Food dye chromatography is a simple way to discover that one visible color may be a mixture of several different pigments. In this project, a small spot of food dye is placed near the bottom of chromatography paper, and a solvent such as water or rubbing alcohol moves upward through the paper. As the solvent climbs, it carries the dye molecules at different speeds, creating separated color bands.
This makes chromatography useful for comparing dyes, testing mixtures, and understanding how scientists separate substances in real labs.
The separation happens because each dye molecule interacts differently with the paper and the solvent. A dye that dissolves well in the solvent and sticks weakly to the paper travels farther, while a dye that sticks strongly to the paper travels a shorter distance. Students can measure these distances and calculate the Rf value, which helps identify and compare pigments.
Changing variables such as dye type, solvent type, paper type, or starting spot size can change the final chromatogram.
Key Facts
- Paper chromatography separates mixtures based on how strongly substances are attracted to the paper and the solvent.
- Rf = distance traveled by dye band / distance traveled by solvent front.
- The solvent front is the highest point reached by the solvent on the chromatography paper.
- A larger Rf value means the dye traveled farther relative to the solvent front.
- The starting dye spot must be above the solvent level so it does not dissolve directly into the beaker.
- Different solvents, such as water and alcohol, can separate the same food dye mixture in different ways.
Vocabulary
- Chromatography
- A laboratory technique used to separate the parts of a mixture based on how they move through a material with a solvent.
- Solvent
- A liquid that dissolves and carries substances through the chromatography paper.
- Solvent front
- The farthest point reached by the solvent as it moves up the paper.
- Rf value
- A ratio that compares how far a dye traveled to how far the solvent traveled.
- Chromatogram
- The final pattern of separated spots or bands left on the chromatography paper.
Common Mistakes to Avoid
- Putting the dye spot below the solvent level is wrong because the sample can wash off into the beaker instead of traveling up the paper.
- Using a very large dye spot is wrong because it can smear and make the separated bands hard to measure.
- Forgetting to mark the solvent front immediately is wrong because the solvent can evaporate and make the Rf calculation inaccurate.
- Comparing Rf values from different solvents as if they are identical is wrong because the solvent changes how far each dye travels.
Practice Questions
- 1 A blue dye band travels 4.2 cm from the starting line, and the solvent front travels 7.0 cm. Calculate the Rf value.
- 2 In a paper chromatography test, the solvent front moves 8.5 cm. A red band has an Rf value of 0.60. How far did the red band travel from the starting line?
- 3 A green food dye separates into yellow and blue bands in water, but only one wide green band appears in alcohol. Explain what this suggests about how the dyes interact with the two solvents and the paper.