Year 7 Science · Unit 2 · Lesson 7
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Learning Goals
1. Paper chromatography has a mobile phase and a stationary phase. Name what each one is, and say which one moves.
Order the steps: distilling salt water
Number the steps from 1 (first) to 6 (last) to show how distillation turns salt water into pure water.
| Order (1–6) | Step |
|---|---|
| Pure water drips into Flask B. | |
| Steam leaves the flask and travels into the condenser. | |
| Heating stops before Flask A boils dry, leaving solid salt behind in it. | |
| Salt water is poured into Flask A. | |
| Cold water flowing around the condenser cools the steam back into a liquid. | |
| Flask A is heated until the water boils. |
Cause and effect: how the dyes separate
Fill the two empty boxes. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
2. Explain why an Rf value can be used like a fingerprint to identify a dye, and why two Rf values can only be compared if the same solvent was used.
Read the data: a lolly colouring
Scenario: which colouring mix?
A food scientist checks the green colouring in a batch of lollies. The supplier uses one of the three approved colouring mixes in Table 2. She runs paper chromatography on the colouring. The solvent moves 12.0 cm, and the colouring leaves three spots, at 10.2 cm, 5.4 cm and 2.4 cm from the start line.
| Dye A | Dye B | Dye C | Dye D |
|---|---|---|---|
| 0.85 | 0.62 | 0.45 | 0.20 |
Table 2. The supplier’s approved colouring mixes
| Mix 1 | Mix 2 | Mix 3 |
|---|---|---|
| Dyes A and C | Dyes A, C and D | Dyes B, C and D |
Source: made-up dyes and results for this worksheet.
3. Divide each spot’s distance by the distance the solvent moved. Show all three divisions.
Rf values (no unit)
4. Which dyes are in the colouring, and which mix did the supplier use? Explain why each of the other two mixes can be ruled out.
5. A food lab has two jobs. Job A: collect and weigh the ethanol-enriched distillate from an ethanol-water mixture. Job B: find out whether the blue colouring in an ice block contains a banned dye. Choose a technique for each job and explain how it works and why it suits the job. Then explain why chromatography could not do Job A.
Question 1
Mobile phase: the solvent, which moves up the paper (1). Stationary phase: the paper, which stays still (1). [2 marks]
Order the steps: distilling salt water
Reading down the table as printed, the correct step numbers are 5, 3, 6, 1, 4, 2. In order: pour salt water into Flask A; heat until it boils; steam enters the condenser; cold water turns it back to liquid; pure water drips into Flask B; stop heating before Flask A boils dry, leaving the salt.
Cause and effect: how the dyes separate
Box 1: the solvent soaks up the paper, dissolves the dyes and carries them up (1). Box 2: the dyes have different solubilities, so the more soluble ones travel further and the less soluble ones are held back by the paper (1). [2 marks]
Question 2
Each substance has its own Rf value in a given solvent, so a matching Rf identifies it (1). A different solvent dissolves each dye by a different amount, so the dyes travel different distances and the Rf values change (1). [2 marks]
Read the data: a lolly colouring
3. 10.2 ÷ 12.0 = 0.85 (1); 5.4 ÷ 12.0 = 0.45 (1); 2.4 ÷ 12.0 = 0.20 (1). [3 marks]
4. Dyes A, C and D, so Mix 2 (1). Not Mix 1: it has no Dye D, yet there is a spot at 0.20 (1). Not Mix 3: it has no Dye A (0.85) and would show Dye B at 0.62 (1). [3 marks]
Question 5
Marking guide: 1 Job A distillation and Job B chromatography; 1 ethanol is enriched in the vapour and distillate; 1 the condenser cools the ethanol-enriched vapour, which condenses to liquid in Flask B (it is not pure ethanol); 1 dyes travel different distances because of different solubilities; 1 matching the banned dye’s known Rf shows it is there; 1 chromatography leaves the parts on the paper, so there is nothing to collect and weigh. [6 marks]
Model answer: Job A: ordinary distillation. The distillate is enriched in ethanol and can be collected in Flask B to be weighed, but it is not pure ethanol because ethanol and water form an azeotrope. Job B: chromatography. Each dye has a different solubility, so it travels a different distance; a spot with the banned dye’s Rf (same solvent) shows it is there. Chromatography cannot do Job A: the parts stay on the paper, so nothing can be poured out and weighed.
Common error: dividing the solvent distance by the spot distance, giving an “Rf” bigger than 1. Rf is always between 0 and 1.