CCEA GCSE Chemistry 1.9 Chemical Analysis Exam Questions

About CCEA GCSE Chemistry 1.9 Chemical Analysis

Chemical Analysis is a Unit 1 topic in CCEA GCSE Chemistry about separating mixtures and identifying substances. You need to know what a pure substance is in chemistry, what formulations are, and how to choose and describe separation methods including filtration, crystallisation, simple and fractional distillation and paper chromatography. The topic also covers how drinking water is treated. A major part is the chemical tests for metal ions, anions and common gases, which form a prescribed practical. These tests reappear in gas chemistry and in Unit 3 practical questions. Examiners frequently ask for exact colours of flame tests and precipitates, and Rf value calculations from chromatograms.

What you need to know

• Purity and formulations A pure substance contains only one element or compound and has a sharp, fixed melting and boiling point. A formulation is a mixture designed for a specific use, with measured quantities of each component, such as paints and medicines.
• Separation techniques Filtration separates an insoluble solid from a liquid. Crystallisation obtains a soluble solid from solution. Simple distillation separates a solvent from a solution; fractional distillation separates miscible liquids with different boiling points.
• Chromatography Draw the baseline in pencil and keep it above the solvent level. Rf = distance moved by spot / distance moved by solvent front. A pure substance gives a single spot; matching Rf values help identify substances.
• Flame tests Lithium crimson, sodium yellow-orange, potassium lilac, calcium brick-red, copper blue-green.
• Tests for cations with sodium hydroxide Cu2+ blue precipitate, Fe2+ green precipitate, Fe3+ red-brown precipitate. Mg2+, Al3+ and Zn2+ give white precipitates; the aluminium and zinc precipitates dissolve in excess sodium hydroxide. Warming NH4+ with sodium hydroxide gives ammonia, which turns damp red litmus blue.
• Tests for anions Halides: add dilute nitric acid then silver nitrate: chloride white, bromide cream, iodide yellow precipitate. Sulfate: add dilute hydrochloric acid then barium chloride: white precipitate. Carbonate: add dilute acid: effervescence, gas turns limewater milky.
• Water treatment Drinking water is treated by sedimentation, filtration through sand and gravel, and chlorination to kill microorganisms.

Common mistakes in 1.9 Chemical Analysis

• Drawing the chromatography baseline in ink. Ink dissolves in the solvent; use pencil.
• Giving the wrong precipitate colours, such as writing yellow for bromide. Bromide gives a cream precipitate and iodide a yellow one.
• Forgetting to acidify before testing for halides or sulfate. The acid removes carbonate ions that would give a false positive.
• Saying a pure substance means nothing has been added to it, as in everyday language. In chemistry it means a single element or compound.
• Calculating Rf by dividing solvent distance by spot distance. Rf is always less than 1.

Try three 1.9 Chemical Analysis exam-style questions