12.3 Chromatography (Cambridge (CIE) O Level Chemistry): Flashcards
Exam code: 5070
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- FrontPaper Chromatography
Define a chromatogram.
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Cards in this collection (14)
Define a chromatogram.
A chromatogram is the visual output of a paper chromatography run, showing the separated components as distinct spots on the chromatography paper after the solvent front has moved.
In paper chromatography, the starting line is drawn in ______ rather than ink, to prevent the ink from running along with the ______.
In paper chromatography, the starting line is drawn in pencil rather than ink, to prevent the ink from running along with the sample.
Why must the pencil line be above the level of the solvent in paper chromatography?
The pencil line must be above the solvent level so that the samples do not dissolve directly into the solvent and get washed away before they can separate.
In paper chromatography, substances with higher ______ in the solvent travel ______ up the paper.
In paper chromatography, substances with higher solubility in the solvent travel further up the paper.
Separation also depends on how strongly each substance is adsorbed to the paper.
True or False?
A pure substance will produce a single spot in paper chromatography.
True.
A pure substance contains only one component and appears as a single spot, whereas a mixture produces multiple spots as its components separate.
The solvent rises up the chromatography paper by ______ action, carrying the dissolved sample components with it.
The solvent rises up the chromatography paper by capillary action, carrying the dissolved sample components with it.
How is a reference spot used to identify an unknown component in paper chromatography?
A reference spot of a known compound is run alongside the unknown sample. If both spots travel to the same distance up the paper, the substances are likely identical.
What is a locating agent?
A locating agent is a substance applied to a chromatogram after the run to make colourless components visible by reacting with them to produce a coloured product.
The retention factor (Rf) is calculated by dividing the distance travelled by the ______ by the distance travelled by the ______.
The retention factor (Rf) is calculated by dividing the distance travelled by the substance by the distance travelled by the solvent.
What does the Rf value allow a chemist to do?
The Rf value of a substance is a constant ratio under the same conditions, so a chemist can compare an unknown substance's Rf value with those of known compounds to identify it.
A locating agent is applied to a chromatogram to make ______ substances visible after the chromatography run has been ______.
A locating agent is applied to a chromatogram to make colourless substances visible after the chromatography run has been completed.
True or False?
The Rf value of a substance has units of centimetres.
False.
The Rf value is calculated as a ratio of two distances, so it is a dimensionless number with no units.
The Rf value of a particular compound is always the ______ under the same conditions, which is why it can be used for ______ of unknown substances.
The Rf value of a particular compound is always the same under the same conditions, which is why it can be used for identification of unknown substances.
Rf values are reproducible only when conditions such as solvent, paper type, and temperature are carefully controlled.
Why must a locating agent be applied after the chromatography run, not before?
A locating agent must be applied after the run because applying it before would cause it to run along with the sample, interfering with the separation and making the results impossible to interpret.
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