Qualitative Analysis with NaOH and NH3

Spec Mapping — OCR H432 Module 5.3.1 — Transition elements (qualitative analysis), covering the reactions of aqueous solutions of Cu2+, Fe2+, Fe3+, Mn2+ and Cr3+ with aqueous sodium hydroxide and with aqueous ammonia, including the colours and identities of the metal-hydroxide precipitates, the behaviour of these precipitates in excess base (both NaOH and NH3), the amphoteric behaviour of Cr(OH)3 (and by analogy Al(OH)3), and the use of these observations to identify unknown transition-metal cations (refer to the official OCR H432 specification document for exact wording).

Qualitative analysis with NaOH and NH3 is the OCR-A laboratory entry point to the chemistry of first-row transition-metal aqueous ions. The five cations Cu2+, Fe2+, Fe3+, Mn2+ and Cr3+ each give a distinctive coloured hydroxide precipitate when treated with aqueous NaOH, and behave differently when more base is added: some precipitates dissolve in excess NH3 (forming ammine complexes), some dissolve in excess NaOH (the amphoteric ones), and some persist unchanged. The combined NaOH-then-excess and NH3-then-excess test is the OCR analogue of the more general "qualitative-analysis flowchart" that GCSE chemistry introduces — it identifies the metal ion present in an unknown solution by colour pattern alone, without instrumentation. This lesson builds the colour grid you must memorise for Paper 1 (where qualitative analysis is examined on a 1-3 mark question almost every year), explains the underlying chemistry (charge-induced acidity of coordinated water, formation of ammine complexes, amphoteric solubility), and walks through specimen questions in the OCR format.

Key Definition — Aqueous transition-metal cations all exist in water as the hexaaqua complex [M(H2O)6]n+ (typically n = 2 or 3 for first-row cations at OCR A-Level). When OH- or NH3 is added, hydroxide ions abstract protons from the coordinated water molecules to form successively M(H2O)5(OH)+, M(H2O)4(OH)2+, and ultimately the neutral metal hydroxide M(OH)n(H2O)6-n which is insoluble and precipitates from solution. The number of OH- required equals the cation charge n, and the precipitate is always electrically neutral.

By the end of this lesson you should be able to:

• Write equations for the reaction of Cu2+, Fe2+, Fe3+, Mn2+, Cr3+ aqueous complexes with NaOH(aq) and with NH3(aq)
• Predict the colours of the resulting hydroxide precipitates and the colours of any complexes formed on dissolution in excess base
• Explain the difference between reaction with dilute NH3 (precipitate forms) and excess NH3 (ammine complex forms for Cu2+, Cr3+, Zn2+)
• Describe the amphoteric behaviour of Cr(OH)3 (and by analogy Al(OH)3, Zn(OH)2), and write equations for dissolution in excess NaOH to form [Cr(OH)6]3-, [Al(OH)4]-, [Zn(OH)4]2-
• Use observations to identify unknown metal ion mixtures from a step-by-step NaOH/NH3 protocol
• Write balanced equations showing the conversion of [M(H2O)6]n+ to M(OH)n(H2O)6-n, including the air-oxidation of Fe(OH)2 to Fe(OH)3 and of Mn(OH)2 to MnO(OH)2
• Explain why [Fe(H2O)6]3+ is appreciably acidic in water (pH ~ 2–3) while [Na(H2O)6]+ is not

The Theory: Why Hydroxide Precipitates Form

When you add OH- to an aqueous solution of a transition metal ion like [Fe(H2O)6]3+, the hydroxide ions abstract protons from the coordinated water molecules:

[Fe(H2O)6]3+ + OH- -> [Fe(H2O)5(OH)]2+ + H2O [Fe(H2O)5(OH)]2+ + OH- -> [Fe(H2O)4(OH)2]+ + H2O [Fe(H2O)4(OH)2]+ + OH- -> Fe(OH)3(H2O)3 + H2O (insoluble)

The neutral species Fe(OH)3(H2O)3 has zero charge and is no longer held in solution - it precipitates as a gelatinous solid. The overall equation is:

[Fe(H2O)6]3+(aq) + 3 OH-(aq) -> Fe(OH)3(H2O)3(s) + 3 H2O(l)

The number of OH- needed equals the charge on the original complex (+2 needs 2 OH-, +3 needs 3 OH-). The charge on the final precipitate is always zero because it has the same number of OH- as the metal's oxidation state.

Colour Summary Table

Memorise this table for the qualitative analysis of unknown metal ions:

Ion (aqueous, octahedral)	Colour of solution	Hydroxide precipitate formula	Precipitate colour	Dissolves in excess NH3?	Dissolves in excess NaOH?
[Cu(H2O)6]2+	Pale blue	Cu(OH)2(H2O)4	Pale blue	Yes - deep blue solution	No
[Fe(H2O)6]2+	Pale green	Fe(OH)2(H2O)4	Green (darkens to brown in air)	No	No
[Fe(H2O)6]3+	Pale yellow / brown	Fe(OH)3(H2O)3	Rusty brown	No	No
[Mn(H2O)6]2+	Very pale pink	Mn(OH)2(H2O)4	Off-white (darkens to brown in air)	No	No
[Cr(H2O)6]3+	Violet / green	Cr(OH)3(H2O)3	Grey-green	Yes - purple solution	Yes - dark green solution

This table answers the majority of OCR qualitative analysis questions.

The Five Key Reactions

1. Copper(II): [Cu(H2O)6]2+

[Cu(H2O)6]2+(aq) + 2 OH-(aq) -> Cu(OH)2(H2O)4(s) + 2 H2O(l)

Observation: pale blue solution turns to pale blue gelatinous precipitate.

With excess ammonia:

Cu(OH)2(H2O)4(s) + 4 NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 2 H2O(l) + 2 OH-(aq)

Observation: precipitate dissolves to give a deep royal blue solution.

With excess NaOH: no change, precipitate persists.

2. Iron(II): [Fe(H2O)6]2+

[Fe(H2O)6]2+(aq) + 2 OH-(aq) -> Fe(OH)2(H2O)4(s) + 2 H2O(l)

Observation: pale green solution -> dirty green precipitate.

On standing in air, the precipitate slowly oxidises to iron(III) hydroxide:

4 Fe(OH)2(H2O)4(s) + O2(g) -> 4 Fe(OH)3(H2O)3(s) + H2O(l) (the brown outer layer visible after a few minutes)

With excess NH3: no change. With excess NaOH: no change.

3. Iron(III): [Fe(H2O)6]3+

[Fe(H2O)6]3+(aq) + 3 OH-(aq) -> Fe(OH)3(H2O)3(s) + 3 H2O(l)

Observation: pale yellow / brown solution -> rusty brown precipitate.

With excess NH3: no change, precipitate persists. With excess NaOH: no change, precipitate persists.

Note: [Fe(H2O)6]3+ itself is mildly acidic (because the high charge polarises coordinated water), so the solution has a low pH even before NaOH is added.

4. Manganese(II): [Mn(H2O)6]2+

[Mn(H2O)6]2+(aq) + 2 OH-(aq) -> Mn(OH)2(H2O)4(s) + 2 H2O(l)

Observation: nearly colourless solution -> off-white / cream precipitate. On standing in air the precipitate oxidises to MnO(OH)2 (brown) or eventually MnO2 (black). Mn2+ is so pale that the initial colour is sometimes described as "very pale pink" but is often hard to see.

With excess NH3: no change (unless Mn2+ is present at very high concentration, in which case a small amount may dissolve). With excess NaOH: no change.

5. Chromium(III): [Cr(H2O)6]3+

[Cr(H2O)6]3+(aq) + 3 OH-(aq) -> Cr(OH)3(H2O)3(s) + 3 H2O(l)

Observation: violet (or green) solution -> grey-green gelatinous precipitate.

With excess NH3:

Cr(OH)3(H2O)3(s) + 6 NH3(aq) -> [Cr(NH3)6]3+(aq) + 3 OH-(aq) + 3 H2O(l)

Observation: precipitate dissolves to give a purple solution.

With excess NaOH (chromium hydroxide is amphoteric):

Cr(OH)3(H2O)3(s) + 3 OH-(aq) -> [Cr(OH)6]3-(aq) + 3 H2O(l)

Observation: precipitate dissolves to give a dark green solution.

Chromium(III) is the only first-row transition metal whose hydroxide dissolves in both excess NH3 and excess NaOH. This makes Cr3+ easy to identify in a mixture.

Test Summary Decision Table

Imagine you have an unknown solution. The observations from adding NaOH and NH3 are enough to tell you which metal ion is present:

Observation	Identify
Pale blue ppt, dissolves in excess NH3 to deep royal blue	Cu2+
Green ppt, darkens in air to brown, no dissolution in excess	Fe2+
Rusty brown ppt, no dissolution	Fe3+
Off-white ppt, darkens to brown in air, no dissolution	Mn2+
Grey-green ppt, dissolves in both excess NH3 and excess NaOH	Cr3+

Explanation of Amphoteric Behaviour (Cr, Al, Zn)

Amphoteric hydroxides can act as both acids AND bases. Cr(OH)3 is the OCR example:

• Acts as a base (reacts with H+): Cr(OH)3 + 3 H+ -> [Cr(H2O)6]3+ (forms cation).
• Acts as an acid (reacts with OH-): Cr(OH)3 + 3 OH- -> [Cr(OH)6]3- (forms anion, deprotonates remaining waters).

The ions of some other amphoteric metals you may have met:

• Al3+: [Al(H2O)6]3+ -> Al(OH)3(H2O)3 (white ppt) -> [Al(OH)4]- (colourless) in excess NaOH
• Zn2+: [Zn(H2O)6]2+ -> Zn(OH)2(H2O)4 (white ppt) -> [Zn(OH)4]2- in excess NaOH (also dissolves in excess NH3 to give [Zn(NH3)4]2+)

But OCR A-Level focuses on Cr(OH)3 for the amphoteric example in transition metals.

The Meaning of "Slightly Acidic" Solutions

Why is [Fe(H2O)6]3+ acidic? The high charge (+3) on the small Fe3+ ion polarises the O-H bonds of coordinated water, making them easier to break (the O becomes more positive, so the H becomes more acidic):

[Fe(H2O)6]3+ + H2O <=> [Fe(H2O)5(OH)]2+ + H3O+

Iron(III) salts such as FeCl3 give an acidic solution (pH ~2-3) because of this hydrolysis. A similar effect occurs with [Cu(H2O)6]2+ but is weaker (pH ~4-5). [Na(H2O)6]+ and [Ca(H2O)6]2+ show almost no acidity because the charge density is low.

This is why the addition of base (OH- or NH3) to a transition metal solution is so effective - the coordinated waters are already partially deprotonated.

Practical: Identifying an Unknown Solution

A common practical assessment asks you to identify an unknown metal ion from a mixture using NaOH and NH3 tests. Procedure:

1. Place ~1 cm3 of the unknown in a test tube.
2. Add NaOH(aq) a drop at a time with shaking. Note colour and nature of any precipitate.
3. Add excess NaOH. Note if the precipitate dissolves and the colour of the resulting solution.
4. Repeat with a fresh sample using NH3(aq) instead.
5. Compare observations with the table above.

If the unknown contains a mixture (e.g. Fe2+ and Cr3+), you may need additional tests (flame tests, colorimetry - see lesson 9) to distinguish.

Relationship to Colorimetry

Hydroxide precipitates are often analysed quantitatively by colorimetry (lesson 9). For example:

• Cr3+ is quantified by oxidising it first to Cr2O7^2- (orange) and measuring absorbance.
• Cu2+ is quantified directly as [Cu(NH3)4(H2O)2]2+ (deep royal blue, strong absorption at ~600 nm).
• Fe3+ is quantified as the red [Fe(SCN)(H2O)5]2+ thiocyanate complex.

So qualitative analysis (what metal is present?) leads naturally into quantitative analysis (how much is present?).

Exam Tips

• Always state the formula of the precipitate (e.g. Cu(OH)2(H2O)4, not just "Cu(OH)2") to show you understand it contains coordinated water.
• State the colour of both the solution and the precipitate - two marks in OCR exams.
• For "dissolves in excess NH3", name the product complex ion and its colour.
• Memorise the table of colours - there is no shortcut.
• Remember Cr3+ is the amphoteric one that dissolves in both NH3 and NaOH.

Common Exam Mistakes

• Forgetting that Fe(OH)2 darkens to brown in air.
• Writing Cu2+ + OH- -> Cu(OH)2 without including the water molecules.
• Saying Cu(OH)2 dissolves in excess NaOH - it does not.
• Confusing Mn(II) (pale pink / colourless) with Mg(II) (colourless) - Mn2+ is a transition metal, Mg2+ is not.
• Writing the wrong charge on the product in excess NH3: [Cu(NH3)6]2+ instead of [Cu(NH3)4(H2O)2]2+.
• Forgetting amphoteric behaviour of Cr(OH)3: it dissolves in BOTH excess NaOH and excess NH3.

The Hydroxide Precipitate Pattern (Visual Decision Tree)

graph TD
  A[Unknown solution] --> B{Add NaOH dropwise}
  B -->|Pale blue ppt| C[Cu2+]
  B -->|Green ppt darkening brown| D[Fe2+]
  B -->|Rust-brown ppt| E[Fe3+]
  B -->|Off-white ppt darkening brown| F[Mn2+]
  B -->|Grey-green ppt| G[Cr3+]
  C --> H{Add excess NH3}
  H -->|Deep royal blue solution| I[Confirm Cu2+ as Cu(NH3)4 H2O 2 squared+]
  D --> J{Add excess NH3}
  J -->|No change| K[Confirm Fe2+]
  G --> L{Add excess NaOH}
  L -->|Dark green solution| M[Cr OH 6 cubed- amphoteric confirms Cr3+]
  G --> N{Add excess NH3}
  N -->|Purple solution| O[Cr NH3 6 cubed+ confirms Cr3+]

The decision tree reduces the five-cation identification to two solution drops and three observations. With practice an OCR student can identify any of the five unknown ions in under thirty seconds.

Worked Example 1: Identifying an Unknown Mixture

Question (5 marks): A solution is suspected to contain either Fe2+ or Cr3+. (a) Describe two tests using NaOH(aq) and NH3(aq) that would distinguish them. State the observations expected for each test for each ion. (b) Explain the amphoteric behaviour of one of the products.

Answer:

Test 1: Add a few drops of NaOH(aq) and observe the precipitate colour.

• Fe2+ gives a dirty green precipitate (Fe(OH)2(H2O)4) which slowly darkens to brown on standing in air (oxidation to Fe(OH)3).
• Cr3+ gives a grey-green precipitate (Cr(OH)3(H2O)3) which does not darken on standing.

Test 2: Add excess NaOH(aq).

• Fe2+ precipitate persists unchanged.
• Cr3+ precipitate dissolves to give a dark green solution of [Cr(OH)6]3-.

The Cr3+ behaviour in part (b) is amphoteric: Cr(OH)3 dissolves in excess NaOH (acting as an acid, deprotonating to form [Cr(OH)6]3-) and would also dissolve in dilute strong acid (acting as a base, accepting H+ to reform [Cr(H2O)6]3+). The dual reactivity confirms amphoteric character. Fe(OH)2 is purely basic and dissolves only in acid, not in excess base.

Worked Example 2: Writing the Balanced Equations

Question (6 marks): Write balanced ionic equations for: (a) the reaction of [Cu(H2O)6]2+ with three equivalents of NH3(aq) — excess ammonia, but limited so substitution stops at four ammines; (b) the reaction of [Cr(OH)3(H2O)3] with three equivalents of OH-(aq); (c) the air-oxidation of Fe(OH)2(H2O)4 to Fe(OH)3(H2O)3.

Answer:

• (a) [Cu(H2O)6]2+(aq) + 4 NH3(aq) → [Cu(NH3)4(H2O)2]2+(aq) + 4 H2O(l). Note that only the four equatorial water ligands are replaced (Jahn–Teller distortion leaves the two axial waters in place).
• (b) Cr(OH)3(H2O)3(s) + 3 OH-(aq) → [Cr(OH)6]3-(aq) + 3 H2O(l). The amphoteric dissolution: Cr(OH)3 acts as an acid by deprotonating three coordinated water molecules.
• (c) 4 Fe(OH)2(H2O)4(s) + O2(g) + 2 H2O(l) → 4 Fe(OH)3(H2O)3(s). The Fe2+ is oxidised to Fe3+ by atmospheric O2; this is what gives "rusted" iron hydroxide samples their characteristic brown outer coating.

Specimen Question (OCR H432 Paper-1 Format)