8.1.4Chemical Analysis
Required Practical: Investigating Chromatography — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 9
Section 1 of 7Method
1
Draw a pencil line about 1 cm above the bottom of a sheet of chromatography paper; pencil is used because it will not dissolve in the solvent.
2
Place small, separate spots of each ink or dye, plus any known reference substances, on the line.
3
Pour a shallow layer of solvent such as water into a beaker, making sure its level is below the pencil line.
4
Hang the paper in the beaker so that its bottom edge dips into the solvent; cover the beaker with a lid to stop evaporation.
5
Allow the solvent to travel up the paper by capillary action until it is near the top, then remove the paper and mark the solvent front straight away with pencil.
6
Leave the chromatogram to dry and measure the distances travelled by each spot and by the solvent.
Section 2 of 7How separation happens
1Solvent rises
- Solvent rises The mobile phase, the solvent, moves up the paper and carries the dyes with it.
2Competition
- Competition Each dye is attracted to the paper (stationary phase) and dissolves in the solvent to different extents.
3Different speeds
- Different speeds Substances that dissolve well and stick weakly move far; those that stick strongly stay low.
4Separate spots
- Separate spots Each component ends at its own height.
Section 3 of 7Calculating Rf
| Quantity | Meaning | Example |
|---|---|---|
| Distance moved by substance | Measured from the start line to the centre of the spot | 3.6 cm |
| Distance moved by solvent | Measured from the start line to the solvent front | 6.0 cm |
| Rf = substance ÷ solvent | Always between 0 and 1, with no units | 3.6 ÷ 6.0 = 0.60 |
Section 4 of 7Interpreting a chromatogram
Pure or mixture
- Pure or mixture A pure substance gives one spot; a mixture gives two or more.
- Spots that match a reference at the same height and have equal Rf values are the same substance.
Dissolved dyes
- Dissolved dyes A dye that does not dissolve in the solvent stays on the line, so a different solvent may be needed.
Uses
- Uses Analysing food colourings, checking for banned additives and testing for drugs.
Section 5 of 7Safety and quality
Good practice
Safety
- Safety Wear eye protection and use flammable solvents away from flames.
Fair test
- Fair test Use the same solvent, paper and time for each comparison.
Accuracy
- Accuracy Small spots, a start line above the solvent and an accurate marking of the solvent front prevent errors.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Chromatography | A technique for separating the components of a mixture using a mobile and a stationary phase. |
| Mobile phase | The solvent that moves through the paper. |
| Stationary phase | The paper, which does not move. |
| Solvent front | The furthest point reached by the solvent. |
| Rf value | The distance moved by a substance divided by the distance moved by the solvent. |
| Chromatogram | The finished paper showing the separated spots. |
Section 7 of 7Exam technique
1
Drawing the start line in pen, whose ink would separate along with the sample.
2
Letting the solvent level cover the start line so the spots dissolve into the beaker.
3
Forgetting to mark the solvent front before it dries.
4
Giving Rf units or a value greater than 1.
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