✓ Revise Right course
With sodium hydroxide, copper(II) ions form a blue precipitate, iron(II) green and iron(III) brown. Free GCSE AQA notes with key terms and 8 questions answered.
Coloured Precipitates and Chemical Equations is part of Identification of ions by chemical and spectroscopic means (chemistry only) in GCSE AQA Higher Chemistry.
3 min
Reading time
17
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
With sodium hydroxide, copper(II) ions form a blue precipitate, iron(II) green and iron(III) brown. Free GCSE AQA notes with key terms and 8 questions answered.
AQA GCSE Chemistry mock draft, 26–31 Oct. Draft coverage: Chemistry Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside Identification of ions by chemical and spectroscopic means (chemistry only) while connecting the detail back to the wider Topic 8: Chemical analysis topic.
Read the core explanations from Coloured Precipitates and Chemical Equations before testing yourself from memory.
Picture this: you have three clear solutions in test tubes. They look identical, but one contains copper ions, one has iron(II) ions, and one has iron(III) ions. How can you tell them apart?
The answer is sodium hydroxide solution. When you add a few drops to each tube, something dramatic happens — coloured solids appear out of nowhere. This is your key to identifying metal ions.
The test works because metal ions react with hydroxide ions to form insoluble metal hydroxides. Each metal hydroxide has its own distinctive colour — that's your identification fingerprint.
You need to memorise three colour combinations. Think of them as a traffic light system, but with different metals:
Copper(II) ions + NaOH → Blue precipitateThe blue is unmistakable — like copper sulphate crystals. This forms copper(II) hydroxide, Cu(OH)₂.
Iron(II) ions + NaOH → Green precipitateA muddy green colour forms iron(II) hydroxide, Fe(OH)₂. Think 'green for two' to remember iron(II).
Iron(III) ions + NaOH → Brown precipitateA rusty brown colour forms iron(III) hydroxide, Fe(OH)₃. This looks like rust because rust contains iron(III) compounds.
At the molecular level, this is an ion-swapping reaction. The metal ions (positive) attract the hydroxide ions (negative) to form an insoluble compound that can't stay dissolved.
The general pattern is always the same: Metal ion + Hydroxide ion → Metal hydroxide (precipitate) + Spectator ions
Examiners love asking for balanced equations. Here's the systematic approach that works every time:
Worked example: Writing Precipitation Equations
Write the balanced equation for copper(II) sulphate reacting with sodium hydroxide
Answer: $CuSO_4(aq) + 2NaOH(aq) \rightarrow Cu(OH)_2(s) + Na_2SO_4(aq)$
Notice the pattern: you always need 2 moles of NaOH for every mole of metal ion with a 2+ charge, and 3 moles for every mole of metal ion with a 3+ charge.
Here are the three equations you must know for your exam. Notice how the coefficients follow the charge pattern:
This topic appears frequently in Paper 2. Examiners test both your practical knowledge and equation-writing skills. Here's how to maximise your marks:
Exam precision note: identify the ion from the colour first, then support it with the balanced equation. In practical questions, write that sodium hydroxide is added to the test tube and that the coloured precipitate is the observation, not a colour change in the sodium hydroxide itself.
A precipitation test is really two ideas working together. The practical observation tells you which ion is present because each hydroxide has a characteristic colour, and the balanced equation explains why a solid forms by matching the metal-ion charge to the number of hydroxide ions needed.
This is why the colour and the coefficient pattern should be learned together. Copper(II) and iron(II) each need two hydroxide ions, while iron(III) needs three. The equation structure reinforces the same charge information that appears in the ion name.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A precipitate is an insoluble solid that forms when two solutions are mixed together. In the sodium hydroxide test, metal ions react with hydroxide ions to form insoluble metal hydroxides that appear as precipitates.
The balanced equation is CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq). Two NaOH are needed to provide the 2 OH⁻ ions in copper(II) hydroxide, which forms a blue precipitate.
A precipitate forms because the positive metal ions attract the negative hydroxide ions to form a metal hydroxide that is insoluble, so it can't stay dissolved. The solid appears to 'fall out' of solution, and its colour identifies the metal ion.
With sodium hydroxide, copper(II) ions form a blue precipitate of Cu(OH)₂, iron(II) ions form a green precipitate of Fe(OH)₂, and iron(III) ions form a brown precipitate of Fe(OH)₃. These colours identify the metal ions.
The number of hydroxide ions matches the charge on the metal ion: 2+ ions such as copper(II) and iron(II) need 2 hydroxide ions, while 3+ ions such as iron(III) need 3. So 2 moles of NaOH react per mole of a 2+ ion and 3 moles per mole of a 3+ ion.
Iron(II) hydroxide is a muddy green precipitate, which can be remembered as green for two, while iron(III) hydroxide is a rusty brown precipitate, like rust, which contains iron(III) compounds.
Iron(III) ions react with three hydroxide ions to form a rusty brown precipitate of iron(III) hydroxide: Fe³⁺ + 3 OH⁻ → Fe(OH)₃. Three hydroxide ions are needed because the ion has a 3+ charge.
Iron(II) ions react with two hydroxide ions to form a muddy green precipitate of iron(II) hydroxide: Fe²⁺ + 2 OH⁻ → Fe(OH)₂. Two hydroxide ions match the 2+ charge, as for copper(II).