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Tests for transition metal ions

Edexcel IAL Chemistry · WCH16 Practical Skills in Chemistry II

The reactions of aqueous transition metal ions with sodium hydroxide and ammonia are taught in the Unit 5 lesson. In Unit 6 you use them as tests, describing exactly what is seen and what it shows.

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The procedure

To a small volume of the solution, add the reagent dropwise and note any precipitate, then add it in excess and note whether the precipitate dissolves. Leave it to stand to see if it changes in air. Repeat with aqueous ammonia.

Record the colour and the state: 'blue precipitate', 'deep blue solution'.

Ion (colour of solution)	NaOH(aq), few drops	Excess NaOH	Excess NH3(aq)
Cu2+ (blue)	blue precipitate	no change	dissolves: deep blue solution
Fe2+ (pale green)	green precipitate, turns brown on the surface on standing	no change	no change
Fe3+ (yellow to orange-brown)	orange-brown (red-brown) precipitate	no change	no change
Cr3+ (green)	green precipitate	dissolves: dark green solution	dissolves slowly: purple solution
Mn2+ (very pale pink)	off-white precipitate, darkens in air	no change	no change
Co2+ (pink)	blue precipitate, turns pink on standing	no change	dissolves: straw-coloured solution
Ni2+ (green)	green precipitate	no change	dissolves: blue-violet solution
Zn2+ (colourless)	white precipitate	dissolves: colourless solution	dissolves: colourless solution

Matches the Unit 5 table. A few drops of ammonia give the same precipitates as NaOH.

Follow-up tests

Hydrogen peroxide oxidises: green Fe(OH)2 turns orange-brown Fe(OH)3; in excess alkali, dark green [Cr(OH)6]3- is oxidised to yellow chromate(VI), CrO4²-, which turns orange (dichromate) when acidified.
Potassium iodide with Cu2+: a white precipitate of CuI in a brown solution of iodine (the mixture looks off-white or beige). Fe3+ also oxidises I- to brown I2.
Concentrated HCl with Cu2+: blue solution turns yellow-green as [CuCl4]2- forms.

Avoiding the common mix-ups

Mn2+ gives an off-white precipitate that darkens slowly; Fe3+ gives an orange-brown one straight away (October 2024 report).
A green precipitate that stays green in air (Ni2+, Cr3+) is different from one that turns brown (Fe2+). Cr(OH)3 dissolves in excess NaOH; Ni(OH)2 does not.
If a precipitate does not change on standing, the ion is not easily oxidised by air.

Worked example

A green solution gives a green precipitate with NaOH(aq) that does not dissolve in excess but turns brown on the surface on standing. Identify the ion and explain the brown colour.

Green precipitate, insoluble in excess: Fe2+ (or Ni2+).
Turning brown in air: oxidation by oxygen, so the ion is easily oxidised.
Fe(OH)2 is oxidised to Fe(OH)3.

Answer: Fe2+; air oxidises Fe(OH)2 to orange-brown Fe(OH)3.

Key idea: Add dropwise, then excess, then stand. Cu2+ deep blue in excess NH3; Cr3+ and Zn2+ dissolve in excess NaOH; Fe2+ green turns brown; Fe3+ orange-brown; Mn2+ off-white darkens. H2O2 oxidises Fe(II) and Cr(III).

Check yourself

Which ion gives a blue precipitate with a few drops of ammonia that dissolves in excess to give a deep blue solution?
- Co2+
- Ni2+
- Fe2+
- Cu2+
Show answer
Cu2+. [Cu(NH3)4(H2O)2]2+ is deep blue.
Which ion gives a green precipitate with NaOH(aq) that dissolves in excess NaOH to give a dark green solution?
- Fe2+
- Cr3+
- Ni2+
- Cu2+
Show answer
Cr3+. Cr(OH)3 is amphoteric and forms [Cr(OH)6]3-.
A white precipitate forms with NaOH(aq) and dissolves in excess NaOH and in excess NH3(aq). Which ion is present?
- Mg2+
- Ca2+
- Mn2+
- Zn2+
Show answer
Zn2+. Zn(OH)2 is amphoteric and also forms an ammine complex.
Which observation shows Mn2+ rather than Fe3+?
- An off-white precipitate with NaOH that slowly darkens in air
- An orange-brown precipitate with NaOH
- A deep blue solution with excess ammonia
- A precipitate that dissolves in excess NaOH
Show answer
An off-white precipitate with NaOH that slowly darkens in air. Fe(OH)3 is orange-brown at once; Mn(OH)2 starts off-white.
Why does the green precipitate from Fe2+ and NaOH turn brown on standing?
- The precipitate dries out
- It reacts with carbon dioxide
- Oxygen in the air oxidises iron(II) hydroxide to iron(III) hydroxide
- The sodium ions are oxidised
Show answer
Oxygen in the air oxidises iron(II) hydroxide to iron(III) hydroxide. Fe(II) is easily oxidised to Fe(III).
Hydrogen peroxide is added to the dark green solution formed from Cr3+ and excess NaOH, and the mixture is warmed. What colour forms?
- Yellow
- Purple
- Deep blue
- Colourless
Show answer
Yellow. Cr(III) is oxidised to chromate(VI), CrO4²-, which is yellow.
Describe what is seen when potassium iodide solution is added to copper(II) sulfate solution.
Show answer
A white precipitate (of copper(I) iodide) forms in a brown solution (of iodine).. 2Cu2+ + 4I- -> 2CuI + I2.

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Last updated 8 November 2026. Spotted a mistake? Tell us.