Chromatography and qualitative analysis — OCR A-Level Chemistry

Chromatography separates components of a mixture by distributing them between a mobile phase and a stationary phase. In thin-layer chromatography (TLC) the stationary phase is a thin layer of solid adsorbent, such as silica, coated on a plate, and the mobile phase is a liquid solvent that creeps up the layer by capillary action. In column chromatography the stationary phase is packed in a tube and the mobile phase is run through it. In gas chromatography the mobile phase is a carrier gas and the stationary phase is a solid or a liquid coated on a solid support inside a heated column. Separation depends on how strongly each component is adsorbed on the stationary phase compared with how readily it moves with the mobile phase, so components travel at different rates and separate.

A one-way TLC chromatogram shows spots of separated components along a plate after a solvent has travelled up it. The Rf value of a component is the distance that component's spot has moved from the origin divided by the distance the solvent front has moved from the same origin, measured along the direction of travel. Because Rf is a ratio of two distances it has no units and lies between 0 and 1. A component that is strongly adsorbed stays near the origin and has a low Rf; one that moves readily with the solvent has a high Rf. Comparing the Rf of an unknown spot with that of a known standard run under the same conditions helps identify the component.

A gas chromatogram plots detector response against time for the components of a vaporised mixture carried through a column by a mobile gas phase. Interpretation means reading the trace to identify how many components are present and which is which. Each peak corresponds to a component emerging from the column; the number of peaks indicates the number of separable components. The position of a peak along the time axis gives its retention time, and the area under a peak relates to the amount of that component. You compare retention times with those of known standards run under identical conditions to assign identities, and you compare peak areas to judge relative proportions. This statement is the umbrella for the sub-clauses, including retention times, that follow it.

Retention time is the time a component spends in the column before it emerges and is detected, measured from injection to the peak maximum on the chromatogram. Different components have different retention times under fixed conditions, so retention time is used for qualitative identification. To identify an unknown, you run a standard of the suspected compound under identical conditions (same column, carrier gas flow and temperature) and compare its retention time with that of the unknown peak. A match supports identification; a mismatch rules the compound out. Retention time depends on conditions, so comparisons are only valid when conditions are kept the same. Retention time does not by itself give the amount; peak area does that.

Chromatography separates the components of a mixture between a mobile phase and a stationary phase. The amount of each component present controls how large a signal it gives, while the proportion describes its share of the whole mixture. In paper or thin-layer chromatography the separated spots can be located and their sizes compared; in gas chromatography the detector response produces a peak whose area is proportional to the amount of that component. Comparing peak areas or spot sizes therefore gives the amounts, and expressing each as a fraction of the total gives the proportions. A mixture giving two peaks of equal area contains equal amounts of the two components, so each is 50% by the detector's response.

Qualitative tests identify which functional group a molecule contains by a visible change. Alkenes decolourise bromine water from orange to colourless. Aldehydes reduce Tollens' reagent to a silver mirror and reduce Fehling's solution from blue to a brick-red precipitate, whereas ketones do not. Carboxylic acids liberate carbon dioxide from sodium carbonate, giving effervescence. Alcohols can be distinguished by the change seen when warmed with acidified potassium dichromate(VI): a primary or secondary alcohol turns the orange solution green, while a tertiary alcohol does not. Each test must be linked to the specific observation, because the colour change is the evidence that earns credit.

This outcome covers small-scale test-tube chemistry used to identify which functional group an unknown organic compound contains. The learner selects a reagent, adds it to a small sample in a test tube, records what is seen (colour change, gas, precipitate, dissolving) and matches that observation to a functional group. The skill is qualitative: the evidence is an observation, not a measurement. Good technique matters, so the learner uses clean test tubes, small volumes, a water bath for warming where needed, and adds reagents carefully. The learner must also recognise that one positive test narrows possibilities but a conclusion is stronger when several tests agree and other groups are ruled out.

This outcome covers using bromine to test for an alkene on a test-tube scale. The learner adds bromine water to a small sample and observes the orange-brown colour being decolourised, which indicates a carbon-carbon double bond. The reaction is an electrophilic addition in which bromine adds across the double bond. The learner should distinguish this from an alkane, which gives no rapid decolourisation in the dark. The test is qualitative: the evidence is the loss of colour, not a measurement. The learner also notes that the test is carried out at room temperature and that excess bromine or light can complicate the observation.

This test identifies a haloalkane by warming it with aqueous silver nitrate in ethanol. The haloalkane undergoes nucleophilic substitution: the silver ion assists departure of the halide, and the halide ion that leaves immediately forms a silver halide precipitate. The colour of that precipitate identifies which halogen is present: silver chloride is white, silver bromide is cream and silver iodide is pale yellow. Ethanol is the solvent because the haloalkane and the aqueous silver nitrate do not mix well; ethanol brings them into one phase so the substitution can occur at a useful rate. The rate of precipitate formation also reflects the carbon–halogen bond strength, so iodoalkanes react fastest and chloroalkanes slowest. The test therefore gives both identity (from precipitate colour) and a rough reactivity order.

This test distinguishes a phenol from a carboxylic acid by its acidity. A phenol is a weak acid: the phenoxide ion formed when it loses a proton is stabilised by delocalisation of the negative charge into the aromatic ring, but this stabilisation is less than that of a carboxylate ion, so phenol is only weakly acidic. A carboxylic acid is strong enough to react with carbonate ions, releasing carbon dioxide, whereas phenol is not. Adding sodium carbonate solution to a phenol therefore produces no effervescence, while a carboxylic acid gives visible bubbles of carbon dioxide. The test uses this difference: no reaction with CO₃²⁻ indicates a phenol rather than a carboxylic acid. Phenol does react with sodium metal and with sodium hydroxide, so those reagents do not distinguish it from a carboxylic acid.

2,4-DNP (2,4-dinitrophenylhydrazine, Brady's reagent) is a qualitative test for the carbonyl group in aldehydes and ketones. The compound is dissolved in a minimum of ethanol, then a few drops of 2,4-DNP reagent are added and the mixture is shaken. A positive result is a yellow, orange or red-orange precipitate of the 2,4-dinitrophenylhydrazone derivative. The test therefore confirms that a carbonyl compound is present but does not distinguish an aldehyde from a ketone; that needs Tollens' or Fehling's reagent. The precipitate can be recrystallised and its melting point compared with literature values to identify the specific carbonyl compound. In a multiple-choice question, select the observation that names the coloured precipitate and the group detected, and reject options claiming a silver mirror or a colour change in solution only.

Tollens' reagent is ammoniacal silver nitrate, [Ag(NH₃)₂]⁺, made by adding aqueous ammonia to silver nitrate until the precipitate just dissolves. It is warmed gently with the aldehyde, and the aldehyde is oxidised to the corresponding carboxylate while the silver(I) complex is reduced to metallic silver. A positive result is a silver mirror on the inside of the test tube, or a grey-black precipitate of silver if the tube is not clean. Ketones do not react, so Tollens' reagent distinguishes aldehydes from ketones. In a multiple-choice question, choose the option that names the silver mirror and identifies the aldehyde as the species oxidised, and reject options describing a coloured 2,4-DNP precipitate or claiming the aldehyde is reduced.

Acidified dichromate(VI), usually written as K₂Cr₂O₇/H₂SO₄ or as Cr₂O₇²⁻ with H⁺, is an orange oxidising agent that turns green when it is reduced to Cr³⁺. You use this colour change to identify which functional group is present. Warm a primary alcohol with acidified dichromate and distillation collects an aldehyde, while refluxing gives a carboxylic acid; the orange solution turns green in both cases. A secondary alcohol is oxidised to a ketone and again turns the solution green. An aldehyde is oxidised to a carboxylic acid, so it also turns the reagent from orange to green. A ketone is not oxidised, so the reagent stays orange. The test therefore distinguishes aldehydes from ketones and confirms that a primary or secondary alcohol is present.

Carboxylic acids can be identified by adding a carbonate, such as sodium carbonate solution or solid calcium carbonate. A carboxylic acid is a weak acid but is strong enough to react with the carbonate ion CO₃²⁻, donating a proton to it. The reaction produces a carboxylate salt, carbon dioxide gas and water. For example, ethanoic acid reacts with sodium carbonate to give sodium ethanoate, carbon dioxide and water. The observable sign is effervescence as carbon dioxide is released; the gas turns limewater milky, confirming CO₂. This test distinguishes a carboxylic acid from most other organic compounds, though it is not unique to carboxylic acids because any acid strong enough to protonate CO₃²⁻ gives the same effervescence. In qualitative analysis the effervescence with carbonate is therefore used alongside other tests to confirm a carboxylic acid.