packed solid stationary phase in a cylindrical column and a moving solvent. |
| Adsorbent | Solid material such as silica gel or alumina that interacts with mixture components. |
| Eluent | The solvent allowed to flow through the column. |
| Fractions | Separate portions of liquid collected from the column. |
| Fractional distillation | Separates miscible liquids with different, especially close, boiling points. |
| Fractionating column | Provides surfaces for repeated condensation and vaporization. |
| Repeated cycles | Make the rising vapour richer in the more volatile component. |
| Crude oil | Complex mixture of hydrocarbons separated into fractions according to boiling points. |
Test Yourself
A chapter test covering chromatography, column chromatography, fractional distillation and crude-oil separation will be added here.
Coming Soon