Chemical tests
Chemical Tests
SB p.190A colourless gas pops when you hold a flame to it. Another colourless gas turns cloudy water milky. A clear solution turns a dull green solid a brilliant blue. None of these substances can be identified by looking at them — but each one answers a chemical-test, a procedure whose observable result has exactly one cause. This chapter is the analytical toolkit of International GCSE chemistry: five gas tests, the flame test and its five colours, the sodium hydroxide tests for metal cations and the ammonium ion, the silver nitrate, barium chloride and acid tests for three families of anions, and the two tests for water — one chemical, one physical. Every test follows the same logic: add the reagent, watch for the positive result, and name the ion or gas it proves.
Seven objectives organise the chapter, and every one of them is examined in both papers. The gas tests (2.44) are pure observation questions. The flame test (2.45) and its colour table (2.46) are method-and-recall questions. The cation tests (2.47) and anion tests (2.48) are the descriptive essay questions of this chapter — full marks live in the sequence of additions and the colour of each result. The two water tests (2.49 and 2.50) close the chapter by distinguishing presence of water from purity of water, a distinction the examiner likes to probe. Learn each test as a three-line script — reagent, result, conclusion — and the whole chapter compresses into one page you can reproduce under exam conditions.
Four earlier chapters load this one. From Chapter 12 (u2:c12) you need the halogens and their coloured silver salts in embryo; the halide tests here are the payoff of Group 7 chemistry. From Chapter 11 (u2:c11) you need the alkaline metals — lithium, sodium and potassium — whose flame colours and hydroxide solubility decide two of the tests. From Chapter 17 (u2:c17) you need the solubility rules, because every positive result in the cation and anion tests is an insoluble salt precipitating out of solution; the term precipitate was defined there. From Chapter 5 (u1:c05) you need balanced equations with state symbols — (s), (l), (g), (aq) — because writing the ionic equation for a precipitate is a standard follow-up mark here.