Explicit science instruction
In paper chromatography, components move differently because they partition differently between a mobile phase and a stationary phase. Stronger relative attraction to the mobile phase generally increases travel distance under the model.
An Rf-style value is distance traveled by solute divided by distance traveled by solvent front. It is a ratio, so both distances must use the same units.
Rf values can support comparison only when experimental conditions are comparable; changing solvent, paper, temperature, or measurement method can shift the result.
Vocabulary: chromatography; stationary phase; mobile phase; retention; Rf.
Worked example with reasoning
A spot travels 3.0 cm while the solvent front travels 6.0 cm. Rf=3.0/6.0=0.50. A second spot with Rf=0.80 traveled farther relative to the same front.
Guided question
A solute travels 4.2 cm and the solvent front 7.0 cm. Calculate Rf and interpret what the ratio means.
Independent work
- Calculate Rf for four supplied spots.
- Rank spots by relative retention.
- Explain why Rf values from different solvent systems should not be compared casually.
Misconception check
Rf is not a universal identity number for a compound; it depends on conditions.
Formative understanding check
What does an Rf of 0.60 mean geometrically?
Reveal response
The solute traveled 60% as far as the solvent front under that set of conditions.
Mentor evidence conference
Ask Archaemenes to separate direct analytical evidence from inference and audit the uncertainty statement.
Teacher guidance
Foundation: Use a distance-ratio organizer.
Core: Calculate and interpret chromatographic ratios.
Extended: Analyze how measurement uncertainty near the origin or solvent front affects Rf.