2.2.4States of Matter and Mixtures
Core Practical: Investigating Inks
5 min read 7 parts Lesson 6 of 7 1 quiz
Section 1 of 7Aim and background
Aim
- Aim To separate and identify the dyes in a mixture of food colourings using chromatography, and to separate the solvent using simple distillation.
Why two methods
- Why two methods An ink is a solvent with dyes dissolved in it.
- Distillation separates the solvent, which has the lower boiling point; chromatography separates the dyes.
Section 2 of 7Part 1: simple distillation
1
Pour the ink into a flask and add a few anti-bumping granules so that it boils smoothly.
2
Set up the flask, thermometer, condenser and receiving beaker, with cold water entering the bottom of the condenser.
3
Heat gently and record the temperature when the liquid distils over steadily.
4
The solvent evaporates first, condenses in the condenser and is collected, while the dyes remain in the flask.
5
Compare the boiling point recorded with the known value for water to identify the solvent.
Section 3 of 7Part 2: chromatography method
1
Draw a pencil line 2 cm from the bottom of a rectangle of chromatography paper.
2
Use a separate capillary tube to place small spots of food colourings A, B, C and D on the line, and one spot of unknown mixture U. Keep each spot no wider than 2 to 3 mm and label them in pencil.
3
Pour water to a depth of less than 1 cm into a beaker and clip the paper to a wooden spill laid across the top, so the bottom edge just touches the water but the pencil line stays above it.
4
Leave undisturbed until the solvent has risen at least three quarters of the way up the paper.
5
Remove the paper and immediately mark the solvent front with a pencil, then let it dry.
6
Measure the distance from the start line to the solvent front and to the centre of each spot.
Section 4 of 7Variables, results and safety
| Aspect | Detail |
|---|---|
| Independent variable | The type of food colouring or ink sample |
| Dependent variable | Distance moved by each spot, giving Rf |
| Control variables | Same paper, same solvent, same start line height and run time |
| Results table | Columns for sample, distance moved by spot, distance moved by solvent and Rf |
| Analysis | Calculate Rf = distance moved by substance ÷ distance moved by solvent, and compare U with A to D |
| Hazard | Solvents such as ethanol or propanone are flammable and harmful: keep away from flames, avoid skin contact and use a fume cupboard if needed |
Section 5 of 7Evaluation
1Compare spots
- Compare spots Check which known colourings give spots at the same height as the unknown.
2Compare Rf
- Compare Rf A matching Rf supports the visual match.
3Conclude
- Conclude Chromatography with Rf values can identify the dyes in an unknown mixture when reference samples are available.
4Improve
- Improve Use smaller spots, a covered beaker and repeat runs for consistent results.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Solvent | The liquid that dissolves the dyes and carries them up the paper. |
| Capillary action | The rise of liquid through the narrow gaps in paper. |
| Anti-bumping granules | Small pieces added to a liquid to make it boil smoothly. |
| Reference sample | A known substance used for comparison. |
Section 7 of 7Exam technique
1
Starting the solvent level above the pencil line so the spots dissolve into the beaker.
2
Drawing the start line in ink, which separates into its own dyes.
3
Letting the solvent run off the top of the paper so the solvent front cannot be marked.
4
Placing large spots that overlap when the dyes spread.