Year 7 Science · Unit 2 · Lesson 7
Challenge Worksheet
Learning Goals
Find the error
A student wrote these five sentences in a lab report. Some are correct and some contain one error. Write “correct”, or rewrite the sentence so it is correct.
| # | Student’s sentence | Correct, or the fixed sentence |
|---|---|---|
| 1 | The thermometer bulb sits in the boiling liquid at the bottom of Flask A. | |
| 2 | Cold water enters the condenser at its lower end and leaves at its upper end. | |
| 3 | The liquid with the higher boiling point is the first to collect in Flask B. | |
| 4 | An Rf value is the distance the solvent moved divided by the distance the spot moved. | |
| 5 | The dye that is most soluble in the solvent travels furthest up the paper. |
Read the data: watching the thermometer
Scenario: distilling ethanol and water
Ethan distils a mixture of ethanol and water. This is a simplified classroom data set, not actual ethanol-water distillation data: the readings show an ethanol-enriched distillate first and a changing vapour mixture later. He reads the thermometer at the top of Flask A every two minutes. A temperature reading cannot show that the ethanol is pure.
| Time (min) | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 |
|---|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) | 21 | 48 | 72 | 78 | 78 | 78 | 78 | 88 | 100 | 100 |
Source: made-up results for this worksheet.
1. Between which times was the distillate enriched in ethanol? Explain what the simplified Table 1 suggests.
2. Ethan wants to collect an ethanol-enriched distillate first, then a separate fraction. At about what time should he swap Flask B for an empty flask, and what does the rising temperature suggest? Could a thermometer show that the first fraction is pure ethanol? What would the new flask collect if he kept heating?
Read the data: the note at the break-in
Scenario: whose pen?
Police find a handwritten note at a break-in. A forensic scientist runs paper chromatography on ink from the note, using water as the solvent: the water moves 8.0 cm and the ink leaves two spots, at 2.0 cm and 5.6 cm. She also tests three pens taken from suspects. A second lab tested a fourth pen, S, but used ethanol as its solvent.
| Pen | Solvent | Rf values of its spots |
|---|---|---|
| P | water | 0.25 and 0.70 |
| Q | water | 0.25, 0.55 and 0.70 |
| R | water | 0.40 and 0.70 |
| S | ethanol | 0.25 and 0.70 |
Source: made-up case for this worksheet.
3. What are the Rf values of the note’s two spots? Show your working.
Rf values (no unit)
4. Which of pens P, Q and R has the same ink as the note? Explain why each of the other two is ruled out.
5. Pen S also gave Rf values of 0.25 and 0.70. Explain why this is not evidence that pen S has the same ink as the note, and what the scientist should do about it.
6. A news report says: “Chromatography has proved that the owner of pen P wrote the note.” Evaluate this claim. Use the results in Table 2 and what chromatography can and cannot show.
Find the error
1. The bulb sits at the top of Flask A, where the vapour leaves, so it reads the vapour’s temperature. 2. Correct. 3. The liquid with the lower boiling point collects first. 4. Rf is the spot distance divided by the solvent distance (so it is never more than 1). 5. Correct. [5 marks: 1 per sentence]
Read the data: watching the thermometer
1. From about 6 to 12 minutes (1). In this simplified model, the temperature stays near ethanol's lower boiling point, suggesting the distillate is enriched in ethanol (1). [2 marks]
2. At about 12–14 minutes, as the reading rises above 78 °C (1). This suggests the vapour composition is changing and the distillate is becoming less ethanol-rich; it does not prove that all ethanol has boiled off or that pure ethanol was collected (1). The new flask would collect a mixture increasingly enriched in water as heating continues (1). [3 marks]
Read the data: the note at the break-in
3. 2.0 ÷ 8.0 = 0.25 (1); 5.6 ÷ 8.0 = 0.70 (1). [2 marks]
4. Pen P: the same two Rf values, 0.25 and 0.70, in the same solvent (1). Q has an extra dye (0.55) that the note does not have (1). R has a dye at 0.40 and none at 0.25, so one of its dyes is different (1). [3 marks]
5. Rf values can only be compared in the same solvent; in ethanol the dyes dissolve differently, so the same numbers could belong to different dyes (1). Test pen S again with water as the solvent (1). [2 marks]
Question 6
Marking guide: 1 position (the claim goes too far); 1 matching Rf values (0.25 and 0.70 in water) show pen P’s ink has the same dyes as the note; 1 so pen P could have written it, and Q and R did not; 1 other pens of the same brand contain the same ink; 1 chromatography tests the ink, not the writer: anyone could have used pen P; 1 other evidence, for example testing more pens of that brand, fingerprints or handwriting. [6 marks]
Model answer: The claim goes too far. The note’s spots (Rf 0.25 and 0.70 in water) match pen P, so pen P’s ink has the same dyes, and pens Q and R can be ruled out. But every pen of that brand may hold the same ink, and chromatography identifies the ink, not the person holding the pen. The police need other evidence, such as fingerprints on the note or handwriting, before saying who wrote it.
Common error: comparing Rf values measured in different solvents, as with pen S.