The method and the results are on the chemical analysis lab. Do the analysis yourself first, then check the answers below.
Quick answer: Chemical analysis identifies what a sample contains. These answers go with the Chemistry 9-10 lab on chemical analysis: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
These model answers use the sample data to show how Rf values are calculated, how spots are matched between samples, and how to evaluate the method. Compare your numbers carefully with the working shown.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C8
U has spots at 0.30, 0.60 and 0.80. Pens P and Q together give exactly these Rf values. R has 0.50, which does not appear in U, so R is not in U.
Question 3 Shared dye
The spot at Rf = 0.30 is shared. In the same solvent a substance has a fixed Rf, so P and Q contain the same dye in this spot.
Question 4 Is R pure
No. One spot is consistent with a single dye but two dyes can have the same Rf in one solvent. Run R in a second solvent and see if it still gives only one spot.
Question 5 Pencil baseline
Pencil lead is insoluble, so it does not move. If the baseline were under the water, the ink would dissolve into the beaker instead of travelling up the paper.
Question 6 Marking the front
The solvent evaporates from the paper, so the front would be lost. The front distance is needed for Rf = distance of spot ÷ distance of front.
Question 7 Effect of a different solvent
The Rf values would change because the dyes dissolve differently, but the same dyes would still travel the same relative order, so the conclusion about which dyes are in U would stay the same if all strips used the same new solvent.
Question 8 Improve reliability
Repeat each strip three times and use mean Rf values, and measure to the centre of each spot using the same method each time. Use strips cut from the same sheet of paper.
