Chemistry · Simulation and virtual lab Free
Paper chromatography: separate dyes and find Rf values
Spot food colours, inks or leaf extract on a paper, let the solvent rise, then measure how far each spot moved and identify the dyes by their Rf values.
- You will learn to
- Separate the components of food colours, inks and leaf pigments by paper chromatography.
- Measure distances and calculate Rf values.
- Identify the dyes in an unknown mixture and explain common mistakes.
- You will measure
- Distance moved by each spot and by the solvent front (cm); then Rf values.
- Time needed
- About 30–40 min
- Equipment
- Nothing extra. Real version: chromatography or filter paper, beaker with a lid, pencil, ruler, capillary tubes, food colours, pens, water, ethanol, leaf extract.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To separate the components of coloured mixtures by paper chromatography, to calculate Rf values, and to identify the dyes in an unknown mixture by comparing them with pure reference dyes.
What to do
- Choose Food colours, water, 1 cm of solvent, pencil line. Tap Put the paper in the solvent.
- Take the paper out when the solvent front is about 10 cm above the start line. Tap Measure and record the spots.
- Compare the spots of sweet X with the red, blue and yellow reference dyes. Which dyes does X contain? Read the box under the table.
- Choose Inks from pens. Run it with water, then with ethanol. Which ink only separates in ethanol?
- Make the mistakes on purpose: solvent 2.5 cm deep (above the line), then a start line drawn in ink. What goes wrong each time?
- S5: choose Leaf pigments and identify carotene, xanthophyll and chlorophylls a and b from their Rf values.
Results
Questions
Why is the start line drawn in pencil and not in ink?
Pencil (graphite) does not dissolve in the solvent; ink would dissolve, move up the paper and mix with the spots.
Why must the solvent level be below the start line?
Otherwise the spots dissolve into the solvent in the beaker instead of being carried up the paper, and nothing is separated.
How is an Rf value calculated?
Rf = distance moved by the spot ÷ distance moved by the solvent front, both measured from the start line. It is always between 0 and 1 and has no unit.
Why do different dyes travel different distances?
Each dye has a different solubility in the solvent and a different attraction to the paper. A dye that is more soluble and less attracted to the paper moves further.
Sweet colouring X gives two spots with Rf 0.40 and 0.62 in water. The pure red dye has Rf 0.40 and the blue dye 0.62. What is in X?
X is a mixture of the red and the blue dyes.
Why does the permanent marker not move in water but separate in ethanol?
Its dyes are insoluble in water but soluble in ethanol. Chromatography needs a solvent in which the substances dissolve.
For teachers: expected results and how the model works
Rf values in the model: water: yellow 0.85, blue 0.62, red 0.40, violet 0.22; ethanol: yellow 0.62, blue 0.38, red 0.20, violet 0.52; leaf pigments (petroleum ether + propanone): carotene 0.95, xanthophyll 0.70, chlorophyll a 0.60, chlorophyll b 0.45. Each spot differs by up to ±0.02 so that measured values look real. Sweet X = red + blue. The permanent marker stays on the start line in water (insoluble) and separates in ethanol into violet, blue and yellow. The solvent front rises fast at first, then more slowly (about 10 cm in 13 min for water, 16 min for leaf extract, 20 min for ethanol); use clock × 40. If the solvent reaches the top before the paper is taken out, the front cannot be measured and the run must be repeated. Solvent above the line: the spots wash into the beaker. Ink start line: a blue band runs up the paper.
Exam practice Exam set
National-exam style questions on this simulation (15 marks). Try each one on paper first, then open the marking scheme.
A spot moved 4.2 cm while the solvent front moved 7.0 cm from the start line. (a) Calculate the Rf value. (b) Explain why the Rf value has no unit. (3 marks)
Marking scheme: (a) Rf = 4.2 ÷ 7.0 (1) = 0.60 (1). (b) It is a ratio of two distances in the same unit, so the units cancel (1).
Describe how you would use paper chromatography to show that a black ink is a mixture. Include a labelled diagram. (6 marks)
Marking scheme: Draw a pencil line 1.5–2 cm from the bottom of the paper (1); put a small concentrated spot of ink on it (1); stand the paper in a beaker with solvent below the line and cover it (1); let the solvent rise until near the top, then remove and mark the solvent front (1); several coloured spots show the ink is a mixture (1). Diagram: beaker, paper, start line above solvent, lid, spots (1).
A food colour gives one spot in water and one spot in ethanol. A student concludes it is a pure substance. Is the conclusion certain? Explain. (2 marks)
Marking scheme: Not certain (1): two dyes could have the same Rf value in these solvents; trying more solvents, or comparing with pure samples, would make it more reliable (1).
In a chromatogram of leaf pigments, carotene has Rf 0.95 and chlorophyll b has Rf 0.45. Explain the difference. (2 marks)
Marking scheme: Carotene is more soluble in the non-polar solvent and less attracted to the paper (1), so it travels further; chlorophyll b is more strongly held by the paper / less soluble (1).
Give two uses of chromatography in everyday life. (2 marks)
Marking scheme: Checking food colours and additives; detecting drugs in blood or urine; forensic tests on inks; separating plant pigments; testing water for pollutants (any two, 1 each).
Donat Sciences and MathsChemistry · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Paper chromatography: separate dyes and find Rf values
Aim: To separate the components of coloured mixtures by paper chromatography, to calculate Rf values, and to identify the dyes in an unknown mixture by comparing them with pure reference dyes.
Apparatus (real laboratory): chromatography or filter paper, beaker with a lid, pencil, ruler, capillary tubes, food colours, pens, water, ethanol, leaf extract.
What to do (with the simulation on www.donatstem.co.rw)
- Choose Food colours, water, 1 cm of solvent, pencil line. Tap Put the paper in the solvent.
- Take the paper out when the solvent front is about 10 cm above the start line. Tap Measure and record the spots.
- Compare the spots of sweet X with the red, blue and yellow reference dyes. Which dyes does X contain? Read the box under the table.
- Choose Inks from pens. Run it with water, then with ethanol. Which ink only separates in ethanol?
- Make the mistakes on purpose: solvent 2.5 cm deep (above the line), then a start line drawn in ink. What goes wrong each time?
- S5: choose Leaf pigments and identify carotene, xanthophyll and chlorophylls a and b from their Rf values.
Results
| # | Sample | Spot colour | Spot moved (cm) | Solvent moved (cm) | Rf |
|---|
| 1 | | | | | |
| 2 | | | | | |
| 3 | | | | | |
| 4 | | | | | |
| 5 | | | | | |
| 6 | | | | | |
| 7 | | | | | |
| 8 | | | | | |
| 9 | | | | | |
| 10 | | | | | |
| 11 | | | | | |
| 12 | | | | | |
Questions
- Why is the start line drawn in pencil and not in ink? [2]
- Why must the solvent level be below the start line? [2]
- How is an Rf value calculated? [2]
- Why do different dyes travel different distances? [2]
- Sweet colouring X gives two spots with Rf 0.40 and 0.62 in water. The pure red dye has Rf 0.40 and the blue dye 0.62. What is in X? [2]
- Why does the permanent marker not move in water but separate in ethanol? [2]
- A spot moved 4.2 cm while the solvent front moved 7.0 cm from the start line. (a) Calculate the Rf value. (b) Explain why the Rf value has no unit. [3]
- Describe how you would use paper chromatography to show that a black ink is a mixture. Include a labelled diagram. [6]
- A food colour gives one spot in water and one spot in ethanol. A student concludes it is a pure substance. Is the conclusion certain? Explain. [2]
- In a chromatogram of leaf pigments, carotene has Rf 0.95 and chlorophyll b has Rf 0.45. Explain the difference. [2]
- Give two uses of chromatography in everyday life. [2]
Donat Sciences and MathsChemistry · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Paper chromatography: separate dyes and find Rf values
Expected results
Rf values in the model: water: yellow 0.85, blue 0.62, red 0.40, violet 0.22; ethanol: yellow 0.62, blue 0.38, red 0.20, violet 0.52; leaf pigments (petroleum ether + propanone): carotene 0.95, xanthophyll 0.70, chlorophyll a 0.60, chlorophyll b 0.45. Each spot differs by up to ±0.02 so that measured values look real. Sweet X = red + blue. The permanent marker stays on the start line in water (insoluble) and separates in ethanol into violet, blue and yellow. The solvent front rises fast at first, then more slowly (about 10 cm in 13 min for water, 16 min for leaf extract, 20 min for ethanol); use clock × 40. If the solvent reaches the top before the paper is taken out, the front cannot be measured and the run must be repeated. Solvent above the line: the spots wash into the beaker. Ink start line: a blue band runs up the paper.
Answers
- Why is the start line drawn in pencil and not in ink? [2]
Pencil (graphite) does not dissolve in the solvent; ink would dissolve, move up the paper and mix with the spots.
- Why must the solvent level be below the start line? [2]
Otherwise the spots dissolve into the solvent in the beaker instead of being carried up the paper, and nothing is separated.
- How is an Rf value calculated? [2]
Rf = distance moved by the spot ÷ distance moved by the solvent front, both measured from the start line. It is always between 0 and 1 and has no unit.
- Why do different dyes travel different distances? [2]
Each dye has a different solubility in the solvent and a different attraction to the paper. A dye that is more soluble and less attracted to the paper moves further.
- Sweet colouring X gives two spots with Rf 0.40 and 0.62 in water. The pure red dye has Rf 0.40 and the blue dye 0.62. What is in X? [2]
X is a mixture of the red and the blue dyes.
- Why does the permanent marker not move in water but separate in ethanol? [2]
Its dyes are insoluble in water but soluble in ethanol. Chromatography needs a solvent in which the substances dissolve.
- A spot moved 4.2 cm while the solvent front moved 7.0 cm from the start line. (a) Calculate the Rf value. (b) Explain why the Rf value has no unit. [3]
(a) Rf = 4.2 ÷ 7.0 (1) = 0.60 (1). (b) It is a ratio of two distances in the same unit, so the units cancel (1).
- Describe how you would use paper chromatography to show that a black ink is a mixture. Include a labelled diagram. [6]
Draw a pencil line 1.5–2 cm from the bottom of the paper (1); put a small concentrated spot of ink on it (1); stand the paper in a beaker with solvent below the line and cover it (1); let the solvent rise until near the top, then remove and mark the solvent front (1); several coloured spots show the ink is a mixture (1). Diagram: beaker, paper, start line above solvent, lid, spots (1).
- A food colour gives one spot in water and one spot in ethanol. A student concludes it is a pure substance. Is the conclusion certain? Explain. [2]
Not certain (1): two dyes could have the same Rf value in these solvents; trying more solvents, or comparing with pure samples, would make it more reliable (1).
- In a chromatogram of leaf pigments, carotene has Rf 0.95 and chlorophyll b has Rf 0.45. Explain the difference. [2]
Carotene is more soluble in the non-polar solvent and less attracted to the paper (1), so it travels further; chlorophyll b is more strongly held by the paper / less soluble (1).
- Give two uses of chromatography in everyday life. [2]
Checking food colours and additives; detecting drugs in blood or urine; forensic tests on inks; separating plant pigments; testing water for pollutants (any two, 1 each).