Quick answer: Chemical analysis identifies what a sample contains. This Chemistry 9-10 lab on chemical analysis gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Chromatography separates substances that dissolve differently. In this lab you separate the dyes in felt-tip inks, calculate Rf values for every spot, and use them to decide which pen inks were mixed to make an unknown ink. You also evaluate how reliable the evidence is.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C8
Aim
To use paper chromatography and Rf values to find which felt-tip pen inks are present in an unknown ink U.
Equipment
- chromatography paper strips, pencil and ruler
- felt-tip pens P, Q, R and a sample of unknown ink U
- 250 cm³ beaker, water as solvent, a pencil to hang the strips, tape
- safety goggles
Safety
Wear goggles and clean up spills at once. Water is the only solvent in this lab; if a teacher provides ethanol as a solvent, keep it away from flames and use a lid. Do not taste any ink.
Method
- Draw a pencil baseline 1.5 cm from the bottom of each strip.
- Place a small, single dot of each ink on the baseline of its own strip, and let it dry.
- Pour water into the beaker to a depth of about 1 cm.
- Tape the strip to the pencil so that the bottom edge just touches the water, with the baseline above the water.
- Leave the strip until the solvent front is near the top. Remove it, mark the solvent front immediately and let the paper dry.
- Measure the distances from the baseline to the centre of each spot and to the solvent front.
Results
Measured distances from the baseline:
| Strip | Solvent front (cm) | Spot distances (cm) |
|---|---|---|
| P | 8.0 | 2.4 and 4.8 |
| Q | 8.0 | 2.4 and 6.4 |
| R | 7.5 | 3.75 |
| U | 8.0 | 2.4, 4.8 and 6.4 |
Analysis questions
Question 1 Calculate all Rf values
Calculate the Rf value of every spot in the table.Question 2 Which pens are in U
Decide which pen inks are present in U and justify your decision.Question 3 Shared dye
Which spot is shared by P, Q and U? What does this suggest?Question 4 Is R pure
R gives one spot. Does that prove R is a pure dye? Suggest one further test.Question 5 Pencil baseline
Explain why the baseline is drawn in pencil and must be above the water level.Question 6 Marking the front
Why is the solvent front marked as soon as the strip is removed?Question 7 Effect of a different solvent
Predict what would happen to the Rf values if the solvent were changed, and say whether the conclusion would change.Question 8 Improve reliability
Suggest two ways of improving the reliability of the results.
