New
New
Year 10
OCR
Higher

Chromatography: separating a mixture of inks

I can identify the composition of inks using paper chromatography.

New
New
Year 10
OCR
Higher

Chromatography: separating a mixture of inks

I can identify the composition of inks using paper chromatography.

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Lesson details

Key learning points

1. Chromatograms can identify unknown substances through comparison or Rf values.
2. Appropriate equipment should be used during comparisons, and calculations should consider significant figures.
3. A fair comparison of chromatograms depends on stationary and mobile phase choices, sample concentration and temperature.
4. Chromatograms can be used to identify pure and impure substances.

Common misconception

Pupils can struggle to interpret information that contains several samples on one chromatogram.

Stress a systematic approach to interpreting chromatograms - use rulers. Live model this - using a visualiser helps enormously.

Keywords

• Solvent front - The solvent front is the furthest distance the solvent travels along the stationary phase during chromatography.

• Sample line - The sample line is where the sample undergoing chromatography is placed on the stationary phase.

• Rf value - The Rf value is a ratio describing the distance a component of a sample travelled, in relation to the distance the solvent travelled under the same conditions.

• Chromatogram - A chromatogram is the resultant pattern on the stationary phase after chromatography has been carried out.

Content guidance

• Equipment requiring safe usage.

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).

Starter quiz

6 Questions

Q1.
Which of the following is not part of preparing a chromatography paper?
drawing a sample line
adding a small volume of solvent
marking the solvent front
Q2.
Why do we use pencil to draw the sample line in chromatography?
pencil marks dissolve in the solvent
pencil is more visible than ink
Correct answer: pencil marks do not travel with the solvent
pencil reacts with the inks to help them separate
Q3.
What is the purpose of the stationary phase in chromatography?
to move the solvent
to absorb the solvent
Correct answer: to hold the substances as they separate
to change the colour of the inks
Q4.
True or false? The solvent front should be marked after the paper is removed from the container.
false
Q5.
True or false? If two substances have the same Rf value then they must be the same substance.
true
Q6.
Calculate the Rf value of a spot that travels 55 mm up from the sample line, where the chromatogram shows a solvent front of 86 mm.

Exit quiz

6 Questions

Q1.
True or false? Rf values can be calculated from a chromatogram of unknown samples. These Rf values can be compared against sample data to identify unknown substances.
false
Q2.
Which of these does not need to remain constant for a fair comparison of Rf values between two chromatograms?
the type of paper used
the samples being analysed
Correct answer: the size of the paper
the temperature of the solvent
the concentration of the solvent
Q3.
If a substance is insoluble in the chosen solvent, which of the following would happen?
the substance will move with the solvent front
the substance will have an Rf value above 1
Correct answer: the substance will stay at the sample line
the substance will have a negative Rf value
Q4.
What is the Rf value of a solvent front that travels 8.0 cm and a substance spot that travels 20 mm?