The periodic table - 3.1.4 Qualitative analysis — OCR A-Level Chemistry

Qualitative analysis on a test-tube scale identifies ions in an unknown compound from observable changes rather than measurements. In this section the cation focus is the ammonium ion: warm the unknown with sodium hydroxide solution and test the gas produced with damp red litmus paper, which turns blue if ammonia is present. The anion focus is the carbonate ion: add dilute acid and pass any gas through limewater, which turns milky or cloudy when carbon dioxide is produced. Choose the reagent, observe the change and name the ion the change confirms.

This statement sets the approach for qualitative analysis on a test-tube scale: use small-scale tests, add reagents carefully and observe changes to identify ions in an unknown compound. The processes and techniques include dissolving or suspending the sample, adding the chosen reagent, warming where a gas must be released, and testing any gas with the correct indicator or reagent. For the ammonium ion, warm the sample with sodium hydroxide solution and test the gas with damp red litmus paper, which turns blue. For the carbonate ion, add dilute acid and pass the gas through limewater, which turns milky.

This heading introduces the anion tests in qualitative analysis: the identification of negatively charged ions in aqueous solution by characteristic reactions. The specification lists carbonate, sulfate and halide anions, each confirmed by a distinct observation. Carbonate ions are detected by adding dilute acid, when carbon dioxide gas is released and turns limewater milky. Sulfate ions are detected by adding barium chloride or barium nitrate solution, when a white precipitate of barium sulfate forms. Halide ions are detected by adding silver nitrate solution, when a precipitate of the silver halide forms; the colour differs between chloride, bromide and iodide. A multiple-choice question on this heading asks you to select the correct reagent, ion or observation, so learn each test as a reagent–observation pair rather than a single fact.

Carbonate ions, CO₃²⁻, are identified by adding an aqueous acid, which supplies H⁺(aq). The reaction releases carbon dioxide gas, CO₂(g), and this gas is confirmed by bubbling it through limewater, which turns milky or cloudy. The ionic equation is CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l), so two hydrogen ions are needed for each carbonate ion. A multiple-choice question may ask for the gas produced, the ionic equation, or the observation that confirms the gas. Learn the balanced equation and the limewater test together, and check that charges balance: the left side has a total charge of zero, matching the neutral products.

Sulfate ions in solution are identified by adding aqueous barium ions, usually as barium chloride solution, acidified with dilute hydrochloric acid. Barium ions and sulfate ions combine to form barium sulfate, an insoluble white precipitate: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The acid prevents false positives from carbonate ions, which would also give a white precipitate with barium ions but dissolve with effervescence. A positive result is a white precipitate that does not dissolve when excess dilute acid is added. The ionic equation and the observation together identify SO₄²⁻.

This entry points to the halide tests covered in 3.1.3 g. To identify Cl⁻, Br⁻ and I⁻, acidify the sample with dilute nitric acid, then add silver nitrate solution, AgNO₃(aq). A precipitate of the silver halide forms: silver chloride is white, silver bromide is cream and silver iodide is yellow. Confirm by adding dilute ammonia: silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide is insoluble in ammonia. The ionic equation is Ag⁺(aq) + X⁻(aq) → AgX(s). Nitric acid removes carbonate interference.

The ammonium ion, NH₄⁺, is identified by adding warm sodium hydroxide solution, NaOH(aq), to the sample. The hydroxide ions react with the ammonium ion in a reversible acid–base step: NH₄⁺ + OH⁻ ⇌ NH₃ + H₂O. Warming shifts the equilibrium and lowers the solubility of ammonia, so NH₃ gas is released. The gas is detected by its pungent smell and by turning damp red litmus paper blue, because NH₃ is an alkaline gas that dissolves to form OH⁻ ions. The test therefore depends on warming, on using NaOH(aq) rather than a dilute acid, and on testing the gas above the mixture rather than the solution itself. A positive result confirms NH₄⁺; no precipitate is expected, which distinguishes it from many metal cations.