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Science lesson plan · 8th grade (Year 9 in England) · 45 minutes

Pure substances, mixtures and chromatography

Materials and their properties

CCSS NGSS.MS-PS1-2England: KS3 Chemistry: Pure and impure substancesEngland: KS4 Combined Science: Chemical analysis

Objective and success criteria

Students will: Pure substances, mixtures and 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: Separating mixtures

Leads to: Calculate and use Rf values in chromatography

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 · 5 min

  • Quick retrieval from earlier learning (Separating mixtures). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Which method is best to separate sand from water? (sieving / filtering / evaporating / using a magnet) → filteringPrompt: Is anything dissolved? Is anything magnetic? Are the bits different sizes?
  2. Separating mixtures: which statement is NOT true? (A magnet can pull salt out of a mixture. / A magnet can separate iron from sand. / Filtering can separate an insoluble solid from a liquid. / Evaporation can get a dissolved solid back from a solution.) → A magnet can pull salt out of a mixture.Prompt: Can a filter catch something that is dissolved?
  3. Which method is best to separate stones from soil? (sieving / filtering / evaporating / using a magnet) → sievingPrompt: Is anything dissolved? Is anything magnetic? Are the bits different sizes?

Hook · 5 min

  • Pass round objects made of different materials and ask why each one was chosen.
  • Share the objective: Pure substances, mixtures and chromatography. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: In a chromatogram the solvent moved 5 cm and a dye spot moved 1 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)
  • Step 1: 1 ÷ 5 = 0.2
  • Step 2: So the answer is: 0.2
  • Answer: 0.2
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 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. In a chromatogram the solvent moved 12 cm and a dye spot moved 2.4 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.) → 0.2Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent
  2. Why is the start line on chromatography paper drawn in pencil, not pen? (Pencil (graphite) does not dissolve in the solvent / Pencil makes the dyes move faster / Pen is too expensive / Pencil is easier to see) → Pencil (graphite) does not dissolve in the solventPrompt: What would happen to ink from a pen?

You do: independent practice · 10 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 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. Purity and chromatography: which statement is NOT true? (An Rf value can be greater than 1. / Air is a mixture of gases. / 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.
  2. In a chromatogram the solvent moved 12 cm and a dye spot moved 2.4 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.) → 0.2Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent

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: Separating mixtures.
  • 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

  • Move on to: Calculate and use Rf values in chromatography.
  • 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-8.

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

eurekami

Pure substances, mixtures and chromatography

Name

Date

Score/ 8

  1. Question 1

    In a chromatogram the solvent moved 10 cm and a dye spot moved 4 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)

    Answer

  2. Question 2

    An ink gives three separate spots on a chromatogram. What does this show?

    • A. The ink is insoluble
    • B. The paper was dirty
    • C. The ink is a mixture of at least three dyes
    • D. The ink is a pure substance

  3. Question 3

    Purity and chromatography: which statement is NOT true?

    • A. A pure substance has a fixed melting point.
    • B. Mixtures can be separated by physical methods.
    • C. In chromatography, a more soluble dye usually travels further.
    • D. Orange juice is a pure substance.

  4. Question 4

    In a chromatogram the solvent moved 5 cm and a dye spot moved 4 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)

    Answer

  5. Question 5

    In a chromatogram the solvent moved 10 cm and a dye spot moved 2.5 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)

    Answer

  6. Question 6

    In chemistry, what does "pure" mean?

    • A. clean and safe to drink
    • B. very expensive
    • C. made of only one substance
    • D. natural, with nothing added by people

  7. Question 7

    In a chromatogram the solvent moved 12 cm and a dye spot moved 2.4 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)

    Answer

  8. Question 8

    In a chromatogram the solvent moved 10 cm and a dye spot moved 8 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)

    Answer

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Answer key: Pure substances, mixtures and chromatography

  1. 1. 0.4

  2. 2. C. The ink is a mixture of at least three dyes

  3. 3. D. Orange juice is a pure substance.

  4. 4. 0.8

  5. 5. 0.25

  6. 6. C. made of only one substance

  7. 7. 0.2

  8. 8. 0.8