A student warms a white solid, X, with sodium hydroxide solution and holds damp red litmus paper at the mouth of the test tube. The litmus paper touches the inside of the tube and turns blue. She concludes that X contains ammonium ions.
Which is the best evaluation of her conclusion?
Not valid: sodium hydroxide on the glass may have turned the litmus blue
Valid: only ammonia gas can turn damp red litmus paper blue
Not valid: the litmus paper should have been dry, as ammonia is soluble
Valid: the paper must be damp, so touching the wet glass does not matter
35 exam-style questions on Edexcel GCSE Chemistry 10.1 Qualitative analysis: tests for ions, covering 10.1.1 Principles of ion tests and flame tests, 10.1.2 Tests for cations and for ammonia, 10.1.3 Tests for anions, 10.1.4 Identification of ions in unknown salts, and 10.1.5 Instrumental methods of analysis. Each one has a worked solution and a mark scheme showing where the marks go.