Paper Chromatography: Separating Colours 🎨
What is Paper Chromatography?
Paper chromatography is a simple, low‑cost technique that separates a mixture of soluble coloured substances on a strip of paper. Think of it as a colour‑sorting game where each colour moves at its own pace, revealing the hidden components of a mixture.
Analogy: Imagine you have a bag of mixed marbles (the mixture). If you slide the bag across a smooth floor (the paper) while a gentle breeze (the solvent) pushes it, the lighter marbles will travel further than the heavier ones. Paper chromatography works on a similar principle, but with molecules instead of marbles.
Materials & Setup
- Whatman paper or any thin, white filter paper
- Solvent (mobile phase) – e.g., a mixture of water, ethanol, and acetone (common for coloured inks)
- Glass or plastic tray (to hold the solvent)
- Capillary tube or a small dropper
- Marker or pencil for drawing a baseline
- Ruler (to measure distances)
- Safety goggles and gloves (always wear protective gear!)
Procedure
- Draw a horizontal line 1–2 cm from the bottom of the paper using a pencil.
- Using a capillary tube, apply a tiny spot of the mixture (≈ 0.5 mm) on the line.
- Place the paper in the tray so that the spot is above the solvent level (do not let the spot touch the solvent).
- Cover the tray to prevent evaporation and let the solvent rise by capillary action.
- When the solvent front has travelled about 8–10 cm, remove the paper and let it dry.
- Measure the distance from the baseline to each coloured spot and to the solvent front.
- Calculate the Rf value for each spot:
Formula: \$R_f = \frac{distance\; travelled\; by\; solute}{distance\; travelled\; by\; solvent}\$
Example: If a spot travelled 4 cm and the solvent front travelled 10 cm, \$R_f = \frac{4}{10} = 0.40\$.
Theory Behind Separation
In paper chromatography, the separation is governed by two competing processes:
- Adsorption – molecules stick to the paper (which is mainly cellulose). Molecules that bind strongly move slowly.
- Solubility in the solvent – molecules that dissolve well in the solvent travel faster.
Each component of the mixture has a unique balance of these two properties, giving it a distinct Rf value. The higher the Rf, the more soluble the component in the solvent and the less adsorbed on the paper.
Example: Separating Ink Components
Suppose you have a pen ink that contains three coloured dyes: blue, red, and yellow. After running paper chromatography:
| Dye | Rf Value |
|---|
| Blue | 0.65 |
| Red | 0.45 |
| Yellow | 0.30 |
Here, blue travels farthest because it is most soluble in the solvent, while yellow stays closest to the baseline due to stronger adsorption.
Exam Tips for IGCSE Chemistry 0620
Remember:
- Define mobile phase (solvent) and stationary phase (paper).
- Explain the Rf value and how it is calculated.
- Use the analogy of marbles to illustrate the balance between adsorption and solubility.
- Show a simple diagram (drawn with a pencil) of the paper, solvent front, and spots.
- Include a table of Rf values for the components you studied.
- Answer the question: “Why does a component with a higher Rf value appear farther from the baseline?” – because it is more soluble in the solvent.
Use clear, concise language and label all parts of your diagram. Good luck! 🍀