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Adding a small amount of hydroxide to many aqueous metal ions removes protons from coordinated water and forms a metal hydroxide precipitate. Copper(II) gives blue, iron(II) green and iron(III) brown precipitates under ordinary test conditions. Aluminium hydroxide is white and amphoteric; excess hydroxide dissolves it to form a soluble aluminate complex. Observations depend on reagent amount, so record both the initial addition and the effect of excess. Atmospheric oxidation can alter an iron(II) sample over time.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Predict reactions of aqueous metal ions with hydroxide and ammonia
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is the usual colour of iron(III) hydroxide?
Brown. Record it as a precipitate, not just a colour.
After watching 2: What does excess ammonia commonly produce with aqueous copper(II)?
A deep-blue soluble complex. Ammonia acts as a ligand.
What you'll be able to do
Adding a small amount of hydroxide to many aqueous metal ions removes protons from coordinated water and forms a metal hydroxide precipitate. Copper(II) gives blue, iron(II) green and iron(III) brown precipitates under ordinary test conditions. Aluminium hydroxide is white and amphoteric; excess hydroxide dissolves it to form a soluble aluminate complex. Observations depend on reagent amount, so record both the initial addition and the effect of excess. Atmospheric oxidation can alter an iron(II) sample over time.
Ammonia can act as a base, initially generating hydroxide and precipitating a metal hydroxide, or as a ligand when present in excess. A copper(II) hydroxide precipitate dissolves in excess ammonia to form a deep-blue complex. This distinguishes it from some metal hydroxides that remain insoluble. A full explanation should identify whether proton transfer, ligand substitution or electron transfer is occurring. Keeping those mechanisms separate makes it possible to explain an unfamiliar sequence rather than guessing solely from colour.
Use separate small samples for incompatible tests. Carbonate reacts with acid to give carbon dioxide, confirmed by limewater turning cloudy. Sulfate forms white barium sulfate after suitable acidification. Acidified silver nitrate identifies halides; ammonia distinguishes the precipitates. Ammonium ions release ammonia when warmed with sodium hydroxide, detected with damp red litmus turning blue. Do not add carbonate, sulfate or halide ions through the test reagents themselves. State the observation and a supporting equation, not merely that a reaction occurs.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: An unknown gives a white precipitate with NaOH that dissolves in excess. Why is one test insufficient to identify aluminium uniquely?
A single shared observation is evidence, not a unique identification.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.