Chromatography: phases, spot positions and Rf
| English | 中文 | Pinyin |
|---|---|---|
| mobile phase/ˈməʊbaɪl feɪz/ | 流动相 | liú dòng xiāng |
| stationary phase/ˈsteɪʃənəri feɪz/ | 固定相 | gù dìng xiāng |
What would explain this observation?
- Two dyes begin at the same pencil line but finish at different heights. They were carried by the same solvent; their different distributions between phases cause the separation.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Chromatography separates a mixture and provides evidence for identifying components. The mobile phase 流动相 moves through the stationary phase 固定相. In paper chromatography the paper supports the stationary phase while solvent moves up the sheet. Components distribute differently between the phases: a component carried more in the mobile phase moves farther under the stated conditions. The solvent front marks the farthest point reached by the solvent, not the highest dye spot.
- mobile phase: The moving phase carrying components through the stated chromatographic system; stationary phase: The phase that remains in place while components distribute between it and the mobile phase.
What distances are used to calculate Rf?
Rf is distance moved by the substance divided by distance moved by the solvent. Measure both from the origin line, using the centre of the spot for the substance. The ratio has no unit and ordinarily lies between zero and one in this model. Compare unknown and reference spots using the same paper, solvent and conditions. A different solvent can change the separation and Rf; it is not a universal identifying number for one substance.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Rf is distance moved by the substance divided by distance moved by the solvent. Measure both from the origin line, using the centre of the spot for the substance. The ratio has no unit and ordinarily lies between zero and one in this model. Compare unknown and reference spots using the same paper, solvent and conditions. A different solvent can change the separation and Rf; it is not a universal identifying number for one substance.
- Read each lane separately and count distinct spots. Multiple spots show multiple detectable components. A single spot is consistent with a pure compound, but a mixture may have components that overlap or are not detected in that solvent. Use another suitable solvent or independent evidence to strengthen a purity judgement. Plot measured spot centres clearly and state when broad or streaked spots make the centre uncertain; do not choose a convenient edge.
Which two habits make the investigation or model in this case more defensible?
Read each lane separately and count distinct spots. Multiple spots show multiple detectable components. A single spot is consistent with a pure compound, but a mixture may have components that overlap or are not detected in that solvent. Use another suitable solvent or independent evidence to strengthen a purity judgement. Plot measured spot centres clearly and state when broad or streaked spots make the centre uncertain; do not choose a convenient edge.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: solvent front is 60 mm from the origin and an unknown spot centre is 36 mm from it. Rf=36/60=0.60. A reference at 36 mm in the same run supports the same detectable component. Another unknown spot at 18 mm has Rf=18/60=0.30, so the unknown lane contains at least two separated components. Matching one spot does not identify the whole mixture.
A spot centre travels 28 mm and the solvent front 40 mm from the origin. Calculate Rf. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A spot centre travels 28 mm and the solvent front 40 mm from the origin. Calculate Rf.
The result is 0.7 . Known: solvent front is 60 mm from the origin and an unknown spot centre is 36 mm from it. Rf=36/60=0.60. A reference at 36 mm in the same run supports the same detectable component. Another unknown spot at 18 mm has Rf=18/60=0.30, so the unknown lane contains at least two separated components. Matching one spot does not identify the whole mixture.
Check the conclusion and its limits
- Do not measure from the paper edge, divide solvent distance by spot distance or attach millimetres to Rf. Higher travel is not evidence that a molecule weighs more or that the paper is moving. This both-tier objective includes calculation and appropriately rounded answers; an unreviewed advanced chromatographic technique is not required.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Rf for a substance must be identical in every solvent. This claim is false: Do not measure from the paper edge, divide solvent distance by spot distance or attach millimetres to Rf. Higher travel is not evidence that a molecule weighs more or that the paper is moving. This both-tier objective includes calculation and appropriately rounded answers; an unreviewed advanced chromatographic technique is not required.
Chromatography: phases, spot positions and Rf: Rf is distance moved by the substance divided by distance moved by the solvent. Measure both from the origin line, using the centre of the spot for the substance. The ratio has no unit and ordinarily lies between zero and one in this model. Compare unknown and reference spots using the same paper, solvent and conditions. A different solvent can change the separation and Rf; it is not a universal identifying number for one substance.
Rf for a substance must be identical in every solvent.
Do not measure from the paper edge, divide solvent distance by spot distance or attach millimetres to Rf. Higher travel is not evidence that a molecule weighs more or that the paper is moving. This both-tier objective includes calculation and appropriately rounded answers; an unreviewed advanced chromatographic technique is not required.
The moving phase carrying components through the stated chromatographic system: write the technical term.
mobile phase means The moving phase carrying components through the stated chromatographic system.