Tests for Ions, Gases, and Water Exam Questions

(a) Crimson/red [1]

(b) Yellow/orange-yellow [1]

(c) Lilac/purple [1]

(d) Orange-red/brick red [1]

(e) Blue-green/green [1]

Examiner note: To carry out a flame test: clean a nichrome wire loop by dipping in concentrated hydrochloric acid and holding in a blue Bunsen flame until no colour is seen. Then dip the wire in the sample and hold it in the flame. Sodium contamination is very common — it produces a strong yellow flame that can mask other colours. Viewing through blue cobalt glass filters out the sodium yellow and reveals the lilac of potassium.

(a) Hold a burning/lighted splint near the gas — hydrogen burns with a squeaky pop [1]

(b) Insert a glowing splint into the gas — the splint relights [1]

(c) Bubble the gas through limewater (calcium hydroxide solution) — the limewater turns milky/cloudy [1]

(d) Hold damp litmus paper in the gas — the litmus paper is bleached/turns white (it may first turn red then bleach) [1]

Examiner note: For chlorine, damp blue litmus paper turns red first (because chlorine dissolves in water to form an acid) and then is bleached white. Some candidates confuse this with ammonia, which turns damp red litmus paper blue. The tests for H2, O2, CO2, and Cl2 must be recalled precisely — vague descriptions lose marks.

• Solution A: Cu2+ (copper(II)) [1]
• Solution B: Fe2+ (iron(II)) [1]
• Solution C: Al3+ (aluminium) [1]
• Solution D: Fe3+ (iron(III)) [1]

Examiner note: The precipitates are metal hydroxides. Aluminium hydroxide and zinc hydroxide are amphoteric — they dissolve in excess NaOH. To distinguish Al3+ from Zn2+, note that aluminium hydroxide is white and dissolves in excess NaOH, while zinc hydroxide is also white and dissolves in excess NaOH — so use another test (flame test or add excess ammonia solution: Zn(OH)2 dissolves in excess NH3 but Al(OH)3 does not).

• Test for water: add the liquid to anhydrous cobalt chloride paper — it turns from blue to pink [1] OR add the liquid to anhydrous copper sulfate — it turns from white to blue [1]
• Test for purity: measure the boiling point — pure water boils at exactly 100 degrees C at standard atmospheric pressure [1] OR measure the freezing point — pure water freezes at exactly 0 degrees C [1]

Examiner note: Cobalt chloride paper and anhydrous copper sulfate detect the presence of water, but they do not prove the water is pure — any water-containing liquid (even a solution) would give the same result. To prove purity, you must check the boiling or freezing point against the known values for pure water.

(a) Add dilute acid (e.g., HCl) — the solution fizzes/effervesces. Pass the gas through limewater — it turns milky, confirming CO2 is produced [1]

(b) Add dilute hydrochloric acid (to remove carbonate impurities), then add barium chloride solution — a white precipitate of barium sulfate forms (insoluble) [1]

(c) Add dilute nitric acid (to remove carbonate impurities), then add silver nitrate solution — a white precipitate of silver chloride forms [1]

(d) Add dilute nitric acid, then add silver nitrate solution — a yellow precipitate of silver iodide forms [1]

Examiner note: For sulfate testing, you must acidify with hydrochloric acid first to remove carbonates (which would also form a white precipitate with barium chloride). For halide testing, acidify with nitric acid (not hydrochloric acid, which would add chloride ions). Silver chloride is white, silver bromide is cream, silver iodide is yellow.

(a) Ammonia (NH3) [1]

(b) The powder contains an ammonium compound / ammonium ions (NH4+) [1]

(c) NH3 + H2O → NH4+ + OH- (or NH3 + H2O ⇌ NH4+ + OH-) [1]

Examiner note: The test for ammonium ions: warm the sample with dilute sodium hydroxide solution and test the gas with damp red litmus paper. Ammonia is the only common alkaline gas, so it is the only gas that turns damp red litmus paper blue. Heating an ammonium compound with NaOH decomposes it: NH4Cl + NaOH → NaCl + H2O + NH3.

• Yellow flame indicates sodium (Na+) [1]
• No precipitate with NaOH means there is no transition metal or aluminium cation [1]
• White precipitate with silver nitrate (after acidification) indicates chloride ions (Cl-) [1]
• The compound is sodium chloride (NaCl) [1]

Examiner note: This is a classic identification question that combines multiple tests. Work through each test systematically: the flame test identifies the cation, the NaOH test confirms it is not a transition metal, and the silver nitrate test identifies the anion. State each conclusion clearly and combine them for the final identification.

• Acid is added to remove carbonate ions, which would otherwise react with the test reagent and give a false positive (e.g., barium carbonate or silver carbonate precipitates are both white) [1]
• Nitric acid is used before silver nitrate because hydrochloric acid contains chloride ions, which would react with silver nitrate to form silver chloride, giving a false positive result [1]
• Hydrochloric acid is used before barium chloride because nitric acid would also work, but HCl is the conventional choice and does not interfere with the sulfate test (barium chloride is the test reagent, and the chloride is already present) [1]

Examiner note: The choice of acid is crucial and is a common source of lost marks. Using HCl with AgNO3 means you are adding Cl- ions, which will form AgCl regardless of whether the sample contains chloride — a guaranteed false positive. This is one of the most frequently tested points in analytical chemistry.