Year 7 Science · Unit 2 · Lesson 9

Distillation and Chromatography

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Learning Goals

Warm Up

Because… chain

Fill in the missing effects. Each cause leads to the next step in separating ink dyes by paper chromatography.

Ink contains multiple dye molecules dissolved together
→
A solvent soaks up the paper and begins to move
→
Different dyes have different solubilities in the solvent
→
Each dye stops at a different height and leaves a visible spot
→

Overall outcome:

Your Turn

Scenario

A forensic scientist at the Australian Federal Police lab is analysing an unknown ink sample from a document used in a fraud case. She runs paper chromatography and measures the distance each dye spot travels. The solvent front travels 10.0 cm. The table below shows her results alongside the known Rf values for four standard dyes in the same solvent.

DyeKnown Rf value
Dye A (cyan)0.85
Dye B (yellow)0.62
Dye C (magenta)0.45
Dye D (black pigment)0.22

The unknown ink produced spots at: 8.5 cm, 4.5 cm and 2.2 cm from the baseline.

(a) Calculate the Rf value for each spot in the unknown ink. Show your working for at least one calculation.

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(b) Which dyes are present in the unknown ink sample? Predict which standard dye pen could have made this mark. Justify your answer using the Rf values.

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Show What You Know

1. Explain why Rf values are useful for identifying unknown substances. Why must the same solvent always be used when comparing Rf values?

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2. Distillation and chromatography both separate mixtures, but they work very differently. Name one type of mixture that distillation is better for, and one type that chromatography is better for. Explain why each technique suits its chosen mixture.

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Wrap Up

In one sentence, what was the main idea of this lesson?

HSCScience · Year 7 Science · Unit 2 · Lesson 9 · Apply Worksheet