Starter quiz
- Which of the following is not part of preparing a chromatography paper?
- drawing a sample line
- adding a small volume of solvent
- heating the paper ✓
- marking the solvent front
-
- Why do we use pencil to draw the sample line in chromatography?
- pencil marks dissolve in the solvent
- pencil is more visible than ink
- pencil marks do not travel with the solvent ✓
- pencil reacts with the inks to help them separate
-
- What is the purpose of the stationary phase in chromatography?
- to move the solvent
- to absorb the solvent
- to hold the substances as they separate ✓
- to change the colour of the inks
-
- True or false? The solvent front should be marked after the paper is removed from the container.
- True ✓
- False
- True or false? If two substances have the same Rf value then they must be the same substance.
- True
- False ✓
- 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.
- '0.64' ✓
Exit quiz
- 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.
- True ✓
- False
- 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
- the size of the paper ✓
- the temperature of the solvent
- the concentration of the solvent
-
- 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
- the substance will stay at the sample line ✓
- the substance will have a negative Rf value
-
- What is the Rf value of a solvent front that travels 8.0 cm and a substance spot that travels 20 mm?
- '0.25' ✓
- True or false? The temperature does not affect the outcome of the chromatography process.
- True
- False ✓
- A chromatogram is collected and two different substances have similar Rf values. What might have caused this? They must have similar ______ in the solvent used.
- 'solubilities' ✓
Worksheet
Presentation
Video
Lesson Details
Key learning points
- Chromatograms can identify unknown substances through comparison or Rf values.
- Appropriate equipment should be used during comparisons, and calculations should consider significant figures.
- A fair comparison of chromatograms depends on stationary and mobile phase choices, sample concentration and temperature.
- 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.