Purity And Chromatography
Bayani Gaba-gaba
Squeeze a drop of black food colouring onto a strip of damp paper and wait. Within a few minutes the black has gone, and in its place a ladder of separate colours is climbing the paper: a pink, a blue, sometimes a yellow you would never have guessed was in there. Nothing has reacted. Nothing has been destroyed. The dyes were always there, sitting on top of one another, and all the paper did was make them travel at different speeds.
That is chromatography, and it is one of the most useful things a laboratory can do. This lesson starts with what a mixture actually is and why the substances in one can always be pulled apart again without any chemistry happening at all. Then it takes you through running a paper chromatogram yourself, precaution by precaution, and through the small calculation that turns a coloured smudge into an Rf value: a number you can compare with a reference and use to name a substance you have never seen before, or to show that a sample which was supposed to be one substance is quietly two.
Manufura
- A mixture consists of two or more elements or compounds not chemically combined together. The chemical properties of each substance in the mixture are unchanged. It is possible to separate the substances in a mixture by physical methods, including distillation, filtration and crystallisation.
- Paper chromatography can be used to analyse substances present in a solution, eg food colourings and inks/dyes. Students should be able to describe how to carry out paper chromatography separations and how the components of a mixture can be identified using Rf values. They have to be aware that solvents other than water can be used.
- Chromatography involves a stationary and a mobile phase and separation depends on the relative solubility of the components. Students should be able to suggest chromatographic methods for distinguishing pure from impure substances.`
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- Which statement about a mixture is correct? A. The substances in it are chemically combined together B. The substances in it keep their own chemical properties C. It can only be separated by a chemical reaction D. It must always contain exactly two substances Answer: B
- Which method would you use to separate an insoluble solid from the liquid it is mixed with? A. Crystallisation B. Filtration C. Fractional distillation D. Paper chromatography Answer: B
- In paper chromatography, why is the baseline drawn in pencil rather than in ink? A. Pencil lines are easier to see on wet paper B. Pencil does not dissolve in the solvent, so the line stays in place C. Ink reacts chemically with the chromatography paper D. Pencil makes the samples travel further up the paper Answer: B
- In paper chromatography, which is the mobile phase? A. The chromatography paper B. The solvent C. The spots of dye D. The lid on the container Answer: B
- A spot on a chromatogram has moved 6.0 cm from the baseline. The solvent front has moved 15.0 cm from the baseline. What is the Rf value of the spot? A. 0.25 B. 0.40 C. 2.50 D. 9.00 Answer: B
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