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Science lesson plan · 8th grade (Year 9 in England) · 30 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 · 4 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 stones from soil? (sieving / filtering / evaporating / using a magnet) → sievingPrompt: Is anything dissolved? Is anything magnetic? Are the bits different sizes?
  2. Separating mixtures: which statement is NOT true? (A magnet can separate iron from sand. / Evaporation can get a dissolved solid back from a solution. / A magnet can pull salt out of a mixture. / Filtering can separate an insoluble solid from a liquid.) → 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 gravel from sand? (sieving / filtering / evaporating / using a magnet) → sievingPrompt: Is anything dissolved? Is anything magnetic? Are the bits different sizes?

Hook · 3 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 · 7 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 · 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. In a chromatogram the solvent moved 5 cm and a dye spot moved 3.8 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.) → 0.76Prompt: 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 makes the dyes move faster / Pencil (graphite) does not dissolve in the solvent / 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 · 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. Purity and chromatography: which statement is NOT true? (A pure substance has a fixed melting point. / In chromatography, a more soluble dye usually travels further. / Mixtures can be separated by physical methods. / Orange juice is a pure substance.) → Orange juice is a pure substance.Prompt: Think about what 'pure' means in chemistry.
  2. In a chromatogram the solvent moved 10 cm and a dye spot moved 2 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

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Pure substances, mixtures and chromatography

Name

Date

Score/ 6

  1. Question 1

    Purity and chromatography: which statement is NOT true?

    • A. In chromatography, a more soluble dye usually travels further.
    • B. Impurities have no effect on melting point.
    • C. Mixtures can be separated by physical methods.
    • D. Air is a mixture of gases.

  2. Question 2

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

    Answer

  3. Question 3

    In chemistry, what does "pure" mean?

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

  4. Question 4

    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

  5. Question 5

    In a chromatogram the solvent moved 12 cm and a dye spot moved 4.8 cm from the start line. What is the Rf value of the dye? (Give 2 decimal places.)

    Answer

  6. Question 6

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

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

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Answer key: Pure substances, mixtures and chromatography

  1. 1. B. Impurities have no effect on melting point.

  2. 2. 0.4

  3. 3. C. made of only one substance

  4. 4. 0.4

  5. 5. 0.4

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