Chromatography Cambridge IGCSE Co-ordinated Sciences (Double Award) revision
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In plain words
Put a dot of ink near the bottom of a strip of paper and stand the paper in a little water. As the water creeps up it carries the dyes with it, and they separate, because some dyes travel faster than others. One dot becomes several spots, one for each dye.
Three things to know
- Paper chromatography separates a mixture of dissolved substances, such as the dyes in ink. The solvent carries them up the paper, and the more soluble ones travel further.
- A pure substance gives one spot. A mixture gives more than one. Spots at the same height as a known substance are the same substance.
- Rf = distance moved by the substance ÷ distance moved by the solvent. It is always less than 1.
Worked example
On a chromatogram a spot has moved 6.0 cm and the solvent front has moved 8.0 cm. Find the Rf value.
- Rf = distance moved by the spot ÷ distance moved by the solvent.
- = 6.0 ÷ 8.0.
- = 0.75.
Tips and tricks
- An Rf value is always less than 1 and has no units. If yours is bigger than 1, the division is upside down.
- Measure both distances from the pencil start line, to the middle of the spot.
It lands in your notebook with its questions as flashcards.
Chromatography: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
How many spots does a pure dye give on a chromatogram?
A single substance cannot be separated any further.
How is an Rf value calculated?
The spot never travels further than the solvent, so Rf is below 1.
A spot moves 3 cm and the solvent front moves 12 cm. What is the Rf value?
3 ÷ 12.
Why does one dye travel further up the paper than another?
The more soluble a substance is, the further the solvent carries it.
Two spots from different inks reach the same height. What does this suggest?
The same substance has the same Rf value in the same solvent.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 8 marks. Write your answers on paper, then check them.
Chromatography
Cambridge IGCSE Co-ordinated Sciences (Double Award) 0654 · 8 marks · papermunch.org
Name ______________________________ Date ______________
Explain why the start line on a chromatogram is drawn in pencil and not ink.[2]
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Pencil does not dissolve in the solvent. Ink would dissolve and run up the paper with the samples.
A spot moves 4.5 cm while the solvent front moves 9.0 cm. Calculate the Rf value.[2]
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0.50. 4.5 ÷ 9.0.
State how a chromatogram shows that a food colouring is a mixture.[2]
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It separates into more than one spot.
Explain why the solvent level must start below the start line.[2]
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Otherwise the spots would dissolve straight into the solvent in the beaker instead of being carried up the paper.
Answers: Chromatography
- 1. Pencil does not dissolve in the solvent. Ink would dissolve and run up the paper with the samples.
- 2. 0.50. 4.5 ÷ 9.0.
- 3. It separates into more than one spot.
- 4. Otherwise the spots would dissolve straight into the solvent in the beaker instead of being carried up the paper.