Lesson Plan

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Lesson Plan
Grade: Date: 25/02/2026
Subject: Chemistry
Lesson Topic: Interpret simple chromatograms to identify: (a) unknown substances by comparison with known substances (b) pure and impure substances
Learning Objective/s:
  • Describe the principle of paper chromatography and define stationary and mobile phases.
  • Calculate Rf values from measured distances.
  • Identify unknown substances by comparing Rf values with known standards.
  • Assess the purity of a sample by interpreting the number of spots on a chromatogram.
  • Evaluate experimental accuracy and suggest improvements.
Materials Needed:
  • Projector and screen
  • Filter paper strips and capillary tubes
  • Solvent mixture (ethanol : water 1:1) and beakers for chromatography chambers
  • Ruler, pencil, and safety goggles
  • Worksheets with data tables and sample substances (caffeine, theobromine, phenol, etc.)
 Introduction:
Begin with a short video showing coloured inks separating on a paper strip to spark curiosity. Review the concepts of stationary and mobile phases that were covered in the previous lesson. Explain that today students will calculate Rf values, use them to identify unknowns, and determine sample purity.
Lesson Structure:
  1. Do‑now (5') – Quick quiz on key terminology (stationary phase, mobile phase, Rf).
  2. Mini‑lecture (10') – Explain Rf calculation and interpretation using the phenol/unknown example.
  3. Demonstration (10') – Teacher runs a paper chromatography with a known standard (caffeine) while students observe.
  4. Guided practice (15') – In pairs, students set up their own strips with unknown samples, record distances, and calculate Rf values.
  5. Data analysis (10') – Groups compare their Rf values with a provided standards table to identify substances and discuss purity.
  6. Check for understanding (5') – Exit ticket: write one sentence describing how to tell if a sample is pure.
  7. Homework briefing (5') – Assign a worksheet with additional chromatogram problems.
Conclusion:
Recap how Rf values are calculated and used to identify unknowns and assess purity. Collect exit tickets to gauge understanding, and remind students to complete the worksheet for homework, which reinforces the concepts with new data sets.