Chromatography: separating a mixture of inks
Lesson details
Learning outcome
I can identify the composition of inks using paper chromatography.
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.
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.
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.
Teacher tip
Challenge pupils to change mobile phases (e.g. water, ethanol, propanone) or temperature when analysing inks to highlight the effect changing conditions has on the development of chromatograms. Try linking this back to ideas of reproducibility and peer evaluation.
Equipment
See additional materials.
Content guidance
Risk assessment required - chemicals
Risk assessment required - equipment
Supervision
Adult supervision required
Licence
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.Which of the following is not part of preparing a chromatography paper?
Q2.Why do we use pencil to draw the sample line in chromatography?
Q3.What is the purpose of the stationary phase in chromatography?
Q4.True or false? The solvent front should be marked after the paper is removed from the container.
Q5.True or false? If two substances have the same Rf value then they must be the same substance.
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.
Assessment 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.
Q2.Which of these does not need to remain constant for a fair comparison of Rf values between two chromatograms?
Q3.If a substance is insoluble in the chosen solvent, which of the following would happen?
Q4.What is the Rf value of a solvent front that travels 8.0 cm and a substance spot that travels 20 mm?
Q5.True or false? The temperature does not affect the outcome of the chromatography process.
Q6.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.
To help you plan your 10 combined science lesson on: Chromatography: separating a mixture of inks, download all teaching resources for free and adapt to suit your pupils' needs...
To help you plan your 10 combined science lesson on: Chromatography: separating a mixture of inks, download all teaching resources for free and adapt to suit your pupils' needs.
The starter quiz will activate and check your pupils' prior knowledge, with versions available both with and without answers in PDF format.
We use learning cycles to break down learning into key concepts or ideas linked to the learning outcome. Each learning cycle features explanations with checks for understanding and practice tasks with feedback. All of this is found in our slide decks, ready for you to download and edit. The practice tasks are also available as printable worksheets and some lessons have additional materials with extra material you might need for teaching the lesson.
The assessment exit quiz will test your pupils' understanding of the key learning points.
Our video is a tool for planning, showing how other teachers might teach the lesson, offering helpful tips, modelled explanations and inspiration for your own delivery in the classroom. Plus, you can set it as homework or revision for pupils and keep their learning on track by sharing an online pupil version of this lesson.
Explore more key stage 4 combined science lessons from the Separating substances unit, dive into the full secondary combined science curriculum, or learn more about lesson planning.