OCR GCSE Chemistry A C4.2 Identifying the Products of Chemical Reactions Exam Questions

About OCR GCSE Chemistry A C4.2 Identifying the Products of Chemical Reactions

C4.2 covers the chemical tests used to identify gases and ions, along with modern instrumental methods of analysis. On the OCR Gateway Chemistry A course it completes Topic C4 Predicting and identifying reactions and products, assessed in Paper 2 (Foundation) or Paper 4 (Higher). It builds on acid reactions and electrolysis in C3 and the groups in C4.1. This topic is very recall-heavy: examiners expect exact test methods and observations, ask you to identify unknown compounds from a table of results (PAG work), and test ionic equations for precipitates and the advantages of instrumental methods, including interpreting a mass spectroscopy chart against reference data.

What you need to know

• Gas tests Hydrogen burns with a squeaky pop when tested with a lighted splint. Oxygen relights a glowing splint; carbon dioxide turns limewater milky (cloudy); chlorine bleaches damp blue litmus paper (it may turn red first).
• Flame tests Lithium gives a crimson flame, sodium yellow, potassium lilac, calcium orange-red and copper green. Use a clean nichrome wire loop dipped in acid between tests.
• Metal hydroxide precipitates Adding sodium hydroxide solution forms coloured precipitates: Cu2+ gives blue, Fe2+ gives green and Fe3+ gives orange-brown. For example, Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s).
• Carbonates and sulfates Carbonates fizz with dilute acid, giving carbon dioxide that turns limewater milky. Sulfates give a white precipitate with dilute hydrochloric acid and barium chloride solution.
• Halides Add dilute nitric acid then silver nitrate solution: chloride gives a white precipitate, bromide a cream precipitate and iodide a yellow precipitate.
• Instrumental methods Instrumental methods of analysis are sensitive, accurate and fast, and work with very small samples. You should be able to interpret an instrumental result, such as a mass spectroscopy chart, by comparing it with a reference set of data.

Common mistakes in C4.2 Identifying the Products of Chemical Reactions

• Saying chlorine turns litmus paper white without saying it must be damp. Use damp litmus paper.
• Describing the oxygen test as a lit splint that burns brighter. The test is a glowing splint that relights.
• Forgetting to acidify before adding silver nitrate or barium chloride. Acid removes carbonate ions that would also give a precipitate.
• Mixing up iron hydroxide colours. Iron(II) gives green and iron(III) gives orange-brown.
• Saying limewater 'turns cloudy' as a test for any gas. Only carbon dioxide turns limewater milky.

Try three C4.2 Identifying the Products of Chemical Reactions exam-style questions