Colourless chromatography and retention-factor calculations
Explain why separated colourless substances need a locating agent, then calculate dimensionless Rf values from the baseline and marked solvent front.
Extended only (Supplement) · Chemistry 0620, 2026–2028. The diagrams and supplied data support supervised laboratory learning.
Before you begin
- Interpret a simple chromatogram using standards.
- Measure a distance from a specified origin and form a ratio.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the locating agent's role for colourless solutes?
What you will learn
- Describe separation of soluble colourless substances using a suitable solvent and locating agent.
- Calculate and interpret Rf with shared starting baseline and conditions.
Invisible does not mean absent
Colourless solutes can separate on paper even when their spots are not initially visible. A suitable locating agent reveals their positions after separation, according to the chosen supervised method. The solvent separates; the locating agent allows detection. This syllabus requires the role of a locating agent, rather than memorising particular named agents.
The supplied coloured diagram illustrates spot positions. A colourless version would need detection before those positions could be read. Lack of a visible untreated spot does not establish that no substance travelled. The starting spots remain above the solvent reservoir, and the solvent front must be marked for subsequent distance measurements.
Measure both distances from the baseline
Rf is distance travelled by the substance divided by distance travelled by the solvent front. Measure the substance to the centre of its spot from the baseline and the solvent front from that same baseline. Use the same distance unit in numerator and denominator, so Rf has no unit. Do not use the distance from the spot to the solvent front as the numerator.
With a front 8.0 cm above baseline, A's 2.0 cm travel gives Rf=0.25, B's 5.0 cm gives 0.625 and C's 6.4 cm gives 0.80. In this model, spots cannot lie beyond the front, so a result greater than 1 flags a distance or setup issue. Compare Rf values only under the same solvent and other relevant conditions; Rf is not a universal constant independent of the chromatographic method.
Pause and explain
A spot moves 4.0 cm and the solvent front 8.0 cm from the same baseline. What is Rf?
Worked example
A colourless unknown is revealed by a suitable locating agent. Its spot centre is 3.6 cm above the baseline; the marked solvent front is 6.0 cm above it. Calculate Rf.
Show the worked solution
- The locating agent reveals the position after separation; it does not replace the separating solvent.
- Use both distances from the same baseline: 3.6/6.0.
- The units cancel, giving 0.60; compare with standards under the same conditions.
Answer Rf=0.60, with no unit.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how a colourless soluble sample can be separated and its spots located.
Give me a hint
Separate the roles of solvent and detection.
Compare my reasoning
- Use a suitable solvent in the supervised paper-chromatography method to separate components.
- The spots may initially be invisible because the components are colourless.
- Use a suitable locating agent to reveal the separated positions, with the front already marked.
Suitable solvent separates the colourless solutes; a suitable locating agent reveals their spots afterwards.
Look for these in your work
- Separation and detection are distinct.
- Specific locating-agent names are not treated as required recall.
Calculate A, B and C's Rf values from the supplied 8.0 cm solvent-front diagram.
Give me a hint
Divide each baseline-to-spot distance by the same front distance.
Compare my reasoning
- A: 2.0/8.0=0.25.
- B: 5.0/8.0=0.625.
- C: 6.4/8.0=0.80; all are dimensionless and specific to the stated run conditions.
A 0.25; B 0.625; C 0.80, with no units.
Look for these in your work
- All distances use the baseline.
- The values and absence of units are correct.
A paper's bottom edge is 1.5 cm below the baseline. A spot centre is 5.1 cm above the bottom and the solvent front 7.5 cm above the bottom. Calculate Rf and assess whether a match from a different solvent proves identity.
Give me a hint
First convert both measurements to distances from the actual baseline.
Compare my reasoning
- The substance travels 5.1−1.5=3.6 cm from the baseline.
- The solvent front travels 7.5−1.5=6.0 cm, so Rf=3.6/6.0=0.60.
- A value from a different solvent is not a directly equivalent identification comparison; shared conditions and suitable standards are needed.
Rf=0.60; a different-solvent match alone does not establish identity.
Look for these in your work
- Both origin corrections are made.
- The identity conclusion respects the conditions of the comparison.
Common mistakes
- Assuming colourless solutes cannot separate.
- Treating the locating agent as the separating solvent.
- Measuring the spot-to-front gap instead of baseline-to-spot travel.
- Giving Rf a cm unit or comparing values from unrelated conditions.
What can you explain now?
Which distances define Rf?
Compare with the explanation
Baseline-to-spot-centre distance divided by baseline-to-solvent-front distance.
Both share the starting origin and the same distance unit.
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Make it stick
Explain colourless-spot detection and calculate Rf from baseline distances, including an example where measurements start at the paper's bottom edge.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.