Chromatography and purityIB MYP Chemistry: Revision notes
Section 1
Paper chromatography
Chromatography separates the substances in a mixture, such as the dyes in an ink. In paper chromatography a small spot of the sample is placed on a start line drawn in pencil (ink would dissolve and move). The bottom of the paper is stood in a solvent, such as water or ethanol, with the solvent level below the start line. A lid stops the solvent evaporating.
The solvent soaks up the paper and carries the substances with it. When the solvent front is near the top, the paper is removed and the position of the front is marked straight away. The dried paper is called a chromatogram.
Do not use a pen for the start line, as the ink would run up the paper with the solvent. Use a pencil, because graphite does not dissolve.
Section 2
Why the substances separate
Each substance in the sample is dissolved in the solvent and also attracted to the paper. A substance that is more soluble in the solvent, and less attracted to the paper, moves further up the paper. A substance that is less soluble, or more attracted to the paper, moves less far. The different distances travelled separate the mixture into spots.
Section 3
Reading a chromatogram
Count the spots.
- A pure substance gives one spot.
- A mixture gives two or more spots.
- A spot level with a known reference substance, run on the same paper, shows that the substance is present.
- A spot that matches no reference is an unknown substance.
An insoluble substance does not move from the start line.
Section 4
Rf values
The Rf value compares how far a substance moves with how far the solvent moves:
Rf = distance moved by the spot ÷ distance moved by the solvent front
Both distances are measured from the start line, in the same units, so Rf has no units and is always between 0 and 1.
Worked example: a spot moves 3.6 cm while the solvent front moves 9.0 cm. Rf = 3.6 ÷ 9.0 = 0.40.
For the same substance, solvent and paper, the Rf value is always the same, so Rf values identify substances even when different papers are used. Substances with the same Rf value are likely, but not certain, to be the same substance.
Measure from the start line, not from the bottom of the paper, and divide the smaller distance by the larger one so the answer is less than 1.
Section 5
Melting and boiling points as tests of purity
A pure substance melts at one fixed temperature and boils at one fixed temperature. For example, pure water melts at 0 °C and boils at 100 °C (at normal pressure), and pure benzoic acid melts at 122 °C.
An impure substance:
- melts over a range of temperatures rather than at one point
- melts at a lower temperature than the pure substance
- boils at a higher temperature than the pure substance, and over a range
The more impurities, the greater the difference. Comparing a measured value with the known value shows how pure a sample is.
Section 6
Chromatography investigations (criteria B and C)
In a fair investigation, control the paper, the solvent, the size of the spot, and the time allowed. Measure distances to the centre of each spot.
To make results more reliable, repeat the experiment and calculate a mean. To improve melting point tests, heat slowly near the melting point and use the same mass of each sample.
When you evaluate evidence, ask what it can and cannot show. Matching Rf values show that substances are similar, not that they are certainly the same. Safety: wear eye protection, and use flammable solvents away from flames.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chromatography and purity
- A student uses paper chromatography to analyse a black ink. The solvent front moves 8.0 cm from the start line. The ink separates into three spots, which move 2.0 cm, 4.8 cm and 6.4 cm from the start line. She runs three known dyes on the same paper: dye A moves 2.0 cm, dye B moves 3.2 cm and dye C moves 6.4 cm.Explain how the chromatogram shows that the ink is not a pure substance.2 marks
- A student sets up paper chromatography for a sample of green food colouring. She draws a start line near the bottom of the paper in pencil, puts a small spot of the sample on the line and stands the paper in a beaker containing a shallow layer of solvent. The solvent level is below the start line, and she puts a lid on the beaker.Explain how the different colours in the food colouring become separated.2 marks
- A student tests the purity of two samples of a white solid sold as pure benzoic acid, which has a melting point of 122 °C. She heats a small amount of each sample slowly in a melting point tube and records the range of temperatures over which it melts. Sample 1 melts between 121 °C and 122 °C. Sample 2 melts between 112 °C and 119 °C.Decide which sample is purer, using the data to justify your answer.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).