Lesson focus: chromatographyA way to separate the different coloured substances in a mixture such as ink, as a liquid carries them up paper. separates dissolved components through different relative attractions.
Set up and interpret a fair paper chromatogram with standards.
Build on earlier learning
Evaporation and distillationApply physical-property separation to small amounts of dissolved substances.
Practical
Paper chromatography of inks
- Draw a pencil start line 1 cm from the bottom of the paper.
- Spot each ink on the line and let it dry.
- Stand the paper in water with the line above the water level.
- Remove the paper before the solvent front reaches the top and mark the front.
Record: The chromatogram, the number of spots per ink and which inks share a colour.
You will be successful when you can…
- Set a pencil baseline above solvent.
- Explain separation through competing attractions.
- Compare unknown and standards from one run.
- State limits of identity and purity claims.
Words that carry the science
- chromatography
- Method separating dissolved components by different movement through phases.
- stationary phase
- Material that remains fixed, such as paper.
- mobile phase
- Solvent moving through the stationary phase.
- baseline
- Pencil line where samples are initially placed.
- solvent front
- Furthest position reached by solvent.
Build a fair comparison from baseline to solvent front
01Set up correctly
Draw baselineThe pencil line at the bottom of chromatography paper where you put the spots of ink. in pencil because ink would dissolve. Place small sample spots on the line and keep solvent below it.
02Why spots separate
Components differ in attraction to paper and solubility/attraction to solvent, so they travel different distances. Mark solvent frontThe highest point the solvent reaches as it soaks up the chromatography paper. before drying.
03Interpret cautiously
Matching positions in the same run support a shared component. One spot suggests one detected component under those conditions, not proof of absolute purity.
Paper chromatography
The solvent soaks up the paper and carries each dye with it. Dyes that dissolve better and stick less to the paper travel further.
Controls
Dyes and Rf values are illustrative. X matches A, B and C wherever spots are at the same height.
Identify an unknown
Unknown U gives spots level with standards A and C on the same chromatogram.
- Confirm all samples share baseline, solvent and paper.
- Compare horizontal spot positions.
- U matches A at one height and C at another.
- Conclude U contains components consistent with A and C; do not claim nothing else could be present.
How to make the reasoning visible: Use “evidence supports” rather than “proves”.
Chromatogram decisions
Judge each setup or conclusion.
| Observation | Decision | Reason |
|---|---|---|
| Baseline drawn in ink | Invalid | Ink can dissolve and separate |
| Solvent covers spots | Invalid | Samples wash into solvent |
| U spot matches A | Shared component supported | Same run and travel position |
| One visible spot | Possibly one component | Method may not resolve all substances |
Why must solvent stay below the baseline?
Why is one spot not proof of purity?
Use the table. The unknown ink U has a spot that matches ink A. What does this suggest?
Check your understanding
Answer from memory, then check against the suggested answer.
What is chromatography?
What is stationary phase?
What is mobile phase?
What is a chromatogram?
Why is the baseline drawn in pencil?
Two labs obtain different travel distances for the same dye. Give a controlled-variable explanation.
How could you strengthen identification of an unknown?
Why do different dyes travel different distances up the paper?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Explain how to compare an unknown dye with standards using paper chromatography.
(5)Total for this question: 5 marks
Command word: Include control variables for the set-up and a cautious evidence-based conclusion.