a) Pencil is insoluble (will not dissolve in the solvent), which means that it will not move up the paper with the solvent; the baseline will stay in the same position. Ink is soluble, so will start to rise up the paper, which will make it unclear as to where the baseline was.
b) If a dye/ substance stays on the baseline, it means that the dye/ substance is insoluble in the solvent. For this chromatogram, dye D is insoluble in the water, which is why it stays on the baseline.
c) It implies that dyes A, B and C are pure substances as they only have one spot
d) It tells us that dye C is a mixture as it has 2 different spots
e) Solvent front
h) No, substances will have different Rf values depending on the solvent that is used. This is because substances have different solubilities in different solvents.
a) The two phases are the mobile phase and the stationary phase
b) The different substances move different distances on the chromatogram because they spend different times in the mobile phase and stationary phase.
i) F because it moves the furthest up the paper/ has the highest Rf value
ii) G because it moves the least up the paper (closest to the baseline)/ has the lowest Rf value
3) The Rf value will increase. If the substance is more soluble in ethanol compared to water, the substance will spend more time in the mobile phase compared to the stationary phase. This means that the substance will move a greater distance up the chromatography paper, which results in a greater Rf value.
a) The baseline on the chromatography paper needs to be above the solvent. This is because if the baseline was in the solvent, the inks/ dyes would start to dissolve into the solvent, which would ruin the chromatograph experiment
b) We have a lid to stop the solvent evaporating
c) The student should remove the chromatography paper just before the solvent reaches the top of the paper
H = 0.29 (±0.05)
I = 0.88 (±0.05)
J = 0.58 (±0.05)
b) Longer piece of paper or a different type of paper
6) The student could use a different solvent so that the substances in sample D will hopefully dissolve in the new solvent and therefore rise up the chromatography paper
7) We undertake chromatography with the mixture and the pure substance that we are testing for (known as the reference) on the baseline.
We then work out the Rf values for the reference (pure substance) and any spots in the mixture that are around the same height as the reference. If the Rf values are the same, it tells us that the substance that we are testing for (the reference substance) may be present in the mixture.
We then change the solvent a few times and complete the whole process again. If the Rf values for the reference substance and spot in the mixture are the same in all of the different solvents, it is very likely that the reference substance is present in the mixture. If the Rf values are only the same for some of the solvents, it means that the reference substance is not present in the mixture.
1) Paper chromatography was carried out by a student on the four dyes A, B, C and D. The solvent used was water. The chromatogram is show below.
a) We use pencil to draw the baseline. Why do we use pencil and not ink?
b) Dye D has stayed on the baseline. What does this tell us about dye D?
c) Dyes A, B and D have one spot. What does this imply about the composition of these dyes?
d) Dye C has two spots. What does this tell us about the composition of dye C?
e) What is the name of the line at the top of the chromatogram?
f) Calculate the Rf value for dye A. Give your answer to 2 decimal places.
g) Calculate the Rf value for dye B. Give your answer to 2 decimal places.
h) The student now carries out the chromatography process again, but this time uses ethanol rather than water. Will the Rf values for all of the substances stay the same? Explain your answer.
a) What are the two different phases in chromatography?
b) With respect to these two phases, why do the substances move different distances on the chromatogram?
c) We have the chromatogram below for dyes E, F and G. All of these dyes are pure substances.
i) Which of these dyes spends longer in the mobile phase compared to the stationary phase? Explain your answer.
ii) Which of these dyes spends longer in the stationary phase compared to the mobile phase? Explain your answer.
3) A student carried out chromatography and used water as the solvent. He worked out the Rf value for a particular substance to be 0.46.
The student now carries out the chromatography process again, but this time uses ethanol rather than water. The particular substance is more soluble in ethanol compared to water. What will happen to the Rf value? Explain your answer.
a) When we undertake chromatography, we put the piece of chromatography paper into a beaker containing the solvent. Do we put the chromatography paper into the solvent so that the baseline is in the solvent or above the solvent? Explain your answer.
b) We always have a lid on the container when completing chromatography. Why do we have a lid?
c) A student puts the chromatography paper into the solvent correctly (with reference to the baseline) and puts a lid on the container. The solvent starts to rise up chromatography paper. When should the student remove the chromatography paper?
5) A student completes a chromatography experiment of 3 different pure substances; H, I and J. His chromatogram is show below.
a) Find the Rf values for H, I and J. Give your answers to 2 decimal places.
Note: you can answer this question by measuring the distances on your computer, tablet or phone – you do not need a printed version.
b) How could the student modify the chromatography experiment to get a more accurate Rf value?
6) A student carries out a chromatography experiment and one of the samples stayed on the baseline; sample D stayed on the baseline. The chromatogram is shown below.
Explain a change that the student could make to the chromatography experiment to separate the substances in sample D.
7) How can we use chromatography to prove that a certain substance is present in a mixture?