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Science lesson plan · 9th grade (Year 10 in England) · 30 minutes

Calculate and use Rf values in chromatography

Materials and their properties

CCSS NGSS.HS-PS1-3CCSS NGSS.SEP 5England: GCSE Combined Science: Chemical analysis (required practical: chromatography)

Objective and success criteria

Students will: Calculate and use Rf values in chromatography.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Pure substances, mixtures and chromatography

Leads to: the next skill in this area.

Materials: A tray of everyday objects, Sorting hoops, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 4 min

  • 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 12 cm and a dye spot moved 6 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. An ink gives three separate spots on a chromatogram. What does this show? (The paper was dirty / The ink is a mixture of at least three dyes / The ink is insoluble / The ink is a pure substance) → The ink is a mixture of at least three dyesPrompt: Each different dye moves a different distance.
  3. Purity and chromatography: which statement is NOT true? (Mixtures can be separated by physical methods. / A pure substance has a fixed melting point. / Air is a mixture of gases. / An Rf value can be greater than 1.) → An Rf value can be greater than 1.Prompt: Think about what 'pure' means in chemistry.

Hook · 3 min

  • 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.

I do: model it · 7 min

  • 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.

We do: guided practice · 6 min

  • 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 12 cm from the start line and a spot moved 0.6 cm. What is the Rf value of the spot? Give 2 decimal places. → 0.05Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent
  2. A dye has an Rf value of 0.8 in water. If the solvent front moves 6 cm, how far will the dye move, in cm? → 4.8 cmPrompt: Rearrange Rf = spot distance ÷ solvent distance.

You do: independent practice · 7 min

  • 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. On a chromatogram the solvent front moved 10 cm from the start line and a spot moved 5.1 cm. What is the Rf value of the spot? Give 2 decimal places. → 0.51Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent
  2. A dye has an Rf value of 0.2 in water. If the solvent front moves 4 cm, how far will the dye move, in cm? → 0.8 cmPrompt: Rearrange Rf = spot distance ÷ solvent distance.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Pure substances, mixtures and chromatography.
  • Use concrete materials: a tray of everyday objects.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

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

Free lesson plan from eurekami.com/lesson-plan-builder

eurekami

Calculate and use Rf values in chromatography

Name

Date

Score/ 6

  1. Question 1

    On a chromatogram the solvent front moved 6 cm from the start line and a spot moved 1.4 cm. What is the Rf value of the spot? Give 2 decimal places.

    Answer

  2. Question 2

    A dye has an Rf value of 0.45 in water. If the solvent front moves 12 cm, how far will the dye move, in cm?

    Answercm

  3. Question 3

    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 2.75 cm when the solvent moves 5 cm. Which dye is it?

    • A. dye C
    • B. dye D
    • C. dye B
    • D. dye A

  4. Question 4

    On a chromatogram the solvent front moved 6 cm from the start line and a spot moved 0.9 cm. What is the Rf value of the spot? Give 2 decimal places.

    Answer

  5. Question 5

    A dye has an Rf value of 0.5 in water. If the solvent front moves 12 cm, how far will the dye move, in cm?

    Answercm

  6. Question 6

    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 3 cm when the solvent moves 20 cm. Which dye is it?

    • A. dye A
    • B. dye D
    • C. dye C
    • D. dye B

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Answer key: Calculate and use Rf values in chromatography

  1. 1. 0.23

  2. 2. 5.4 cm

  3. 3. A. dye C

  4. 4. 0.15

  5. 5. 6 cm

  6. 6. A. dye A