What it shows
A green leaf contains several pigments. Paper chromatography separates them because they travel up the paper at different speeds, and the ratio of distances, the Rf value, identifies each one.
The science
The paper is a stationary phase and the solvent is the mobile phase. Each pigment is attracted differently to the paper and dissolves differently in the solvent. A pigment that dissolves well and sticks little moves far. The Rf value is the distance moved by the pigment divided by the distance moved by the solvent, and it is always less than 1. Carotenes, which are orange and non-polar, usually travel the farthest, then the blue-green chlorophyll a and the yellow-green chlorophyll b, with the yellow xanthophylls often staying lowest. The exact order can change a little with the solvent and paper.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Fresh spinach or other leaves, 3 to 4
- Isopropyl rubbing alcohol (isopropanol) 70 per cent or higher, 50 ml
- Strips of filter paper or coffee filter, 2 cm by 15 cm
- A tall glass or jar and a pencil to hang the strip
- A coin or a spoon for crushing the leaf
- A ruler
- A small cup and clear tape
- A lid or foil to cover the jar
Safety
For this project: An adult must supervise. Rubbing alcohol is flammable and its fumes irritate. Work in a ventilated space with no flame, stove or candle nearby. Do not taste it and keep it away from children.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw a pencil line 2 cm from the bottom of the paper strip. Do not use ink, because ink would also travel.
Crush a leaf over the line by rolling a coin firmly along it so a dark green streak is left. Repeat five or six times on the same line, letting it dry in between, so the band is strong.
Pour alcohol into the jar to a depth of 1 cm. It must be below the pencil line.
Hang the strip from the pencil so that its end just touches the alcohol, and cover the jar to stop the vapour escaping.
Wait about 20 to 30 minutes until the solvent is near the top, and do not move the jar.
Remove the strip, mark the solvent front with a pencil straight away and let it dry.
Mark the middle of each coloured band and measure its distance from the start line.
Calculate Rf for each band by dividing its distance by the distance to the solvent front and match colours to the pigments.
What you should see
Several bands: orange-yellow near the top, then blue-green and yellow-green, and yellow lower down, each with a different Rf value.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is an Rf value?
The distance moved by the substance divided by the distance moved by the solvent.
2Why can the Rf value never be more than 1?
A substance cannot move farther than the solvent that carries it.
3Why was the start line drawn with a pencil?
Pencil graphite does not dissolve in the solvent, but ink would run.
4Which pigment usually travels the farthest?
Carotene, because it dissolves best in the solvent and sticks least to the paper.
Take it further
Compare pigments of a green leaf, a red amaranth leaf and a dry autumn leaf to see which pigments are missing.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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