IMSO 2009 Science International - Experimental Examination

**The Secret of Color Separation**

**Original introduction.** Colors can be separated by chromatography. Scientists use chromatography to separate molecules of different masses. In coloring materials, molecules with different masses may have different colors.

Plants have various separable pigments that generate the colors of their leaves and flowers. They may have carotene (orange), xanthophyll (yellow), chlorophyll (green) and anthocyanin (red) pigments. The pigments can be separated by chromatography. This method needs suitable paper as a separation medium and a liquid to dissolve the pigments. An everyday example is food coloring.

A quantitative indication of how far a compound travels in a particular solvent is called the retention factor $R_f$:

$$R_f=\frac{D_1}{D_2}.$$

$D_1$ is the distance the color travels, measured from the point where it was placed to the center of its band. $D_2$ is the total distance traveled by the solvent, measured from the same starting line.



Diagram labels: Start, End, Solvent Front; $D_1$, $D_2$.

You are asked to design experiments to determine the components of marker ink, food coloring and leaves, and calculate their $R_f$ values. The following materials may be needed:

| Material | Quantity |
| --- | --- |
| 70% alcohol | 1 bottle |
| Water | 1 bottle |
| Plastic boxes | 2 |
| Paper clips | 10 pieces |
| Experiment guideline | 1 plastic bag (scrap paper and green marker) |
| Different types of paper | 4 types: rough filter paper; smooth filter paper; scrap paper; copy/HVS paper |
| Spinach leaves | Green and red |
| Food coloring | 3 colors: yellow, green and red |
| Ruler | 1 |
| Watch | 1 |
| Medium plastic bags | 10 |
| Labels | 1 packet |
| Palette (source: “pallet”) | 1 |
| Towel | 1 |
| Water bucket | 1 |
| Toothpicks | 1 packet |
| Coins | 2 |
| Pipettes | 3 |
| Supporting aluminum bars | 2 |

**Experiment 1**

1. Open the experiment guideline.
2. Make a dot with a marker on one piece of scrap paper, 2 cm from the bottom.
3. Prepare a plastic box and fill it with water.
4. Hang the paper with approximately its bottom 1 cm immersed in water. After about 5 minutes, take out the paper, observe it and record your observations.

**Editorial safety note for all experiments:** Work under a teacher’s supervision. Keep 70% alcohol away from flames, sparks and heat; use small supervised quantities with ventilation. Do not taste the materials, avoid eye contact, wash hands, and follow the teacher’s disposal instructions. The leaf species and alcohol type are not specified in the source. Do not substitute an unknown chemical.

**Experiment 2: Comparing four papers**

1. Design an experiment showing separation differences among the four paper types listed in Experiment 1. Draw the setup on the answer sheet.
2. Carry out the experiment using your design. Hang and suspend the paper.
3. After about 10 minutes, remove the papers, observe them and record the results.

Based on your experiment, which paper performs best at separating the colors? Explain.

4. When finished, place the papers into plastic bags. Label both each paper and its bag. Make sure the papers are dry before putting them in the bags.

Use the common materials and safety notes given in Experiment 1.

**Experiment 3: Food-color mixtures**

Always use the paper that gave the best result in the previous experiment.

1. Use the set of food colorings. Choose primary colors and make one secondary color and one tertiary color. On the answer sheet, make a table of the colors used and the resulting colors. Make the tertiary color by mixing one secondary color and one primary color.
2. Observe chromatography of one primary, one secondary and one tertiary color until their movement stops, approximately 10 minutes. Dry the paper for approximately 10 minutes.
3. From your results, record the colors of the bands and calculate $R_f$ for every band. Make sure the paper is dry before calculating.
4. Analyze your $R_f$ data. Are there significant differences between the $R_f$ values of the same color in different bands?
5. After finishing, place the papers into plastic bags and label every paper. Make sure the papers are dry before putting them into the bags.

Use the common materials and safety notes from Experiment 1.

**Experiment 4: Solvents and leaf pigments**

1. Design an experiment showing that chromatography results are influenced by the type of solvent.
2. Draw the design on the answer sheet and carry out the experiment using the red and green “spinach” leaves.
3. Scrape the leaf surfaces to release pigments, using a coin if necessary. Identify every pigment found in the leaves. Allow approximately 15 minutes for results.
4. Calculate their $R_f$ values and record the results on the answer sheet.
5. When finished, place the paper results into plastic bags and label them correctly. Make sure the papers are dry before bagging.
6. Answer from your experiment:

a) Which solvent is better for leaf chromatography?

b) Is the pigment order similar for different leaves?

c) Write the pigment order for each leaf, beginning at the bottom of the paper immersed in solvent.

d) Do the same colors from different leaves have the same $R_f$ values? Write the $R_f$ for every pigment.

Use the materials and safety notes from Experiment 1, especially the precautions for 70% alcohol. The original source does not identify the plant species.

Write all conclusions based on all your experimental results.