Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Chemistry
Lesson Topic: Describe how to test for the purity of water using melting point and boiling point
Learning Objective/s:
  • Describe how melting (freezing) point and boiling point indicate water purity.
  • Explain the effect of dissolved solutes on freezing‑point depression and boiling‑point elevation.
  • Perform the laboratory procedures to determine the freezing and boiling points of water samples.
  • Analyse temperature data to infer the presence and approximate concentration of impurities.
  • Evaluate the limitations of using these colligative‑property measurements for water‑quality assessment.
Materials Needed:
  • Ice‑salt mixture and insulated container
  • Thermometer or digital temperature probe (±0.1 °C)
  • Beakers, hot plate (or Bunsen burner) with stand and clamp
  • Sealed thin‑walled glass tubes
  • Stopwatch
  • Distilled water (reference) and sample water
  • Safety goggles, lab coat, heat‑resistant gloves
  • Worksheet/data table for recording results
  • Projector for brief demonstration (optional)
 Introduction:
Begin with a quick question: “Why does sea water freeze at a lower temperature than tap water?” Connect this to prior learning about colligative properties. Explain that today students will use temperature measurements to assess water purity and will be able to predict impurity concentration. Success will be shown by accurate data collection and clear interpretation of results.
Lesson Structure:
  1. Do‑Now (5’) – Students answer a short worksheet on freezing‑point depression and boiling‑point elevation.
  2. Mini‑lecture & demonstration (10’) – Review pure‑water melting/boiling points, show diagram of the ice‑salt bath and boiling setup.
  3. Group lab: Freezing‑point determination (15’) – Set up ice‑salt bath, place sealed tube with sample, record the temperature at first ice formation; repeat with distilled water.
  4. Group lab: Boiling‑point determination (15’) – Heat 50 mL of sample on a hot plate, record the temperature when a steady boil is maintained for 30 s; repeat with distilled water.
  5. Data analysis & comparison (10’) – Complete the observation table, discuss deviations, optionally calculate molality using colligative formulas.
  6. Concept check (5’) – Exit‑ticket question: “How does a non‑volatile solute affect the boiling point of water and why?”
  7. Wrap‑up (5’) – Summarise key ideas and assign homework.
Conclusion:
Recap that lower freezing points and higher boiling points signal dissolved impurities and that the magnitude of the shift relates to concentration. Collect exit tickets to gauge understanding, and assign a short problem set calculating molality from given temperature changes. Encourage students to consider other methods for detecting microbial contamination.