A white solid, Z, is tested.
- In a flame test it gives a lilac flame.
- When dilute nitric acid and silver nitrate solution are added to a solution of Z, a cream precipitate forms.
(a) Identify Z. [2]
(b) Describe how you would test a solution for sulfate ions and give the result for a positive test. [2]
(c) Describe the test for ammonia gas. [1]
- (a) potassium (ions) / K⁺(1)
- (a) bromide (ions) / Br⁻ (so potassium bromide / KBr scores both marks; reject bromine)(1)
- (b) add dilute hydrochloric acid (allow dilute nitric acid) and barium chloride solution (allow barium nitrate; allow 'acidified barium chloride'; reject sulfuric acid)(1)
- (b) white precipitate (dep on barium compound used)(1)
- (c) damp red litmus (paper) turns blue (allow moist / wet for damp; allow damp universal indicator paper turns blue / purple)(1)
- Answer: (a) potassium bromide, KBr (b) add dilute hydrochloric acid then barium chloride solution: white precipitate (c) damp red litmus paper turns blue
- Examiner tip: Litmus must be DAMP — gases only affect indicator paper when they can dissolve.