RP4: Identifying Inorganic Ions

Required coverage includes Group 2 and ammonium cations and halide, hydroxide, carbonate and sulfate anions. Use labelled, separate portions so one reagent does not contaminate a later test. Record the reagent, conditions and actual observation before writing an inference. A negative result is evidence only if the test conditions and reagent are working.

For the familiar Group 2 flame tests, calcium gives brick-red/orange-red, strontium crimson and barium apple-green; magnesium has no characteristic visible flame colour in this test. Sodium contamination can mask weaker colours. Follow the approved clean-wire procedure and do not infer that every Group 2 ion can be identified reliably from a visible flame alone. Solubility trends can provide supporting comparisons.

Carbonate releases carbon dioxide on addition of dilute acid; passing the gas through limewater gives cloudiness. Hydroxide is alkaline, so an indicator or pH measurement supports its detection, but alkalinity alone cannot distinguish hydroxide from every other basic anion. Interpret the set of possible unknowns and use complementary tests.

For sulfate, acidify with dilute hydrochloric acid then add barium chloride: a white barium sulfate precipitate persists. Acid removes carbonate interference; sulfuric acid would introduce sulfate. For halides use dilute nitric acid then silver nitrate: chloride gives white, bromide cream and iodide yellow precipitates. Hydrochloric acid would introduce chloride. Silver chloride dissolves in dilute ammonia, silver bromide in concentrated ammonia, and silver iodide remains insoluble under those tests.

With aqueous sodium hydroxide and gentle warming, ammonium ions release ammonia, which turns damp red litmus blue. Use the teacher’s ventilation and warming arrangements; do not identify a gas by directly inhaling it. The ionic equation is NH₄⁺ + OH⁻ → NH₃ + H₂O.

Small test portions reduce reagent use and hazardous waste. Barium and silver salts and ammonia require the specified handling and disposal. A matching observation supports an identification within the candidate set; mixtures, low concentration or contamination can create ambiguity. Repeat with fresh apparatus or compare a known reference when the observation is weak rather than inventing a more decisive colour.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.