Science lesson plan · 8th grade (Year 9 in England) · 60 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 · 6 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.
- Which method is best to separate steel paper clips from plastic beads? (sieving / filtering / evaporating / using a magnet) → using a magnetPrompt: Is anything dissolved? Is anything magnetic? Are the bits different sizes?
- Separating mixtures: which statement is NOT true? (Filtering can separate an insoluble solid from a liquid. / A magnet can separate iron from sand. / Distillation can get pure water from salt water. / A magnet can pull salt out of a mixture.) → A magnet can pull salt out of a mixture.Prompt: Can a filter catch something that is dissolved?
- Which method is best to separate sugar from sugar solution? (sieving / filtering / evaporating / using a magnet) → evaporatingPrompt: Is anything dissolved? Is anything magnetic? Are the bits different sizes?
Hook · 6 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 · 12 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 · 12 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- 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.) → 0.4Prompt: Rf = distance moved by the spot ÷ distance moved by the solvent
- An ink gives three separate spots on a chromatogram. What does this show? (The ink is a pure substance / The ink is insoluble / The ink is a mixture of at least three dyes / The paper was dirty) → The ink is a mixture of at least three dyesPrompt: Each different dye moves a different distance.
You do: independent practice · 17 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 · 7 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Why is the start line on chromatography paper drawn in pencil, not pen? (Pen is too expensive / Pencil (graphite) does not dissolve in the solvent / Pencil is easier to see / Pencil makes the dyes move faster) → Pencil (graphite) does not dissolve in the solventPrompt: What would happen to ink from a pen?
- Purity and chromatography: which statement is NOT true? (A pure substance has a fixed melting point. / Mixtures can be separated by physical methods. / In chromatography, a more soluble dye usually travels further. / An Rf value can be greater than 1.) → An Rf value can be greater than 1.Prompt: Think about what 'pure' means in chemistry.
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