• Quick retrieval from earlier learning (Pure substances, mixtures and chromatography). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. In a chromatogram the solvent moved 10 cm and a dye spot moved 5 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.) → 0.5Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent
2. How can you test whether a sample of water is pure? (Check that it boils at exactly 100 °C (at sea-level pressure) / Check that it looks clear / Shake it / Smell it) → Check that it boils at exactly 100 °C (at sea-level pressure)Prompt: Pure substances have a fixed melting point and boiling point.
3. Purity and chromatography: which statement is NOT true? (A pure substance has a fixed melting point. / An Rf value can be greater than 1. / Mixtures can be separated by physical methods. / In chromatography, a more soluble dye usually travels further.) → An Rf value can be greater than 1.Prompt: Think about what 'pure' means in chemistry.

• Pass round objects made of different materials and ask why each one was chosen.
• Share the objective: Calculate and use Rf values in chromatography. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: On a chromatogram the solvent front moved 12 cm from the start line and a spot moved 0.1 cm. What is the Rf value of the spot? Give 2 decimal places.
• Step 1: 0.1 ÷ 12 = 0.008
• Step 2: So the answer is: 0.01
• Answer: 0.01
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. On a chromatogram the solvent front moved 5 cm from the start line and a spot moved 0.5 cm. What is the Rf value of the spot? Give 2 decimal places. → 0.1Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent
2. A dye has an Rf value of 0.5 in water. If the solvent front moves 8 cm, how far will the dye move, in cm? → 4 cmPrompt: Rearrange Rf = spot distance ÷ solvent distance.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Reference Rf values in the same solvent: dye A 0.15, dye B 0.35, dye C 0.55, dye D 0.8. An unknown ink gives one spot that moves 1.75 cm when the solvent moves 5 cm. Which dye is it? (dye D / dye C / dye A / dye B) → dye BPrompt: Calculate the Rf of the unknown spot.
2. On a chromatogram the solvent front moved 5 cm from the start line and a spot moved 3.3 cm. What is the Rf value of the spot? Give 2 decimal places. → 0.66Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.