Chalk Chromatography


Chromatography is an analytical technique which uses the principle of selective adsorption. In this method, a mixture of substances is allowed to pass through a solid that will act as an adsorbent (chalk in the case). some substances in the mixture are more strongly adsorbed on the chalk than others. As they solvent moves up, the more strongly adsorbed substances are held back and thus travel slower while the less strongly adsorbed move on ahead oft hem. In this way, separation of colours occurs. The less strongly adsorbed substances will appear as bands above the more strongly adsorbed ones. In this experiment, chalk chromatography is used to separate the green pigment will be separated in to 2 blotches, a green blotch (chlorophyll) and yellow blotch (xanthophyll)

Chalk Chromaatography



Plain white chalk

Small petri dish



1. Cup up a quantity of green grass in to small pieces with scissors

2. Grind the pieces with ethanol to produce 2 to 3 cm3 of liquid in a mortar. This is to extract the green pigment in the leaves. Make the extract as concentrated as possible

3. Decant the green liquid in to small dish and stand a piece of chalk in the extract

4. The liquid will be drawn up the chalk and coloured bands of the various components will be formed on the chalk. What are colours of these bands? what you can conclude about the “green” in grass

Further Investigations

1. Carry out chalk chromatography on different types of coloured leaves and flowers. Compare their chromatograms

2. Carry out an investigation into food colours used in sweets. Take a half a dozen brown Smarties and place them in dish. Add 2 to 3 cm3 of water to get dark brown extract

3. Experiment with different types of commercial food colouring, ballpoint ink, indicators (ex universal indicator solution) and dyes. The chalk is dotted with the colour to be separated about 1 cm from its base. When the colour dot is dry, add anotherdot on the same spot. Stand the chalk in a small dish of the ethanol (make sure that the ethanol level does not touch the dot

4. Try other solvents like acetone, water, methylated spirit and a solvent system made up of n- butanol ( 3 volumes), ammonia 2 M (1 volume). compare the chromatograms obtained using the different solvents

5. Use different types of chalk. Will all them give the same separations

6. How will the result compare if paper is used as an adsorbent? Cut a small piece of filter paper in to small  spiral. The edges must not touch. Add a drop of food colouring to the points marked *. Placethe spiral on the top of beaker. Add a drop of water (or ethanol) to the each of the points *. When the water has spread, add another drop. Continue until separation is a completed. Which gives a better separation (chalk or paper) for a given colour in the same solvent?

a.Alternatively, you can support the spiral on 2 glass rods placed over a petri dish. Dip the ends of the spiral into water. The water level must not touch the spots of colour

b. Different shapes can also be used

7. Experiment with different grades of paper (toilet paper, Whatman’s filter paper). How do the separations compare

8. Examine the chromatograms under UV light. Are there any changes

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