Lesson Plan

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Lesson Plan
Grade: Date: 25/02/2026
Subject: Chemistry
Lesson Topic: Explain in terms of structure how alloys can be harder and stronger than the pure metals because the different sized atoms in alloys mean the layers can no longer slide over each other
Learning Objective/s:
  • Describe the crystal lattice of pure metals and how slip planes enable deformation.
  • Explain how substitutional and interstitial alloying atoms create lattice strain that impedes dislocation motion.
  • Apply the equation τcritical = τ0 + k c ε to predict the effect of solute concentration and size mismatch on hardness and strength.
  • Compare the mechanical properties of pure metals versus alloys using a structured table.
Materials Needed:
  • Projector or interactive whiteboard for diagrams
  • Printed worksheet with lattice diagrams and comparison table
  • Ball‑and‑stick model kits to build pure‑metal and alloy lattices
  • Sample alloy specimens (e.g., steel rod, brass piece)
  • Whiteboard and markers
  • Exit‑ticket cards for the final check
 Introduction:
Begin with a quick demonstration: show a steel nail and a copper wire, asking students which they think would be harder to bend and why. Recall prior knowledge of metallic bonding and crystal lattices, reminding learners that slip planes allow layers to slide in pure metals. Explain that today’s success criteria are to describe how different‑sized atoms disrupt these slip planes and to use the τ‑critical equation to explain increased hardness.
Lesson Structure:
  1. Do‑now (5'): Students answer a short question on slip planes on a worksheet.
  2. Mini‑lecture (10'): Review crystal structures and introduce substitutional vs interstitial alloying with diagrams.
  3. Interactive modelling (10'): In groups, students build pure‑metal and alloy lattices using ball‑and‑stick kits, observing distortion.
  4. Equation exploration (8'): Derive and discuss τcritical = τ0 + k c ε, using sample values.
  5. Real‑world examples (7'): Discuss bronze, brass, steel and their applications; fill the comparison table.
  6. Check for understanding (5'): Quick quiz via Kahoot or show of hands.
  7. Exit ticket (5'): Write one sentence summarising how atom size mismatch increases strength.
Conclusion:
Summarise that alloying introduces lattice strain which blocks dislocation motion, raising the shear stress needed for slip and thus hardness and strength. Prompt students to reflect on the exit‑ticket answer and clarify any misconceptions. Assign homework to research another alloy and calculate its expected τcritical using given concentration and size‑misfit values. End with a reminder of the next lesson on heat treatment of alloys.