Exploring Mixtures and their Separation · Lesson 7 of 13
Paper Chromatography
“Black ink reveals that it has been a tiny colour crowd all along.”
• Describe the paper chromatography procedure. • Explain why a pencil line is used. • Keep the sample spot above the initial solvent level. • Explain separation through different movement rates. • Apply the method to inks, food colours and plant pigments.
A drop of water falling on a black sketch-pen line may spread into blue, purple or other colours. What looked like one colour was actually a mixture. Paper chromatography allows the hidden components to travel at different rates until they become visible as separate spots.
Paper chromatography is a method that separates soluble components because they interact differently with the moving solvent and the paper and therefore travel at different rates.
Activity: Let Us Investigate
Cut a strip of chromatographic or filter paper about 3 cm wide. Draw a horizontal pencil line about 2 cm from its lower end and place a small black-ink spot at the centre of the line. Pour a shallow layer of water into a tall container.
Suspend the strip vertically so that its lower edge touches the water while the ink spot remains above the water level. Cover the container and watch the water rise through the paper. Remove the strip before the solvent reaches the top and allow it to dry.
| Choice | Reason |
|---|---|
| Pencil starting line | Graphite does not dissolve and travel like many inks |
| Small concentrated sample spot | Produces clearer separated spots |
| Sample above solvent level | Prevents the sample from dissolving directly into the solvent pool |
| Vertical paper | Allows the solvent to move upward evenly |
| Suitable solvent | Must dissolve and carry the components being investigated |
The solvent rises through tiny spaces in the paper. Soluble components are carried with it, but each component has a different balance of attraction to the solvent and attraction to the paper. A component that moves readily with the solvent travels farther; one held more strongly by the paper travels less.
Problem
A chromatogram shows yellow, blue and violet spots after using black ink. What can be inferred?
- 1.The original black spot did not remain as one substance.
- 2.Three differently coloured regions travelled to different heights.
- 3.The different movement rates show that the ink contains at least three coloured components.
- 4.The result identifies components qualitatively; it does not by itself give their exact amounts.
Problem
What goes wrong if the initial ink spot is below the solvent level?
- 1.The ink dissolves directly into the solvent at the bottom.
- 2.It can spread through the container instead of moving from one fixed starting point.
- 3.Separate narrow bands are unlikely to form clearly.
- 4.The experiment should be repeated with the solvent level below the pencil line.
Threads of Curiosity
The name chromatography comes from words meaning colour and writing because the method was first used with coloured substances. The principle is not limited to black ink. Green food colour can be tested using a dilute salt solution as solvent, and extracts of spinach, flower petals or coloured leaves can reveal several pigments.
Water does not dissolve every pigment. In some investigations, alcohol or a suitable solvent mixture is needed. The solvent should be chosen for the particular substances and used only with appropriate safety guidance.
Pause and Ponder
Problem
Evaluate the claim: “The solvent level should begin above the sample spot.”
- 1.The claim is false.
- 2.A submerged spot dissolves into the solvent pool.
- 3.For separation on paper, the spot must remain above the initial solvent level.
- 4.Only the lower edge of the paper should dip into the solvent.
Quiz
Which description best matches Paper Chromatography?
Which term matches this description: Paper chromatography is a method that separates soluble components because they interact differently with the moving solvent and the paper and therefore travel at different rates.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Why is a pen unsuitable for drawing the starting line?
- Explain why different pigment spots appear at different heights.
- What would you change if water did not carry the pigments?
- Design a fair comparison of pigments in green and red leaves.
Key Takeaways
• Paper chromatography separates components by different movement rates. • The sample begins on a pencil line above the solvent. • The solvent carries soluble components upward. • Interactions with paper and solvent determine how far each component moves. • A suitable solvent is essential for a clear separation.