6.3.1 Chromatography and Qualitative Analysis

For TLC, Rf = distance travelled by the solute ÷ distance travelled by the solvent front. Measure both distances from the start line to the centre of the spot or to the solvent front, using the same units.

An Rf value lies between 0 and 1. Under the same experimental conditions, matching Rf values can support an identification and one spot can support purity; extra spots show extra components. In the 2025 OCR paper, a recurring error was measuring from the bottom of the plate instead of the start line.

Each component in a gas chromatogram gives a peak at a retention time. Compare that time with a known standard run under the same conditions; a matching retention time supports the identity of that component.

Retention time is not a universal fingerprint. It depends on the column and operating conditions, so compare like with like. If two components have different retention times, the chromatogram has separated them.

Use the supplied peak integration/area values for amount. For a mixture, the fraction of one component is its peak area divided by the total peak area; multiply by 100 for a percentage.

OCR also expects external calibration. Run standards of known concentration, plot peak area against concentration, then use the sample peak area to read or calculate its concentration from the calibration line. Keep retention time for identity and peak area for amount — they answer different questions.

Add bromine solution to the sample at room temperature. An alkene reacts by addition across C=C, so the orange/brown bromine colour disappears.

Do not add UV light: that belongs to free-radical substitution of alkanes. Also use the whole observation pattern if the unknown could be a phenol, because phenol also decolourises bromine water and gives a white precipitate of 2,4,6-tribromophenol.

Warm the haloalkane with aqueous silver nitrate in ethanol. Ethanol helps the organic haloalkane mix with the aqueous reagent. Hydrolysis releases X⁻, which then forms a silver-halide precipitate with Ag⁺.

The precipitate colours are AgCl white, AgBr cream, AgI yellow. In this organic test, the precipitate appears as the haloalkane hydrolyses; a faster appearance means faster hydrolysis under the same conditions.

Phenol is weakly acidic enough to react with NaOH, forming a phenoxide salt, but it does not release CO₂ from carbonate. A carboxylic acid reacts with both NaOH and carbonate.

So if an O–H-containing unknown reacts with NaOH but gives no effervescence with carbonate, that supports phenol. Effervescence with carbonate points to a carboxylic acid instead.

Add 2,4-DNP (2,4-DNPH). An aldehyde or ketone gives a yellow/orange precipitate, showing that a carbonyl compound is present.

A positive 2,4-DNP result does not distinguish aldehyde from ketone. If that distinction matters, take a fresh portion and use Tollens’ reagent next.

Warm a fresh portion gently with Tollens’ reagent. An aldehyde reduces the silver ions and gives a silver mirror or grey/silver deposit; a ketone gives no reaction under the test conditions.

Pair this with 2,4-DNP when needed: 2,4-DNP says “aldehyde or ketone”, then Tollens tells you whether the carbonyl is an aldehyde.

Warm the sample with acidified dichromate(VI). Primary and secondary alcohols are oxidised, so the solution changes orange → green as Cr(VI) is reduced to Cr(III).

A tertiary alcohol gives no colour change under these usual conditions. State what you actually see: “orange to green” is stronger exam evidence than simply saying oxidation occurs.

Aldehydes are also oxidised by acidified dichromate, this time to carboxylic acids. The dichromate is reduced in the process, so an aldehyde also gives the orange → green colour change.

That means a positive dichromate test alone cannot prove the unknown is an alcohol. OCR’s 2025 report specifically highlighted students who missed the aldehyde reaction; use Tollens or other evidence to separate the possibilities.

Add aqueous carbonate. A carboxylic acid gives effervescence as CO₂ is released; confirm the gas by passing it through limewater, which turns cloudy or milky.

Keep observation and conclusion separate in your answer. OCR’s 2025 report noted that “CO₂ formed” did not answer an observation question by itself — write the bubbles/effervescence, then identify the gas.

Use fresh portions of the unknown where one test could consume or alter it, and choose tests that eliminate possibilities cleanly. Carbonate can pick out a carboxylic acid; 2,4-DNP followed by Tollens can separate aldehydes and ketones; acidified dichromate tests oxidisable alcohols but also aldehydes; silver nitrate in ethanol tests haloalkanes.

Treat bromine with care in a mixed unknown set because both alkenes and phenols can decolourise it. Build the identification from the whole pattern of observations, including precipitates and gas tests, and stop once one structure or functional-group class is uniquely supported.