به انگلیسی

Split the ink, read the spots

Science · Matter & Materials · ages 14-15

Name ______________________ Date ____________

A pen baseline is suitable because its ink cannot move in the solvent.
Circle one: True False
Why can dyes separate as solvent moves through chromatography paper?
- Dyes interact differently with solvent and stationary phase.
- The solvent sorts every dye only by the mass of its molecules.
- The paper changes each dye into a new substance as it moves up.
Measured from the same baseline, a spot centre is 3 cm away and the solvent front is 10 cm away. What is the spot's Rf value?
Answer: ______________
Why should the starting sample spots be above the solvent level in the container?
- To stop any solvent from rising up through the chromatography paper
- To make the pencil line dissolve before it contaminates the sample
- To keep sample out of the solvent pool at the start
A sample produces two well-separated dye spots in a clean chromatography run. What does this show?
- A pure substance whose dye always separates into two colours
- A mixture with at least two detected components
- A sample with exactly two components and no undetected ones
Which observation supports, but does not prove, a match between an unknown ink and a known ink?
- Matching spot colours and Rf values under shared conditions
- Matching pen prices and barrel colours, even when ink patterns differ
- Matching strip lengths, even when paper types and solvents differ
The same unchanged dye has different Rf values with two solvents. Paper and temperature are unchanged. What explains the difference?
- Its identity changed because each dye has one fixed Rf in all solvents.
- Its molecules became heavier because farther-moving dyes have more mass.
- The balance between moving and stationary phases changed.
A spot travels 2 cm and the solvent front travels 8 cm, both from the baseline. Tom calculates Rf = 8 / 2 = 4. Which correction should he make?
- Keep 4, because the solvent-front distance belongs on top.
- Use spot distance / front distance: 2 / 8 = 0.25.
- Add both distances, because their combined travel gives Rf = 10.

Answer key

For grown-ups. Fold this page away before handing over the rest.

Split the ink, read the spots W1-mt_BAcCcz11ls-s1

False · False. Pen ink may dissolve and travel with the sample, adding unwanted spots. Use pencil for the baseline.
Dyes interact differently with solvent and stationary phase. · Different interactions with the mobile and stationary phases can give dyes different travel distances without changing their identities.
0.3 · Rf = spot distance / solvent-front distance = 3 / 10 = 0.3. The centimetres cancel, so Rf has no units.
To keep sample out of the solvent pool at the start · Submerged sample spots may dissolve directly into the liquid in the container instead of travelling from a clear starting point on the paper.
A mixture with at least two detected components · Two separated sample spots show at least two detected components. Other components could overlap or remain undetected.
Matching spot colours and Rf values under shared conditions · A matching pattern under shared conditions supports a possible match. It does not uniquely identify the ink or pen.
The balance between moving and stationary phases changed. · A solvent change can alter the dye's distribution between mobile and stationary phases, changing Rf without changing the dye's identity.
Use spot distance / front distance: 2 / 8 = 0.25. · Tom reversed the ratio. Rf = 2 / 8 = 0.25. His arithmetic gives 4, but that result is outside the valid range.