Leaf chromatography

During the practical activity, learners will build understanding of how paper chromatography experiments are carried out. Use questioning while setting up the practical and analysing the results to develop your learners’ understanding of stationary and mobile phases, as well as what chromatograms can tell us about relative solubilities of pigments and relative attractions towards the chromatography paper.

You may wish to set up an example chromatogram using water, to clearly illustrate learning objective 4.

Learning objectives

1. Carry out paper chromatography to find the pigments present in leaves and calculate their R_f values.
2. Identify stationary and mobile phases in paper chromatography.
3. Describe what the chromatogram can tell us about substances.
4. Explain why water is not always used as the solvent in chromatography.

Scaffolding

Integrated practical instructions are included in the slides.

Two versions of the worksheet are included: scaffolded and unscaffolded. Questions in the scaffolded worksheet provide more prompts about correct use of keywords and structuring answers, whereas the unscaffolded worksheet provides less guidance.

Teachers may decide to remove the results table and guidance for how to calculate R_f values to provide further challenge for learners.

Technician notes

Read our standard health and safety guidance and carry out a risk assessment before running any live practical.

You can make the capillary tubing from lengths of ordinary glass tubing (diameter: 3–4 mm) using a Bunsen burner fitted with a flame-spreading (‘fish-tail’) jet.

A variety of leaves can be used. The best results are obtained from trees or bushes with dark green leaves, e.g. holly.

Use pipettes that do not allow squirting, e.g. those fitted to dropper bottles of universal indicator.

The spot on the chromatography paper should be as small and as concentrated as possible. Encourage learners to be patient and to wait until each application is dry before adding the next.

At least three spots should be obtained and one of these should be yellow due to carotenes.

How far a component moves up the paper is dependent not only on its solubility in propanone but also on its attraction for the cellulose in the chromatography paper. The yellow carotene spot (with a higher R_f value) tends to move up the paper the furthest.

Equipment

Apparatus (per group)

• Safety glasses
• Pestle and mortar
• Chromatography paper
• Beaker, 100 cm³
• Small capillary tube 
• Pencil
• Ruler
• Cut-up leaves, or leaves and scissors

Chemicals (per group)

• Propanone (HIGHLY FLAMMABLE, IRRITANT) (approx. 10 cm³ per group)
• Sand

Preparation

• Approximately 10 cm3 of propanone per group is required. See CLEAPSS Hazcard HC085A, refer to SSERC or contact your local safety advisory body.
• For disposal, dilute in water to below 1M (~ 5 % v/v) and pour down a foul-water drain.
• Propanone, CH3COCH3 (l), (HIGHLY FLAMMABLE, IRRITANT) – The vapour of propanone is HIGHLY FLAMMABLE. Do not have any source of ignition nearby.
• A lid should be used during this experiment if it is not possible to provide fume hood use. An upturned watch glass works well.
• Beware of sharp edges when using capillary tubes.

Method

Wear safety glasses at all times.

1. Finely cut up some leaves and fill a mortar to a depth of about 2 cm.
2. Add a pinch of sand and about six drops of propanone from the teat pipette.
3. Grind the mixture with a pestle for at least three minutes.
4. On a strip of chromatography paper, draw a pencil line 3 cm from the bottom.
5. Use a capillary tube to put the liquid from the leaf extract on to the centre of the line. Keep the spot as small as possible.
6. Allow the spot to dry, then add another spot directly on top. Repeat this five times, letting each drop dry before adding the next. This concentrates the pigment.
7. Secure your chromatogram carefully to ensure that it sits just above the base of the beaker.
8. Place no more than 10 cm³ of propanone into the beaker and hang your chromatogram so that it dips into the propanone but does not submerge the pencil line. Add a lid to your beaker.
9. Allow the propanone to travel about ¾ of the way up the chromatography paper and then remove the chromatogram
10. Mark with a pencil how high up the paper the propanone gets and allow the chromatogram to dry.

Questions

Questions linking the practical experiment to quantitative chemistry topics can be found in the student worksheets. The scaffolded sheet offers more support to allow learners to access the questions. Hints are provided after some of the questions to support learners and guide their answers.

Answers

Answers to the questions in both levels of student sheets and on the lesson slides can be found in the teacher notes.