Coloured cation precipitates: chromium, copper and iron
Compare initial colours and excess-reagent behaviour, including iron(II)'s change near the surface on standing.
Core and Extended · Chemistry 0620, 2026–2028. Interpret the supplied laboratory observations on paper; actual chemical tests require an appropriate supervised laboratory method.
Before you begin
- Distinguish precipitate colour from solution colour.
- Interpret Roman numerals and cation charge in iron(II)/iron(III).
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair represents iron(II) and iron(III) correctly?
What you will learn
- Describe NaOH and ammonia tests for Cr³⁺, Cu²⁺, Fe²⁺ and Fe³⁺.
- Use excess behaviour and observation timing to distinguish similar colours.
Coloured cation comparison reference
Try recalling the result, then open this comparison. Read the test and observation together. Actual testing belongs to an appropriate supervised laboratory method.
Open the comparison reference
Chromium(III) · Cr³⁺
- Aqueous sodium hydroxide
- Green precipitate; soluble in excess.
- Aqueous ammonia
- Green precipitate; insoluble in excess.
Copper(II) · Cu²⁺
- Aqueous sodium hydroxide
- Light blue precipitate; insoluble in excess.
- Aqueous ammonia
- Light blue precipitate; soluble in excess to a dark blue solution.
Iron(II) · Fe²⁺
- Aqueous sodium hydroxide
- Green precipitate; insoluble in excess. Turns brown near the surface on standing.
- Aqueous ammonia
- Green precipitate; insoluble in excess. Turns brown near the surface on standing.
Iron(III) · Fe³⁺
- Aqueous sodium hydroxide
- Red-brown precipitate; insoluble in excess.
- Aqueous ammonia
- Red-brown precipitate; insoluble in excess.
Use the supplied candidate set and conditions. One observation does not establish every ion in a mixture or certify purity.
Compare both the colour and what excess does
Chromium(III), Cr³⁺, gives a green precipitate with NaOH, soluble in excess; with aqueous ammonia its green precipitate is insoluble in excess. Copper(II), Cu²⁺, gives a light blue precipitate with NaOH, insoluble in excess. With ammonia, copper's initial light blue precipitate dissolves in excess to give a dark blue solution.
A dark blue solution after excess ammonia is different from a light blue solid suspended in liquid. Record state and colour together. For green precipitates, excess NaOH helps distinguish chromium(III) from iron(II) in the required candidate set. The reagent name, initial response and excess stage are all needed; colour alone can leave ambiguity.
Record iron's initial colour and later change separately
Iron(II), Fe²⁺, gives a green precipitate insoluble in excess with both NaOH and ammonia. The precipitate turns brown near the surface on standing in the required observations. Iron(III), Fe³⁺, gives a red-brown precipitate insoluble in excess with both reagents.
A delayed brown surface on a precipitate initially observed green should not erase the initial observation and be reported as immediate iron(III) red-brown formation. In the school model, the standing change is associated with oxidation of iron(II) material near the air-exposed surface. Note the timing, location and reagent conditions. A mixed or contaminated sample can need further investigation beyond these simple single-candidate comparisons.
Pause and explain
Which observation distinguishes chromium(III) from iron(II) in their green NaOH precipitates?
Worked example
Unknown G gives a green precipitate that dissolves in excess NaOH but remains insoluble in excess aqueous ammonia. Another unknown gives light blue precipitate that becomes dark blue solution in excess ammonia. Identify the supported candidates.
Show the worked solution
- G's green/excess-NaOH-soluble pattern matches chromium(III), not iron(II).
- The second unknown's ammonia sequence matches copper(II).
- Both conclusions use the full supplied pattern rather than a colour word alone.
Answer G Cr³⁺; the second unknown Cu²⁺ under the stated candidate comparisons.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe both required aqueous-reagent tests for copper(II), including the solution after excess ammonia.
Give me a hint
Keep initial solid and later dissolved colour separate.
Compare my reasoning
- NaOH gives a light blue precipitate insoluble in excess.
- Ammonia initially gives a light blue precipitate.
- In excess ammonia the precipitate dissolves, giving a dark blue solution.
Cu²⁺: NaOH light blue precipitate insoluble in excess; ammonia light blue precipitate soluble in excess to dark blue solution.
Look for these in your work
- Both reagents and excess behaviours are stated.
- The final solution is not confused with a solid precipitate.
Compare chromium(III), iron(II) and iron(III) with NaOH and ammonia.
Give me a hint
Use colour, excess behaviour and the later surface change.
Compare my reasoning
- Cr³⁺ gives green precipitate soluble in excess NaOH but insoluble in excess ammonia.
- Fe²⁺ gives green precipitate insoluble in excess of either reagent, turning brown near the surface on standing.
- Fe³⁺ gives red-brown precipitate insoluble in excess of either reagent.
Cr³⁺: green, NaOH-excess soluble/ammonia-excess insoluble. Fe²⁺: green, both excess insoluble, brown near surface on standing. Fe³⁺: red-brown, both excess insoluble.
Look for these in your work
- All three complete patterns are correct.
- Iron's initial and delayed observations are distinct.
A recorded precipitate is green immediately after NaOH and insoluble in excess. After standing, only its upper surface turns brown. A report labels the initial result 'red-brown precipitate, iron(III)'. Evaluate it using the supplied notes.
Give me a hint
Preserve the observation timeline rather than replacing an earlier colour.
Compare my reasoning
- The immediate green, excess-insoluble result fits iron(II) among the required candidates.
- Brown near the surface on standing matches its stated later observation.
- The report incorrectly rewrites the initial result as iron(III); retain both timed observations and investigate further if the sample is complex.
The supplied pattern supports Fe²⁺ initially; the later brown surface must not be reported as an immediate Fe³⁺ red-brown result.
Look for these in your work
- Initial colour, excess behaviour and timing are retained.
- The candidate inference stays within the supplied simple-test model.
Common mistakes
- Calling every green precipitate iron(II) without checking excess behaviour.
- Describing a dark blue solution as an unchanged light blue precipitate.
- Treating iron(II)'s delayed brown surface as the initial iron(III) result.
- Ignoring reagent, excess and observation timing.
What can you explain now?
Which iron cation initially gives a red-brown precipitate with either reagent?
Compare with the explanation
Iron(III), Fe³⁺.
Iron(II) is initially green and can turn brown near the surface on standing.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall all four coloured-cation patterns, including copper's dark blue solution and iron(II)'s delayed surface change.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.