Starter quiz
- 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
- substances which are not chemically combined and can be physically separated ✓
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- Match each keyword about chromatography to the relevant description.
- solvent⇔dissolves the solute(s) ✓
- solvent front⇔furthest point reached by a solvent along the stationary phase ✓
- stationary phase⇔fixed medium through which a solvent, and components it contains, move ✓
- mobile phase⇔a liquid or gas that moves components it contains to separate them out ✓
- How high up a piece of chromatography paper should the solvent be?
- above the sample line
- below the sample line ✓
- on the sample line
- above the solvent front
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- In chromatography, what information is required to calculate the retention factor of a substance?
- distance travelled by solvent ✓
- distance travelled by component ✓
- type of stationary phase used
- type of mobile phase
- time taken
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- 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
- 6.8 ÷ 11.2 ✓
- 11.2 - 6.8
- 11.2 × 6.8
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- 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?
- change the stationary phase ✓
- change the mobile phase ✓
- change the container used for the chromatography process
- add a lid to the container
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Exit quiz
- Match the method of separation to the relevant statement.
- gas chromatography⇔helium is often used as the mobile phase ✓
- paper chromatography⇔water is typically used as the mobile phase ✓
- thin layer chromatography⇔volatile organic solvents are used as the mobile phase ✓
- Locating agents may be used in __________ to aid the analysis of a chromatogram.
- thin layer chromatography (TLC)
- gas chromatography (GC) ✓
- TLC and GC
- TLC, GC and paper chromatography
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- Which type of chromatography provides information about the amount of each component present in a sample?
- gas chromatography (GC) ✓
- thin layer chromatography (TLC)
- TLC and GC
- TLC, GC and paper chromatography
- paper chromatography
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- Which of these methods uses alumina or silica as the stationary phase?
- gas chromatography (GC)
- thin layer chromatography (TLC)
- TLC and GC ✓
- TLC, GC and paper chromatography
- paper chromatography
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- Why is helium used as a mobile phase in gas chromatography?
- 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.
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- 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?
- pentadecane ✓
- decane
- octane
- none of the above
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Worksheet
Presentation
Video
Lesson Details
Key learning points
- Paper, thin layer (TLC), and gas chromatography (GC) can all distinguish between pure and impure substances.
- Chromatographic processes use a mobile and stationary phase. TLC and GC both use silica/alumina as a stationary phase.
- Thin layer chromatography (TLC) uses locating agents to develop chromatograms and aid analysis of them.
- Chromatograms from gas chromatography (GC) also provide details about the amount of each component in a sample.
- Retention factors (Rf) and retention times both provide quantitative information about a sample's components.
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.
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.