Edexcel GCSE Chemistry Topic 9: Separate Chemistry 2 Exam Questions

About Edexcel GCSE Chemistry Topic 9: Separate Chemistry 2

Topic 9 of Edexcel GCSE Chemistry is separate Chemistry only content assessed on Paper 2. It begins with qualitative analysis: flame tests, sodium hydroxide tests for metal ions, tests for ammonium, carbonate, sulfate and halide ions, and instrumental flame photometry, with a core practical identifying the ions in unknown salts. It then extends organic chemistry to alkenes, addition and condensation polymers, natural polymers, alcohols and carboxylic acids, with a core practical on the combustion of alcohols. Finally it covers nanoparticles and how bulk materials such as ceramics, polymers, composites and metals are chosen for uses. Expect identification questions and polymer structure drawings.

What you need to know

• Tests for ions Flame tests: lithium red, sodium yellow, potassium lilac, calcium orange-red, copper blue-green. Sodium hydroxide gives white precipitates with Al3+, Ca2+ and Mg2+ (aluminium dissolves in excess), blue with Cu2+, green with Fe2+ and brown with Fe3+. Ammonium ions with warm sodium hydroxide give ammonia, which turns damp red litmus blue.
• Anion tests and flame photometry Carbonates fizz with dilute acid giving carbon dioxide. Sulfates give a white precipitate with dilute HCl and barium chloride. Halides with dilute nitric acid and silver nitrate give white (chloride), cream (bromide) and yellow (iodide) precipitates. Flame photometry identifies metal ions and measures their concentration from emission spectra.
• Alkenes and polymers Alkenes contain C=C and decolourise bromine water. Addition polymers such as poly(ethene), poly(propene), poly(chloroethene) (PVC) and PTFE form from alkene monomers. Condensation polymers such as polyesters form from monomers with two functional groups, eliminating a small molecule such as water. Problems include non-biodegradability and limited landfill.
• Biological polymers DNA is a polymer of nucleotides; proteins are polymers of amino acids; starch and cellulose are polymers of sugars.
• Alcohols and carboxylic acids Alcohols contain -OH (methanol, ethanol, propanol, butanol) and can be oxidised to carboxylic acids (-COOH). Ethanol is made by fermentation of carbohydrates with yeast and concentrated by fractional distillation. Core practical: investigate the temperature rise produced by burning a known mass of different alcohols.
• Nanoparticles and materials Nanoparticles are 1 to 100 nm across and have a very high surface area to volume ratio, giving different properties from bulk materials; possible risks are not fully understood. Compare ceramics, polymers, composites and metals to choose materials for particular uses.

Common mistakes in Topic 9: Separate Chemistry 2

• Adding hydrochloric acid in the halide test. Nitric acid must be used, otherwise the added chloride gives a white precipitate.
• Mixing up the litmus colours for ammonia. Ammonia turns damp red litmus blue because it is alkaline.
• Leaving the double bond in the repeating unit of an addition polymer.
• Saying ethanol made by fermentation is pure. Fermentation produces a dilute solution that must be fractionally distilled.
• In the alcohol combustion practical, not keeping the volume of water and distance of the flame the same, making it an unfair test.

Try three Topic 9: Separate Chemistry 2 exam-style questions