| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Chemistry |
| Lesson Topic: State and use the equation for $R_{mathrm{f}}$ : $R_{mathrm{f}}=frac{ ext { distance travelled by substance }}{ ext { distance travelled by solvent }}$ |
Learning Objective/s:
- State the definition of the retention factor (Rf) and write its equation.
- Calculate Rf values from measured distances in paper chromatography.
- Explain how experimental conditions affect Rf values.
- Interpret Rf values to identify substances.
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Materials Needed:
- Chromatography paper strips
- Pencil and ruler
- Suitable solvent (e.g., organic solvent)
- Chromatography chamber (beaker with lid)
- Sample solutions (pigments/ink)
- Stopwatch
- Data‑table worksheet
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Introduction:
Begin with a quick demonstration of a coloured ink spot moving up a strip of paper as the solvent rises, asking students what they observe. Recall that chromatography separates mixtures based on differential affinity for stationary and mobile phases. Today students will learn to calculate the retention factor (Rf) and understand what influences its value.
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Lesson Structure:
- Do‑now (5'): Students answer a short question on what chromatography separates and why (check understanding).
- Mini‑lecture (10'): Explain the principle of paper chromatography and introduce the Rf equation with a labelled diagram.
- Guided lab setup (15'): Demonstrate drawing the origin line, spotting the sample, and placing the strip in the chamber; students set up their own strips.
- Run chromatography (10'): Allow solvent to travel, then mark the solvent front; teacher circulates for safety.
- Measurement & calculation (10'): Students measure Dsubstance and Dsolvent, compute Rf for each spot using the worksheet.
- Data analysis discussion (10'): Compare Rf values, discuss factors (solvent polarity, temperature, paper thickness) that affect them (formative questioning).
- Practice questions (5'): Students solve provided Rf problems individually, then review answers as a class.
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Conclusion:
Summarise that Rf is a simple ratio reflecting how far a component travels relative to the solvent front and that it is characteristic under set conditions. For the exit ticket, each student writes one factor that could change an Rf value on a sticky note. Homework: complete the worksheet with additional chromatography data and reflect on how solvent choice influences Rf.
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