A useful Secondary 2 chromatogram task makes the baseline, solvent front, sample lanes and reference spots easy to identify. Students can then compare the evidence and explain which conclusions the chromatogram supports.
For Hougang Chemistry support, the tutor should prepare more than a spot-count answer. A matching position can support comparison with a reference under the same conditions, while one visible spot is a more limited purity finding than many students assume.
Chemistry here means chemistry within lower-secondary Science. Use the student’s actual G2 or G3 materials and school sequence. The numerical Rf example is a supplied-formula extension where appropriate, rather than a claim that every lower-secondary course requires Rf calculation. All positions below are fictional teaching data.
EDUKATESG · HOUGANG CHEMISTRY · TUTOR PREPARATION
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Choose the preparation route you need, or follow the complete guide.
ROUTE 2 · CHAPTERS 3–4
Follow the worked example
Connect observations and quantities to the chemistry.
Full chapter index · Tutor checklist · Hougang Chemistry learning routes
Full chapter index
Chapters 1–3: purpose, definitions and first example
Chapters 4–7: interpretation and teaching decisions
Chapters 8–10: independent application and continuation
CHAPTER 1 OF 10 · Define the teaching decision
1. Define the interpretation task before drawing spots
Choose whether the student must count resolved components, compare an unknown with references or calculate a distance ratio. Those tasks can share a chromatogram, but they place different demands on reading and reasoning.
For a first task, state that the samples are stable and the displayed spots are genuine detected components, with no contamination or reaction during development. These are simplifying assumptions for the supplied paper example.
Give each lane an unambiguous label. A reference called A and an unknown called X should not change names between the diagram, question and answer key. The tutor should verify those labels before printing.
Decide what each observation can establish within the question model. Three separated spots support three resolved components in the idealised record. They do not automatically reveal the name or amount of every substance.
Prepare at least one question about a limitation. Students should learn to connect evidence to a conclusion and identify what remains unresolved, rather than treat chromatography as a machine that supplies complete identities.
A candidate-reference task can supply two possible substances that migrate to different positions in the stated system. The student’s job is then to use that restricted comparison. Make the candidate set explicit so a classroom identification does not quietly become a universal claim about all possible chemicals.
CHAPTER 2 OF 10 · Define the teaching decision
2. Make the baseline and solvent front explicit
State that the baseline is the origin from which migration distances are measured. The solvent front marks how far the solvent travelled during the run. Both should be identified independently of the bottom and top edges of the paper.
A common preparation mistake is to crop the baseline out of the student image. The tutor may know where it was drawn, but a student cannot make a valid distance measurement from information that has been removed.
If the task uses written coordinates instead of a drawing, explain that all positions share the same coordinate system. The student must subtract the baseline coordinate from each position before calculating a travelled distance.
In an actual suitable paper-chromatography setup, the sample origin needs to begin above the solvent level. This preserves the intended development rather than washing the sample into the solvent reservoir. Teach the purpose at the appropriate school level.
Use the actual school instructions for practical work and solvent handling. The written exercise here focuses on interpretation and does not supply a home procedure or assume that every dye can be developed using water.
Mark the solvent front at the time specified by the actual practical instructions. In the paper version, give its position directly. A student should not be expected to recover a missing front from a neat rectangular border or from the highest sample spot.
Give the baseline coordinate as 1.0 cm and the solvent-front coordinate as 9.0 cm on a vertical scale. Unknown X shows spot centres at 3.0, 5.0 and 7.0 cm.
The solvent travelled 9.0 − 1.0 = 8.0 cm. The detected components travelled 2.0, 4.0 and 6.0 cm from the baseline. The raw coordinate of a spot is different from its distance travelled.
Under the supplied idealised assumptions, X has three resolved detected components. Describe what was separated rather than claim that every possible component in the original sample must have been observed.
Ask students to identify the lowest and highest migrated components without assuming their names. A greater distance in this system does not mean the substance has a greater molecular mass or is automatically more chemically reactive.
Prepare a plain-language explanation before introducing a ratio. The student should understand where the distances begin and end, so the calculation retains physical meaning when the layout changes.
If the original sample stayed at the baseline, ask what the record shows rather than force a non-zero ratio. Its migration distance is zero in that record. This may call for examining the suitability of the system, while the observation alone does not establish that the sample contained no substance.
CHAPTER 4 OF 10 · Follow the worked example
4. Use a supplied Rf relationship when appropriate
For the extension, supply Rf = distance travelled by the substance divided by distance travelled by the solvent front, with both distances measured from the same baseline.
The three ratios are 2.0/8.0 = 0.25, 4.0/8.0 = 0.50 and 6.0/8.0 = 0.75. The units cancel because numerator and denominator are lengths expressed in the same unit.
A result above one for an ordinary spot below the solvent front should trigger a check of the distances or denominator. A student may have divided the solvent distance by the spot distance or measured from the paper edge.
Do not add cm to the final ratio. The substance and solvent distances have length units, while their ratio has no unit. That is a useful distinction between a measured input and a derived quantity.
Where numerical Rf is outside the current taught expectations, retain qualitative spot comparison and provide the calculation only as clearly supported extension. The task should match the student’s actual learning stage.
Use the same length unit in numerator and denominator. Converting 2.0 cm to 20 mm is valid only if the 8.0 cm solvent distance is also expressed consistently when forming the ratio. A unit mismatch can produce an apparently impossible Rf even when the measured distances were sound.
CHAPTER 5 OF 10 · Inspect evidence and repair
5. Compare with references under matched conditions
Suppose reference A produces a spot at Rf 0.25 and reference B produces a spot at Rf 0.75 when developed under the same relevant conditions as X. X’s lowest and highest spots are consistent with those references.
The middle spot at 0.50 remains unassigned by those two references. Do not label it as a familiar dye merely because it is the third spot. A suitable additional reference or other evidence would be needed.
Matching migration provides evidence for an identification, but it is not a unique fingerprint under every possible condition. Different substances can fail to separate in one system. Teach the strength of the inference at the intended level.
Use references on the same run where suitable, or state clearly that the comparison conditions are equivalent. A number copied from a different solvent or stationary phase may not support the intended comparison.
The table below records the fictional coordinates and ratios. It can be used as a checked tutor key while the student version leaves selected entries blank.
The relevant comparison is between corresponding conditions, including the stationary and mobile phases. Equal numerical ratios from unrelated systems do not supply the same evidence as standards developed alongside the unknown. Ask students to state which condition supports the comparison before identifying a component.
| Feature | Coordinate / cm | Distance from baseline / cm | Rf where used |
|---|---|---|---|
| Baseline | 1.0 | 0 | — |
| Solvent front | 9.0 | 8.0 | — |
| Lowest X spot | 3.0 | 2.0 | 0.25 |
| Middle X spot | 5.0 | 4.0 | 0.50 |
| Highest X spot | 7.0 | 6.0 | 0.75 |
CHAPTER 6 OF 10 · Inspect evidence and repair
6. Teach the limits of one visible spot
A pure stable substance can give one spot in a suitable system where it is detected. In a simple school question with explicit idealised assumptions, one spot may be used to classify a sample as one component within that model.
The reverse conclusion needs care outside that simplified setting. One observed spot can also occur when components are not separated or some material is not detected. State that the sample appears as one detected spot under these conditions.
Contrast two prompts: how many spots are shown, and has purity been established conclusively? The first is a direct reading. The second asks about the method’s limits and may need additional information.
Prepare an extension in which a sample gives one spot with one solvent and two separated spots with another suitable solvent. Under the stable-sample assumptions, the second result reveals that the first system did not resolve all the components.
Keep the purpose calm and clear. This does not make chromatography useless; it teaches students to state what a particular run has shown and what a further suitable comparison could establish.
A useful teacher prompt is: which further run could reveal whether the single spot hides more than one component? A different suitable separating system can address that possibility. The student should explain the purpose of the proposed check rather than assume that repeating an identical unresolved result proves purity.
CHAPTER 7 OF 10 · Practise independently
7. Prevent unsupported quantity and mechanism claims
A darker or larger spot does not supply an exact mass percentage without an appropriate measurement and calibration method. Different compounds can produce different responses, and application conditions can also affect appearance.
Ask students which information would be needed to make a quantitative claim. The original question supplies positions and references, not calibrated spot areas or known response factors. Its answer should stay with those data.
Do not explain migration solely by saying that the lightest molecule travels farthest. The separation depends on interactions with the chosen mobile and stationary phases. Use the explanation depth taught in the actual course.
Likewise, a solvent that separates one mixture may not separate another. The task should name the system or provide the relevant comparison conditions rather than treat solvent choice as incidental.
Prepare wrong-answer examples with their corrections visible. A memorable but false shortcut should not remain the most prominent sentence on a revision sheet after the lesson ends.
CHAPTER 8 OF 10 · Practise independently
8. Check transfer with a changed coordinate system
For a fresh run under suitable comparable conditions, give baseline coordinate 0.5 cm, solvent-front coordinate 6.5 cm and a spot centre at 3.5 cm. The substance travelled 3.0 cm and the solvent 6.0 cm, so Rf is 0.50.
The spot’s coordinate differs from the 5.0 cm middle spot in the first example, while the ratio is the same. This helps students see why raw paper positions alone are not enough when comparing different development lengths.
Then supply a deliberately incomplete record with no solvent-front position. The student cannot calculate a numerical Rf from the spot distance alone. Ask what additional information is required.
If a solvent front has not been marked in an actual record, the tutor should acknowledge the missing evidence rather than invent a value to complete the calculation. Good preparation makes that distinction visible.
Record whether the student reconstructs the baseline subtraction independently. A correct ratio after the tutor identifies every distance shows supported performance; a fresh response helps establish independent control.
For another supplied-data check, use a baseline at 2.0 cm, a front at 10.0 cm and a spot at 4.0 cm. The distances are 2.0 and 8.0 cm, giving Rf 0.25. The changed coordinate labels preserve the same underlying proportion as the lowest spot in the first example.
CHAPTER 9 OF 10 · Plan the next learning move
9. Make the feedback useful to students and parents
If the student divides 5.0 by 9.0 in the first example, return to the shared origin. The issue is a coordinate-versus-distance error, even if the division itself is accurate.
If the student identifies an unknown solely because its spot has the same colour as a reference, ask what migration evidence and conditions are available. Colour alone may not discriminate the candidates.
A parent update can say that the student now measures from the baseline and can explain why an unmatched spot remains unidentified. That shows a specific improvement in interpreting evidence.
In small-group teaching, ask everyone to annotate the baseline and front before discussing answers. One student’s pointing can otherwise make a common completed diagram look like individual understanding.
Parents can ask which observation supports a conclusion and what the chromatogram does not yet tell us. A paper record is enough to support that discussion without arranging a solvent experiment at home.
CHAPTER 10 OF 10 · Plan the next learning move
10. Finish the chromatogram preparation audit
Check every lane label, coordinate, distance, ratio and reference condition in the tutor key. A mismatch between diagram and data table can create a false learning difficulty.
Keep the student version and completed version separate, with the same baseline and front information. Remove answer cues deliberately rather than accidentally removing information needed for a valid interpretation.
Choose repair for scale or origin confusion, stabilise through a changed layout and extend through unresolved identity or purity questions when the basic reading is secure.
Match the numerical depth to the actual course. A supplied Rf extension can teach proportional reasoning, while the core lower-secondary task may remain qualitative interpretation of components and references.
The completed brief helps students read a chromatogram as evidence: identify the measurement, compare under appropriate conditions and limit the conclusion. That gives the tutor a precise next task when a student’s answer goes beyond the data.
Keep qualitative and numerical marks distinct in the tutor record. A student might count the resolved spots correctly but measure the distances from the paper edge. Another might calculate all ratios correctly and still overclaim a unique identity. Those responses call for different follow-up tasks.
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Frequently asked questions
Where should the migration distance start?
At the baseline, not automatically at the bottom of the paper. Both the spot and solvent-front distances must use that same origin.
Does Rf have a unit?
No. It is a ratio of two lengths expressed in the same unit. The input distances still need clearly stated units.
Does one spot conclusively prove purity?
It shows one detected spot under that system’s conditions. Components may remain unresolved or undetected. A simplified school question may explicitly use an idealised one-component model.
Does a matching Rf prove a unique identity?
It supports comparison with a reference under matched relevant conditions. Additional evidence may be needed because different substances can share a migration position in one system.
Can spot darkness give an exact mass percentage?
Not from this dataset. Quantitative interpretation would require an appropriate calibrated method and information about the response.
Must every Secondary 2 student calculate Rf?
Use the actual course and taught sequence. Here the formula is supplied as an extension when appropriate, while qualitative interpretation can remain the main task.
Tutor preparation checklist
| Check | Before the lesson |
|---|---|
| Layout | Label lanes, baseline and solvent front. |
| Distances | Measure every migration from the same origin. |
| References | State comparable development conditions. |
| Inference | Separate spot count, identity and purity claims. |
| Transfer | Prepare a changed origin and an incomplete record. |
Continue with connected teaching guides
- Secondary 2 Chemistry Tutor | Designing a Separation-Method Decision Task
- Secondary 2 Chemistry Tutor | Checking Diagrams Before They Become Misconceptions
- Secondary 2 Chemistry Tutor | Preparing a Solubility-Data Task with Explicit Conditions
- Hougang Chemistry Tuition and Tutor Library: Secondary and JC learning routes
For a discussion about support, bring the student’s current Science or Chemistry course details, examination year where relevant and a recent piece of schoolwork showing the difficulty. Confirm current subject provision, grouping and teaching location directly with the provider. This guide describes preparation practice and does not establish class availability.
Sources and scope
MOE: Lower Secondary Science syllabus for G2 and G3. Use the student’s school materials to select the current content and depth.
Further teaching reference: University of Wisconsin–Madison Chemistry: interpreting a chromatogram.
The fictional records, worked calculations and tutor planning tools are original teaching illustrations. They are not student outcome evidence or official marking schemes. Use the applicable syllabus and supervised practical instructions for the actual course and examination year.