Edexcel IAL Chemistry Unit 6 — Practical Skills in Chemistry II

Seventeen WCH16 papers from 2020 to 2026 — 469 parts, 850 marks. One topic dominates: analysis of inorganic and organic unknowns is 31% of the marks — flame tests, anion and cation tests, gas tests, and the organic functional-group tests. Titrations and quantitative analysis are 17%, organic preparation and yield 15%, following a reaction by titrimetric and clock methods 14%, purifying an organic liquid 9% and preparing a transition-metal complex 8%. Ka determination, electrochemical cells and activation energy are each under 2%.

"Give" and "State" are the top command words — the paper asks for the observation, the reagent, the reason. "Explain" 12%, "Identify" 9%, "Suggest" 8%. Only 21% of parts are high-difficulty, the lowest of the three A2 papers. This is the paper where a well-organised student scores 45 out of 50: learn the observations table for every test, the method for every core practical, and the six standard errors.

Four abilities: independent thinking in a practical context; use and application of scientific methods — controlling variables, presenting data, evaluating results, understanding uncertainty; numeracy — graphs, significant figures, accuracy and precision; and use of apparatus — selecting it with reasons, considering range and resolution, identifying hazards. Questions may ask you to recall or interpret tests for ions, gases and functional groups from Units 1 and 2, to manipulate data from titrations, kinetics and electrochemical cells, and to comment on the preparation of inorganic or organic compounds.

Almost a third of the paper. Cations: flame colours for Group 1 and 2, and the sodium hydroxide and ammonia reactions of Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ with the excess observations. Anions: carbonate with acid and limewater, sulfate with barium chloride, halides with silver nitrate then ammonia, ammonium with warm sodium hydroxide. Gases: oxygen, hydrogen, carbon dioxide, chlorine, ammonia, nitrogen dioxide. Organic: bromine water for C=C, PCl₅ for –OH, Fehling's, Tollens' and 2,4-DNPH for carbonyls, iodoform, sodium carbonate for acids, silver nitrate in ethanol for halogenoalkanes.

Core Practicals 9a/9b (iodine–propanone by titration; clock reaction), 10 (activation energy), 11 (Ka of a weak acid), 12 (electrochemical cells), 13a/13b (Fe²⁺/MnO₄⁻ and S₂O₃²⁻/I₂ titrations), 14 (preparation of a transition-metal complex), 15 (analysis of unknowns) and 16 (preparation of aspirin). For each: the method in numbered steps, the measurement and its uncertainty, the calculation, the two largest sources of error and how to reduce them. Organic preparations add the techniques — reflux, distillation, separating funnel, drying, recrystallisation, melting temperature — and percentage yield.

Titration questions: burette readings to 0.05 cm³, concordant titres, mean titre, the unstructured calculation, and percentage uncertainty of each measurement. Rate questions: what a clock reaction measures, why a tangent gives the rate, how to find order from the data given. Preparation questions: why each step is done — why reflux, why wash with sodium carbonate, why dry, why recrystallise from a minimum of hot solvent — and how the melting temperature shows purity. Every one of these is a mark you can secure before the paper is opened.