Chromatography — Chemistry, 14–17 years
Chromatography separates a mixture so its substances can be seen and identified. Chemistry, 14–17 years.
A race through a material
In chromatography, a solvent carries a mixture across a material such as paper. Each substance is attracted differently to the moving solvent and the paper, so some travel farther and others lag behind. The separate spots reveal that the original sample contained more than one substance.
Why separate a mixture
A colour, smell or liquid may look like one substance even when it is a mixture. Separation gives chemists a clearer test: they can compare the positions of components with known samples or collect one component for further analysis. It solves the problem of identifying hidden ingredients.
Calculating an Rf value
A dye spot starts 2.0 cm from the pencil line and travels 3.6 cm. The solvent front travels 6.0 cm. Calculate Rf = distance travelled by spot ÷ distance travelled by solvent front: 3.6 ÷ 6.0 = 0.60. A matching pure dye should give a similar value under the same conditions.
A spot is not a proof
It is sensible to think that two spots at the same height must be the same substance. However, different substances can sometimes travel together, and a changed solvent or paper changes the distances. A matching spot is useful evidence, not complete identification by itself.
Checking real samples
Chromatography is used to check food colours, medicines, inks, perfumes and pollutants in water. Forensic scientists may compare ink from a note with pens found elsewhere. The method is valuable because it works with tiny samples, though important results usually need a second test for confirmation.
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