Sodium Hydroxide Test: Adding dropwise to metal aqua ions results in the formation of insoluble metal hydroxides. For example, forms a blue precipitate of .
Ammonia Test: Aqueous ammonia acts as a base initially, forming the same hydroxide precipitates as . However, with excess ammonia, some ions like undergo ligand substitution to form soluble complexes, resulting in a deep blue solution.
Oxidation on Standing: Certain precipitates, notably , are unstable in air and will slowly oxidize. The initial green precipitate of turns orange-brown at the surface as it converts to .
M2+ vs M3+ Acidity: Metal ions with a charge are more polarizing and acidic than ions. This higher charge density weakens the bonds in the coordinated water molecules, releasing ions into the solution.
Reaction with Carbonates: When is added, ions simply form insoluble metal carbonates (e.g., ). In contrast, ions are acidic enough to react with carbonate ions to produce gas and a hydroxide precipitate.
Observation of Effervescence: The presence of bubbles when adding carbonate solution is a definitive test for ions like or , distinguishing them from transition metals.
Halide Testing: Silver nitrate () acidified with nitric acid () is used to identify halides. produces a white precipitate, a cream precipitate, and a yellow precipitate.
Ammonia Solubility: To further distinguish halides, their solubility in ammonia is tested. dissolves in dilute ammonia, requires concentrated ammonia, and remains insoluble in both.
Sulfate Testing: Barium chloride () acidified with hydrochloric acid () reacts with sulfate ions to form a thick white precipitate of .
Carbonate Testing: Adding a dilute acid to a solid or solution triggers the release of gas, which can be confirmed by bubbling it through limewater, turning it cloudy.
| Ion | NaOH (aq) | (aq) | Initial Color |
|---|---|---|---|
| Blue precipitate | Blue-green precipitate | Pale blue | |
| Green precipitate (turns brown) | Green precipitate | Pale green | |
| Orange-brown precipitate | Brown precipitate + bubbles | Yellow/Brown | |
| White precipitate (redissolves) | White precipitate + bubbles | Colourless |
Acidification is Critical: Always add acid before testing for halides or sulfates to remove any carbonate ions that might produce a false positive precipitate (like or ).
Observation Precision: Distinguish clearly between a 'solution' and a 'precipitate'. Use terms like 'effervescence' instead of 'bubbles' and specify color changes accurately (e.g., 'pale green' to 'green precipitate').
Excess Reagent: Always note if a precipitate redissolves in excess reagent, as this is a primary identifier for amphoteric species like or specific complexes like .
Background Contrast: Use a white tile or paper behind test tubes to observe faint color changes or thin precipitates more clearly.
