Test mixture proposals with matched chromatograms

A matching spot need not identify a substance

The first synthetic plate contains standards A, B, C and D plus an unknown. A and D both travel 8 mm while the solvent front travels 40 mm. The unknown has centers at 8 and 32 mm. Predict whether these observations can distinguish A+C, D+C and A+D+C.

Select each proposal. The activity checks whether every observed unknown position is accounted for and whether every proposed component has a matching observed position. Coincident components can share a spot, so the number of visible positions is not automatically the number of substances. This is a supplied original evidence packet, not an experiment performed in the browser.

Compare relative movement under matched conditions

In planar chromatography, Rf = distance from origin to spot center / distance from origin to solvent front. Both distances use the same baseline. For A in the first plate, 8/40=0.200. The ratio is dimensionless, normally between zero and one in this setting.

The longer record under the same condition has A at 16 mm and front at 80 mm: Rf remains 0.200. Comparing raw spot distances across these two runs would confound relative migration with distance traveled by the solvent. Conversely, a change of mobile phase can change Rf even when the labeled standard remains the same.

Each plate compares its standards and unknown under the same stipulated stationary/mobile conditions. Do not match an unknown in condition 2 against a standard’s Rf from condition 1 as though Rf were a universal property independent of the method.

Keep reading bounds distinct from spot width

The packet assigns ±1 mm to each center and front reading. These are bounded reading uncertainties, not measured band widths or 95% confidence intervals. A conservative ratio interval is (d−1)/(F+1) to (d+1)/(F−1), where d is center distance and F is front distance.

For 8±1 mm and 40±1 mm, this gives 7/41≈0.1707 to 9/39≈0.2308. For 16±1 and 80±1, it narrows to 15/81≈0.1852 to 17/79≈0.2152. The central ratio is unchanged while the relative effect of the same absolute reading bound is smaller.

Standards and unknown share a front on one plate, so their ratio uncertainties are correlated. The matching rule therefore compares center distances directly: two ±1 mm intervals can overlap when their centers differ by at most 2 mm. It does not treat separate Rf intervals as independent probabilities or calculate a confidence score for identity.

Change the condition to discriminate

Keep D+C selected and move from mobile phase 1 to mobile phase 2. In the 40 mm-front record, A is now at 18 mm, D at 14 mm and C at 28 mm. The unknown centers are 18 and 28 mm. D no longer matches the lower observed position within the assigned bounds. A+D+C also predicts an unmatched D position.

This second condition rejects alternatives the first record could not distinguish. A+C remains positionally compatible within the listed candidates, but that does not prove a unique chemical identity: an unlisted compound could co-migrate, an undetected component could be present, or the assumed matching conditions could fail. Retain your first proposal and changed-condition explanation in captures rather than rewriting the first record as if you had known the answer.

Connect migration to molecular interactions

Chromatography relies on distribution between stationary and mobile phases. Greater attraction to the stationary phase relative to the mobile phase tends to reduce migration under those conditions. Changing the mobile phase changes the competition; it may change the relative spacing or ordering of components. Considering only “solubility in the solvent” omits the stationary-phase interaction.

The records do not identify specific solvents, molecular structures or stationary-phase chemistry. They illustrate what matched comparisons can establish, without pretending to calculate migration from structure. A real method would also require controlled loading, development and detection conditions. The plotted marker size and darkness are symbolic, not abundance measurements.

The existing dissolution and filtration ledger separates material by solubility and physical filtration. Chromatography adds differential phase interactions and analytical comparison. This small plate does not demonstrate recovery of the whole mixture or a preparative yield.

Choose a separation principle, not only a piece of equipment

Ordinary filtration can retain suspended particles but does not separate molecularly dissolved components of a true liquid solution. Chromatography can use paper, a thin stationary layer or a packed column; their common principle is differing stationary/mobile interactions, not a universal rule that every polar component moves least. The polarity inference requires the specified phases.

Distillation instead uses differing tendencies to enter the vapor: component interactions affect vapor pressures and therefore vapor composition. Condensing a vapor enriched in one component can produce a composition different from the starting liquid. Enrichment is not a guarantee of complete separation, and mixtures can have nonideal limitations. This is conceptual analysis only, not a heating, collection or solvent-handling procedure. Current CED 3.9.A.1 i–ii, printed p.68, visually checked 31 October 2026.

Original vapor packet: A fictional binary liquid has equal amounts of A and B. At its stipulated comparison condition, the equilibrium vapor has partial pressures 60 and 20 pressure units and obeys the ideal-gas mixture relation. Find its A fraction. Does this show that a single condensed vapor sample is pure A? What if the vapor pressures were equal instead?

Check enrichment versus purity

The vapor A fraction is 60/(60+20)=0.75, compared with the liquid's 0.50: A is enriched, not pure. Condensing that sample without loss or reaction preserves its two-component composition. Equal partial pressures would give an A fraction of 0.50 and no enrichment at that supplied condition. These records provide no operating temperatures, collection design or claim that repeated stages always reach purity.

Transfer and sources

An unfamiliar record has an 18±1 mm spot and a 60±1 mm front. Calculate its ratio bounds, then explain why a matching standard is supportive but not conclusive. Propose an independent analytical comparison that would challenge a competing composition claim.

• Royal Society of Chemistry chromatography resource: relative stationary/mobile phase interactions and separation.
• RSC chromatography teaching chapter: planar migration ratio and phase context.
• AP Chemistry CED, 3.9.A: explain separation results using component interactions.

The candidate-comparison model assumes matched loading/detection, no reaction and no mixture-induced positional shift. It ignores intensities and detection limits. No identity, purity, recovery or physical laboratory completion is certified by a compatible proposal.

Infer polarity only when the phases and comparison are supplied

A separate original virtual plate uses a polar stationary phase and a relatively nonpolar mobile phase. Two neutral analytes have comparable size, no ionization and otherwise matched interaction opportunities. Under these deliberately restricted conditions, the analyte interacting more strongly with the polar stationary phase is retained longer and migrates less. Greater relative polarity is one supported explanation for that stronger attraction, not a conclusion from an unlabeled plate alone.

Changed supplied record: In this matched-phase case A migrates 0.20 and B 0.70 of the solvent-front distance. Infer the more strongly retained analyte and conditional polarity ordering. Then the mobile phase becomes more polar: must those two fractions and their separation remain unchanged?

Check retention versus a universal polarity rule

A is more strongly retained and, under the stated matched-analyte assumptions, is consistent with greater polarity. A more polar mobile phase can compete differently for stationary-phase interactions and change analyte solvation, migration and separation; the original fractions cannot simply be reused. Other stationary phases, ionization or specific binding can reverse a simple ordering. No solvent recipe or physical operation is assigned.

This supplied-phase inference supplements the earlier unknown-phase identity packet; it does not retroactively supply missing phase evidence to that packet. Current CED 3.9.A.1, checked 31 October 2026, requires phase-specific chromatographic reasoning.