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Year 10
OCR
Foundation

Comparing types of chromatography: including gas chromatography

I can identify features of different types of chromatography and describe how the techniques are used to separate and identify components of a mixture.

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New
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Year 10
OCR
Foundation

Comparing types of chromatography: including gas chromatography

I can identify features of different types of chromatography and describe how the techniques are used to separate and identify components of a mixture.

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Lesson details

Key learning points

  1. Paper, thin layer (TLC), and gas chromatography (GC) can all distinguish between pure and impure substances.
  2. Chromatographic processes use a mobile and stationary phase. TLC and GC both use silica/alumina as a stationary phase.
  3. Thin layer chromatography (TLC) uses locating agents to develop chromatograms and aid analysis of them.
  4. Chromatograms from gas chromatography (GC) also provide details about the amount of each component in a sample.
  5. Retention factors (Rf) and retention times both provide quantitative information about a sample's components.

Keywords

  • Stationary phase - A surface along which the solvent (and any dissolved components) travels. It does not move during chromatography.

  • Mobile phase - The movement of a solvent and any dissolved components of a sample along the stationary phase in chromatography.

  • Chromatogram - The resultant pattern of a sample’s components on the stationary phase after chromatography has been carried out.

  • Thin layer chromatography - A type of chromatography that uses a thin layer of an inert substance spread over an unreactive flat surface as the stationary phase and a liquid state substance for the mobile phase.

  • Gas chromatography - A type of chromatography that uses an inert substance packed into a coiled tube as the stationary phase and an unreactive/inert gas as the mobile phase.

Common misconception

Students sometimes struggle to understand how using different mobile and/or stationary phases affects the resulting chromatograms.

Perform chromatography of the same mixture using different mobile phases/stationary phases and discuss the pros/cons of each combination.

Perform paper and TLC chromatography on a sample of grass to highlight why one type of chromatography may be preferable to another.
Teacher tip

Equipment

Grass sample, propanone, capillary tube, dropping pipette, mortar and pestle, water, TLC plate, chromatography, 2x beaker, watch glass, paper, pencil, ruler.

Content guidance

  • Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Which statement best describes a mixture?
two or more elements or compounds which are chemically combined
two or more elements cannot separated by physical or chemical methods
substances which cannot easily be separated physically
Correct answer: substances which are not chemically combined and can be physically separated
Q2.
Match each keyword about chromatography to the relevant description.
Correct Answer:solvent, dissolves the solute(s)

dissolves the solute(s)

Correct Answer:solvent front,furthest point reached by a solvent along the stationary phase

furthest point reached by a solvent along the stationary phase

Correct Answer:stationary phase,fixed medium through which a solvent, and components it contains, move

fixed medium through which a solvent, and components it contains, move

Correct Answer:mobile phase,a liquid or gas that moves components it contains to separate them out

a liquid or gas that moves components it contains to separate them out

Q3.
How high up a piece of chromatography paper should the solvent be?
above the sample line
Correct answer: below the sample line
on the sample line
above the solvent front
Q4.
In chromatography, what information is required to calculate the retention factor of a substance?
Correct answer: distance travelled by solvent
Correct answer: distance travelled by component
type of stationary phase used
type of mobile phase
time taken
Q5.
How can you calculate the Rf value of this unknown component? The solvent front travelled 11.2 cm and the component travelled 6.8 cm on the chromatogram.
11.2 ÷ 6.8
Correct answer: 6.8 ÷ 11.2
11.2 - 6.8
11.2 × 6.8
Q6.
Chromatography was used to separate three unknown substances using the same solvent and stationary phase. Two different substances had the same Rf value. What could be done next to help identify them?
Correct answer: change the stationary phase
Correct answer: change the mobile phase
change the container used for the chromatography process
add a lid to the container

Exit quiz

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6 Questions

Q1.
Match the method of separation to the relevant statement.
Correct Answer:gas chromatography,helium is often used as the mobile phase

helium is often used as the mobile phase

Correct Answer:paper chromatography,water is typically used as the mobile phase

water is typically used as the mobile phase

Correct Answer:thin layer chromatography,volatile organic solvents are used as the mobile phase

volatile organic solvents are used as the mobile phase

Q2.
Locating agents may be used in __________ to aid the analysis of a chromatogram.
thin layer chromatography (TLC)
Correct answer: gas chromatography (GC)
TLC and GC
TLC, GC and paper chromatography
Q3.
Which type of chromatography provides information about the amount of each component present in a sample?
Correct answer: gas chromatography (GC)
thin layer chromatography (TLC)
TLC and GC
TLC, GC and paper chromatography
paper chromatography
Q4.
Which of these methods uses alumina or silica as the stationary phase?
gas chromatography (GC)
thin layer chromatography (TLC)
Correct answer: TLC and GC
TLC, GC and paper chromatography
paper chromatography
Q5.
Why is helium used as a mobile phase in gas chromatography?
Correct answer: Helium is inert.
Helium reacts with the samples easily.
Helium enables all the components of a sample to become visible.
Helium reacts with the alumina or silica in the stationary phase.
Q6.
Retention times were measured for 3 substances using GC. Which substance had the greatest affinity with the stationary phase? Pentadecane (22.8 mins), decane (9.3 mins), octane (4.1 mins) or none?
Correct answer: pentadecane
decane
octane
none of the above