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The practical identifies transition-metal ions in aqueous solution using small test-tube reactions. For the AQA aqueous-ion work, compare the specified metal ions including iron(II), iron(III) and copper(II); aluminium provides a useful comparison but is not itself a transition metal. Keep sample concentration, reagent addition and observation conditions consistent.
Video by Eliot Rintoul. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Record initial and excess-reagent observations
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which observation supports copper(II) with excess ammonia?
A deep-blue solution after the precipitate dissolves. The soluble ammine complex gives the deep-blue appearance.
After watching 2: Why might a green iron(II) hydroxide sample brown on standing?
Oxidation by oxygen in air. Iron(II) can oxidise to iron(III), changing the observed solid.
What you'll be able to do
The practical identifies transition-metal ions in aqueous solution using small test-tube reactions. For the AQA aqueous-ion work, compare the specified metal ions including iron(II), iron(III) and copper(II); aluminium provides a useful comparison but is not itself a transition metal. Keep sample concentration, reagent addition and observation conditions consistent.
Add hydroxide dropwise, record the initial precipitate, then add the specified excess and record whether it dissolves. Copper(II) normally gives a blue precipitate, iron(II) green and iron(III) brown. Iron(II) hydroxide can brown on standing as it oxidises in air, so the time of observation matters. Aluminium gives a white precipitate that dissolves in excess hydroxide through amphoteric behaviour.
Ammonia is a weak base and can remove protons from water ligands, causing hydroxide precipitation. In excess it can also act as a ligand. With copper(II), the initial blue precipitate dissolves in sufficient excess ammonia to form a deep-blue solution of the ammine complex. This is ligand substitution, not reduction of copper(II) to copper metal.
For copper, the simplified precipitation equation is Cu²⁺ + 2OH⁻ → Cu(OH)₂(s). An aqua-complex representation shows the water ligands and proton removal more explicitly. For excess ammonia, [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O. Check atoms and charge. Iron hydroxides do not show the same dissolution in excess ammonia under the usual test conditions.
Use fresh separate portions for hydroxide, ammonia and any carbonate comparison required by the specification. A mixed unknown or coloured starting solution can obscure a precipitate. A known reference, clean background and fresh iron(II) solution can improve interpretation. Do not report an initial “blue solution” as a “blue precipitate”: physical state is part of the evidence.
Ammonia vapour and metal-ion solutions require the specified ventilation, eye protection and waste collection. Adding a large uncontrolled excess from the start can conceal the initial precipitate. A useful improvement is dropwise addition with a recorded sequence, followed by enough excess to test solubility. Repeating a contaminated test with the same tube will not resolve the source of the error.
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: Balance the simplified precipitation equation for iron(III) with hydroxide and predict the observation.
Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s); a brown precipitate.
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.