ChemistryICSE Class 10 · Chapter 4

Analytical Chemistry ICSE Class 10: Identifying Metal Ions with NaOH and NH₄OH

Updated 14 October 2026 · Fact-checked

Analytical chemistry in ICSE Class 10 identifies metal ions by adding sodium hydroxide or ammonium hydroxide drop by drop, then in excess, to salt solutions of iron, copper, zinc and lead. The colour of the hydroxide precipitate, and whether it dissolves in excess, tells the ions apart. Zinc, aluminium and lead also react with caustic alkalis.

What Analytical Chemistry is about

When an alkali is added to a solution of a metal salt, the metal's insoluble hydroxide is usually precipitated. Each hydroxide has a characteristic colour, and some dissolve in excess of the alkali. Together these two observations identify the metal ion.

The CISCE 2027 syllabus covers: (i) the colour of the salt and its solution; the action of sodium hydroxide and ammonium hydroxide, added drop by drop and then in excess, on solutions of salts of Fe (iron(II) and iron(III)), Cu, Zn and Pb, with equations; the special action of ammonium hydroxide on copper salts and of sodium hydroxide on ammonium salts; and (ii) the action of caustic alkalis (NaOH, KOH) on aluminium, zinc and lead, their oxides and their hydroxides, which shows their amphoteric nature. Calcium salts, part of the 2026 course, are not in the 2027 syllabus.

In every answer, the examiners want an observation (colour, 'precipitate', 'soluble or insoluble in excess', 'gas with a pungent smell'), not just a product name.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Analytical Chemistry is in the 4. Analytical Chemistry part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Colours of salts and their solutions

Ion	Typical salt	Colour of solid	Colour of solution
Cu²⁺	CuSO₄·5H₂O	blue (anhydrous CuSO₄ is white)	blue
Fe²⁺	FeSO₄·7H₂O	pale green	pale green
Fe³⁺	FeCl₃	yellowish brown	yellowish brown
Zn²⁺	ZnSO₄, Zn(NO₃)₂	white	colourless
Pb²⁺	Pb(NO₃)₂	white	colourless

So a coloured solution already narrows the choice to copper or iron; a colourless solution suggests zinc or lead (among these four), and the alkali tests decide.

Worked example

A salt solution is pale green. Which metal ion does it probably contain, and what would you add to confirm it?

Try it first, then show the solutionSolution

Pale green is typical of iron(II) salts.
Add NaOH solution: a dirty green precipitate of Fe(OH)₂, insoluble in excess, confirms Fe²⁺.

Answer: Fe²⁺; confirm with NaOH (dirty green precipitate).

Sodium hydroxide on Fe²⁺, Fe³⁺ and Cu²⁺ salts

Drop by drop, each gives a coloured precipitate that does not dissolve in excess NaOH:

Fe²⁺: dirty green precipitate. $F e S O_{4} + 2 N a O H \to F e (O H)_{2} + N a_{2} S O_{4}$
Fe³⁺: reddish brown precipitate. $F e C l_{3} + 3 N a O H \to F e (O H)_{3} + 3 N a C l$
Cu²⁺: pale blue precipitate. $C u S O_{4} + 2 N a O H \to C u (O H)_{2} + N a_{2} S O_{4}$

The dirty green iron(II) hydroxide slowly turns brown at the surface on standing in air, as it is oxidised to iron(III) hydroxide.

Worked example

How would you distinguish iron(II) sulphate from iron(III) chloride using sodium hydroxide solution?

Try it first, then show the solutionSolution

Add NaOH to each.
Iron(II): dirty green precipitate of Fe(OH)₂.
Iron(III): reddish brown precipitate of Fe(OH)₃. Both insoluble in excess.

Answer: Dirty green precipitate (Fe²⁺) versus reddish brown precipitate (Fe³⁺).

Sodium hydroxide on Zn²⁺ and Pb²⁺ salts

Both give a white precipitate that dissolves in excess NaOH, because their hydroxides are amphoteric:

Zn²⁺: white gelatinous precipitate. $Z n S O_{4} + 2 N a O H \to Z n (O H)_{2} + N a_{2} S O_{4}$ ; in excess: $Z n (O H)_{2} + 2 N a O H \to N a_{2} Z n O_{2} + 2 H_{2} O$ (sodium zincate, colourless solution).
Pb²⁺: chalky white precipitate. $P b (N O_{3})_{2} + 2 N a O H \to P b (O H)_{2} + 2 N a N O_{3}$ ; in excess: $P b (O H)_{2} + 2 N a O H \to N a_{2} P b O_{2} + 2 H_{2} O$ (sodium plumbite, colourless solution).

So NaOH alone cannot tell zinc from lead. Ammonium hydroxide can.

Worked example

A colourless solution gives a white precipitate with NaOH that dissolves in excess. Name the two possible cations.

Try it first, then show the solutionSolution

White precipitate, soluble in excess NaOH: amphoteric hydroxide.
Among the syllabus ions: Zn²⁺ and Pb²⁺.

Answer: Zn²⁺ or Pb²⁺.

Ammonium hydroxide on Fe, Cu, Zn and Pb salts

Ammonium hydroxide gives the same coloured precipitates for Fe²⁺ (dirty green), Fe³⁺ (reddish brown) and Pb²⁺ (chalky white), all insoluble in excess. The two ions that behave differently are copper and zinc:

Cu²⁺: pale blue precipitate, which dissolves in excess NH₄OH to give an inky (deep) blue solution of tetraamminecopper(II) sulphate.

$C u S O_{4} + 2 N H_{4} O H \to C u (O H)_{2} + (N H_{4})_{2} S O_{4}$
$C u (O H)_{2} + (N H_{4})_{2} S O_{4} + 2 N H_{4} O H \to [C u (N H_{3})_{4}] S O_{4} + 4 H_{2} O$

Zn²⁺: white gelatinous precipitate, which dissolves in excess NH₄OH to give a colourless solution.

$Z n S O_{4} + 2 N H_{4} O H \to Z n (O H)_{2} + (N H_{4})_{2} S O_{4}$
$Z n (O H)_{2} + (N H_{4})_{2} S O_{4} + 2 N H_{4} O H \to [Z n (N H_{3})_{4}] S O_{4} + 4 H_{2} O$

Pb²⁺: chalky white precipitate, insoluble in excess NH₄OH.

$P b (N O_{3})_{2} + 2 N H_{4} O H \to P b (O H)_{2} + 2 N H_{4} N O_{3}$

Worked example

Distinguish between zinc nitrate and lead nitrate solutions using ammonium hydroxide.

Try it first, then show the solutionSolution

Both give a white precipitate at first.
Add excess NH₄OH: the zinc hydroxide precipitate dissolves (colourless solution); the lead hydroxide precipitate stays.

Answer: White precipitate soluble in excess (Zn²⁺) versus insoluble in excess (Pb²⁺).

Sodium hydroxide on ammonium salts

Warming any ammonium salt with sodium hydroxide solution releases ammonia gas:

$N H_{4} C l + N a O H \to N a C l + H_{2} O + N H_{3}$

$(N H_{4})_{2} S O_{4} + 2 N a O H \to N a_{2} S O_{4} + 2 H_{2} O + 2 N H_{3}$

Observation: a colourless gas with a pungent smell that turns moist red litmus blue and gives dense white fumes with a rod dipped in concentrated HCl ( $N H_{3} + H C l \to N H_{4} C l$ ). No precipitate forms, because the ammonium ion has no insoluble hydroxide. This is the test for the NH₄⁺ ion.

Worked example

How would you distinguish ammonium chloride from sodium chloride using NaOH?

Try it first, then show the solutionSolution

Warm each with NaOH solution.
Ammonium chloride gives ammonia: pungent smell, moist red litmus turns blue.
Sodium chloride gives no gas.

Answer: Only NH₄Cl gives ammonia gas.

Amphoteric metals, oxides and hydroxides with caustic alkalis

Aluminium, zinc and lead, and their oxides and hydroxides, react with hot concentrated NaOH or KOH as well as with acids: they are amphoteric.

Metals (hydrogen is released):
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ (sodium aluminate)
$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ (sodium zincate)
$P b + 2 N a O H \to N a_{2} P b O_{2} + H_{2}$ (sodium plumbite)

Oxides (salt + water):
$A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$
$Z n O + 2 N a O H \to N a_{2} Z n O_{2} + H_{2} O$
$P b O + 2 N a O H \to N a_{2} P b O_{2} + H_{2} O$

Hydroxides:
$A l (O H)_{3} + N a O H \to N a A l O_{2} + 2 H_{2} O$
$Z n (O H)_{2} + 2 K O H \to K_{2} Z n O_{2} + 2 H_{2} O$

With KOH the products are potassium aluminate, zincate and plumbite.

Worked example

Show that zinc oxide is amphoteric, with two equations.

Try it first, then show the solutionSolution

With an acid: $Z n O + 2 H C l \to Z n C l_{2} + H_{2} O$ .
With an alkali: $Z n O + 2 N a O H \to N a_{2} Z n O_{2} + H_{2} O$ .
It forms a salt and water with both.

Answer: ZnO neutralises both an acid and an alkali, so it is amphoteric.

Chemistry in real life: Analytical Chemistry

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Identifying an iron salt

Suppose a pharmacist has two unlabelled bottles of iron tablets dissolved in water: one is meant to be iron(II) sulphate and the other iron(III) chloride.

Describe how a few drops of sodium hydroxide solution could tell them apart, with equations.

Try it first, then show the solutionSolution

Iron(II) gives a dirty green precipitate: $F e S O_{4} + 2 N a O H \to F e (O H)_{2} + N a_{2} S O_{4}$ .
Iron(III) gives a reddish brown precipitate: $F e C l_{3} + 3 N a O H \to F e (O H)_{3} + 3 N a C l$ .
Neither dissolves in excess, so the colour alone decides.

Answer: Dirty green = iron(II) sulphate; reddish brown = iron(III) chloride.

Memory hook: Two (Fe²⁺) is green, three (Fe³⁺) is brown, like rust.

Amphoteric aluminium

Suppose someone tries to clean a blocked drain with a caustic soda (NaOH) drain cleaner and pours it into an aluminium mug to carry it.

Explain why the mug fizzes and gets damaged, with an equation.

Try it first, then show the solutionSolution

Aluminium is amphoteric and reacts with caustic alkalis.
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$
The fizzing is hydrogen gas; the metal dissolves as sodium aluminate.

Answer: Aluminium reacts with NaOH, releasing hydrogen and forming sodium aluminate.

Memory hook: Aluminium, zinc and lead don't trust alkalis either.

Test for ammonium ions

Suppose a farmer wants to check whether a white fertiliser powder contains ammonium ions before buying a large bag.

Describe a simple test with sodium hydroxide and the result expected for an ammonium salt.

Try it first, then show the solutionSolution

Warm a little of the powder with NaOH solution.
If ammonium ions are present, ammonia is released: $N H_{4} C l + N a O H \to N a C l + H_{2} O + N H_{3}$ (for ammonium chloride).
The gas has a pungent smell and turns moist red litmus blue.

Answer: Pungent gas turning moist red litmus blue shows NH₄⁺.

Memory hook: Alkali pushes ammonia out of an ammonium salt.

Topics in this chapter

Action of Sodium Hydroxide on Salt Solutions
Action of Ammonium Hydroxide on Salt Solutions
Amphoteric Metals, Oxides and HydroxidesComing soon
Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis

Analytical Chemistry formula sheet

Copper(II) hydroxide precipitate: $C u S O_{4} + 2 N a O H \to C u (O H)_{2} + N a_{2} S O_{4}$; Pale blue; insoluble in excess NaOH.
Iron(III) hydroxide precipitate: $F e C l_{3} + 3 N a O H \to F e (O H)_{3} + 3 N a C l$; Reddish brown; insoluble in excess.
Sodium zincate: $Z n (O H)_{2} + 2 N a O H \to N a_{2} Z n O_{2} + 2 H_{2} O$; White precipitate dissolves in excess NaOH.
Sodium plumbite: $P b (O H)_{2} + 2 N a O H \to N a_{2} P b O_{2} + 2 H_{2} O$; Chalky white precipitate dissolves in excess NaOH.
Inky blue complex: $C u (O H)_{2} + (N H_{4})_{2} S O_{4} + 2 N H_{4} O H \to [C u (N H_{3})_{4}] S O_{4} + 4 H_{2} O$; Pale blue precipitate dissolves in excess NH₄OH.
Ammonia from an ammonium salt: $N H_{4} C l + N a O H \to N a C l + H_{2} O + N H_{3}$; Warm; moist red litmus turns blue.
Aluminium with NaOH: $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$; Sodium aluminate and hydrogen.

Analytical Chemistry: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Master table: NaOH and NH₄OH on Fe, Cu, Zn and Pb salts

Complete-the-table questions; also a one-page revision chart.

Draw

Rows: Fe²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Pb²⁺, NH₄⁺.
Columns: NaOH few drops | NaOH excess | NH₄OH few drops | NH₄OH excess.
Fe²⁺: dirty green ppt | insoluble | dirty green ppt | insoluble.
Fe³⁺: reddish brown ppt | insoluble | reddish brown ppt | insoluble.
Cu²⁺: pale blue ppt | insoluble | pale blue ppt | dissolves, inky blue solution.
Zn²⁺: white gelatinous ppt | dissolves (colourless) | white gelatinous ppt | dissolves (colourless).
Pb²⁺: chalky white ppt | dissolves (colourless) | chalky white ppt | insoluble.
NH₄⁺: no ppt; NH₃ on warming | - | - | -.

Label

precipitate (ppt)
soluble in excess
insoluble in excess

Watch out: Shade the three 'dissolves' cells under NaOH (Zn, Pb) and NH₄OH (Cu, Zn): the pattern ZL / CZ is easy to remember.

How the board asks Analytical Chemistry

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Observation questions	Section A or B: state what you observe when NaOH (or NH₄OH) is added drop by drop and then in excess to a named solution.	Give colour + 'precipitate' + behaviour in excess, in two short sentences.
Distinguish between two salts	Using a named reagent, distinguish FeSO₄ from FeCl₃, ZnSO₄ from Pb(NO₃)₂, NH₄Cl from NaCl, CuSO₄ from ZnSO₄.	Give one observation for each salt; the difference must be visible.
Identify the cation	An unknown salt solution gives a described result with NaOH and NH₄OH; name the cation (with its charge).	Use the table: colour first, then solubility in excess of each reagent.
Amphoteric equations	Write equations for zinc, aluminium or lead (or their oxides/hydroxides) with NaOH or KOH; name the salt formed.	Zincate Na₂ZnO₂, plumbite Na₂PbO₂, aluminate NaAlO₂; metals give H₂, oxides and hydroxides give water.
Complete the table	A table of salts with columns for NaOH (a few drops, excess) and NH₄OH (a few drops, excess) to fill in.	Memorise one master table (below in the diagrams section).

Tricks to remember Analytical Chemistry

Short memory aids. Say each one aloud once, then use it on a question the same day.

'Green, Brown, Blue, then two Whites'
Fe²⁺ green, Fe³⁺ brown, Cu²⁺ blue, Zn²⁺ and Pb²⁺ white: the order is the same with both alkalis.
Use it when: Precipitate colours.
Soda dissolves Zinc and Lead (ZL)
Excess NaOH dissolves the amphoteric pair Zn and Pb.
Use it when: Excess NaOH column.
Ammonia dissolves Copper and Zinc (CZ)
Excess NH₄OH dissolves the complex-formers Cu (inky blue) and Zn (colourless).
Use it when: Excess NH₄OH column.
Zinc dissolves in both
Zinc is in both lists, so zinc's white precipitate dissolves in excess of either reagent; lead dissolves only in NaOH; copper only in NH₄OH.
Use it when: Distinguishing Zn from Pb, and Cu from everything.

Analytical Chemistry: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Soluble in excess NH₄OH · 1 mark · MCQBoard level

A pale blue precipitate dissolves in excess ammonium hydroxide to give an inky blue solution. The cation is:
(a) Fe²⁺
(b) Zn²⁺
(c) Cu²⁺
(d) Pb²⁺

Try it first, then show the solutionSolution

Pale blue precipitate and inky blue solution: copper.

Answer: (c) Cu²⁺

Example 2: Which reagent? · 1 mark · MCQHarder board level

Which reagent would distinguish zinc sulphate solution from lead nitrate solution?
(a) Sodium hydroxide solution
(b) Ammonium hydroxide solution
(c) Potassium hydroxide solution
(d) Water

Try it first, then show the solutionSolution

Both precipitates dissolve in excess NaOH or KOH.
Only zinc hydroxide dissolves in excess NH₄OH.

Answer: (b)

Example 3: Observation, not product · 2 marks · Short answerBoard level

State one observation when (a) sodium hydroxide solution is added drop by drop and then in excess to lead nitrate solution; (b) ammonium hydroxide is added in excess to iron(III) chloride solution.

Try it first, then show the solutionSolution

(a) A chalky white precipitate forms, which dissolves in excess to give a colourless solution.
(b) A reddish brown precipitate forms, insoluble in excess.

Answer: As above.

Example 4: Unknown salt · 3 marks · Short answerHarder board level

Salt solution S is colourless. With NaOH it gives a white precipitate soluble in excess; with NH₄OH it gives a white precipitate insoluble in excess. With dilute HCl it gives a white precipitate. Name the cation and write the equation for its reaction with excess NaOH.

Try it first, then show the solutionSolution

White, soluble in excess NaOH: Zn²⁺ or Pb²⁺.
Insoluble in excess NH₄OH: Pb²⁺.
White precipitate with HCl (PbCl₂) agrees.
$P b (O H)_{2} + 2 N a O H \to N a_{2} P b O_{2} + 2 H_{2} O$

Answer: Pb²⁺; sodium plumbite forms in excess NaOH.

Example 5: Amphoteric equations · 3 marks · Short answerBoard level

Write balanced equations for the reaction of (a) zinc with sodium hydroxide, (b) aluminium oxide with sodium hydroxide, (c) lead(II) oxide with potassium hydroxide.

Try it first, then show the solutionSolution

(a) $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$
(b) $A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$
(c) $P b O + 2 K O H \to K_{2} P b O_{2} + H_{2} O$

Answer: As above.

Example 6: Three-way identification · 3 marks · Short answerHarder board level

Three test tubes contain copper(II) sulphate, zinc sulphate and ammonium sulphate solutions (labels lost). Using only sodium hydroxide solution, identify each. Give one observation each.

Try it first, then show the solutionSolution

CuSO₄ is already blue; NaOH gives a pale blue precipitate, insoluble in excess.
ZnSO₄ gives a white gelatinous precipitate, soluble in excess.
(NH₄)₂SO₄ gives no precipitate; on warming, a pungent gas (ammonia) turns moist red litmus blue.

Answer: Pale blue precipitate: copper; white soluble in excess: zinc; ammonia on warming: ammonium.

Example 7: Reasoning about solubility in excess · 2 marks · Short answerHOTS / competency

Explain why the white precipitate from zinc sulphate and NaOH dissolves in excess, but the pale blue one from copper(II) sulphate does not.

Try it first, then show the solutionSolution

Zinc hydroxide is amphoteric: it reacts with more NaOH to form soluble sodium zincate, $Z n (O H)_{2} + 2 N a O H \to N a_{2} Z n O_{2} + 2 H_{2} O$ .
Copper(II) hydroxide is basic only and does not react with excess NaOH.

Answer: Zn(OH)₂ is amphoteric; Cu(OH)₂ is not.

Example 8: Ammonium test in a mixture · 4 marks · Long answerHOTS / competency

A white solid is a mixture of zinc chloride and ammonium chloride. Describe what happens when excess NaOH solution is added and the mixture warmed. Write two equations.

Try it first, then show the solutionSolution

A white precipitate of zinc hydroxide forms at first: $Z n C l_{2} + 2 N a O H \to Z n (O H)_{2} + 2 N a C l$ .
In excess NaOH it dissolves to form sodium zincate (colourless solution).
On warming, ammonia is released from the ammonium chloride: $N H_{4} C l + N a O H \to N a C l + H_{2} O + N H_{3}$ .
Ammonia is detected by its pungent smell and by moist red litmus turning blue.

Answer: White precipitate that dissolves in excess, then a pungent gas (ammonia) on warming.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 1 + 2 + 1 marks.

The unlabelled salt

Suppose a student divides a solution of an unknown salt into two parts. To part 1 she adds NaOH: a dirty green precipitate forms, insoluble in excess. To part 2 she adds barium chloride solution: a white precipitate forms, insoluble in dilute HCl.

(i) · 1 mark
Name the cation, with its charge.
Answer
Answer: Fe²⁺
(ii) · 1 mark
Name the anion.
Answer
Answer: Sulphate, SO₄²⁻
(iii) · 2 marks
Write the formula and the equation for part 1.
Answer
Answer: FeSO₄: $F e S O_{4} + 2 N a O H \to F e (O H)_{2} + N a_{2} S O_{4}$
(iv) · 1 mark
On standing in air, the precipitate surface turns brown. Explain.
Answer
Answer: Iron(II) hydroxide is oxidised by air to iron(III) hydroxide, which is reddish brown.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Distinguish between zinc nitrate and lead nitrate solutions using ammonium hydroxide. (2 marks) (2 marks)

Step	Marks
Zinc nitrate: white gelatinous precipitate, soluble in excess NH₄OH	1
Lead nitrate: chalky white precipitate, insoluble in excess NH₄OH	1

State what you observe and write the equations when ammonium hydroxide is added drop by drop and then in excess to copper(II) sulphate solution. (3 marks) (3 marks)

Step	Marks
Pale blue precipitate at first, which dissolves in excess to give an inky blue solution	1
$C u S O_{4} + 2 N H_{4} O H \to C u (O H)_{2} + (N H_{4})_{2} S O_{4}$	1
$C u (O H)_{2} + (N H_{4})_{2} S O_{4} + 2 N H_{4} O H \to [C u (N H_{3})_{4}] S O_{4} + 4 H_{2} O$	1

Write equations to show the amphoteric nature of (a) aluminium, (b) zinc hydroxide, (c) lead(II) oxide, with sodium hydroxide; name one product of (a). (4 marks) (4 marks)

Step	Marks
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$	1
$Z n (O H)_{2} + 2 N a O H \to N a_{2} Z n O_{2} + 2 H_{2} O$	1
$P b O + 2 N a O H \to N a_{2} P b O_{2} + H_{2} O$	1
Sodium aluminate (or hydrogen)	1

Where students lose marks in Analytical Chemistry

The slips examiners see most often, and what to write instead.

Mistake: Writing a product instead of an observation
CISCE examiners' comments, 2025: asked for a significant observation, candidates wrote an equation or the product, or confused white with blue.
Wrong working
Zinc hydroxide is formed.
Right working
A white gelatinous precipitate forms, which dissolves in excess.
Do this: An observation is what you see: colour, precipitate, gas, dissolving.
Usually costs: 1 mark each.
Mistake: Missing the word 'excess'
CISCE examiners' comments, 2025: in an item on the product of ammonia with excess chlorine, candidates missed the word 'excess'. The same slip costs marks in every 'drop by drop and then in excess' observation.
Wrong working
Lead nitrate + NaOH in excess: white precipitate.
Right working
White precipitate that dissolves in excess NaOH to give a colourless solution.
Do this: Underline 'drop by drop' and 'in excess' in the question; answer both.
Usually costs: 1 mark.
Mistake: Cation without its charge
CISCE examiners' comments, 2024: identifying the cation in an unknown salt, candidates wrote only 'Fe', or wrote Fe²⁺ when the result showed Fe³⁺.
Wrong working
Cation: Fe
Right working
Reddish brown precipitate: Fe³⁺; dirty green: Fe²⁺.
Do this: Always write the ion with its charge, and match the charge to the colour.
Usually costs: 1 mark.
Mistake: Incomplete test for ammonia
CISCE examiners' comments, 2025: distinguishing an ammonium salt with NaOH, candidates wrote only 'colourless gas' without naming ammonia or its test.
Wrong working
A colourless gas is evolved.
Right working
Ammonia, with a pungent smell, which turns moist red litmus blue.
Do this: Name the gas and give its test.
Usually costs: 1 mark.
Mistake: Same reaction written for both salts
CISCE examiners' comments, 2025: in 'differentiate using the reagent' questions, candidates wrote the reaction for only one salt or said both were soluble.
Wrong working
Only the zinc salt's result given.
Right working
One observation for each salt, and they must differ.
Do this: Write two lines, one for each salt.
Usually costs: 1 mark.

Practice questions on Analytical Chemistry

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
A reddish brown precipitate insoluble in excess NaOH indicates:
(a) Fe²⁺
(b) Fe³⁺
(c) Cu²⁺
(d) Pb²⁺
Hint
Rust colour.
Answer
(b) Fe³⁺
Q2 · 1 mark · MCQBoard level
The salt formed when zinc oxide reacts with sodium hydroxide is:
(a) sodium plumbite
(b) sodium aluminate
(c) sodium zincate
(d) zinc hydroxide
Hint
Zn → zincate.
Answer
(c)
Q3 · 1 mark · MCQHarder board level
Statement I: Lead hydroxide dissolves in excess sodium hydroxide.
Statement II: Lead hydroxide dissolves in excess ammonium hydroxide.
(a) Both statements are true.
(b) Both statements are false.
(c) I is true, II is false.
(d) I is false, II is true.
Hint
Lead is only in the 'ZL' list.
Answer
(c)
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Ammonium hydroxide can be used to distinguish between copper(II) sulphate and iron(II) sulphate solutions.
Reason (R): Both give precipitates insoluble in excess ammonium hydroxide.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.
Hint
What does copper do in excess?
Answer
(c): copper's precipitate dissolves in excess to give an inky blue solution.
Q5 · 3 marks · Very short answerBoard level
Write the colour of: (a) copper(II) sulphate crystals, (b) iron(II) sulphate solution, (c) the precipitate formed when NaOH is added to zinc sulphate solution.
Hint
Salt, solution, precipitate.
Answer
(a) Blue (b) Pale green (c) White (gelatinous)
Q6 · 2 marks · Short answerBoard level
Using sodium hydroxide solution, distinguish between: (a) FeCl₃ and FeSO₄; (b) NH₄Cl and NaCl.
Hint
One observation each.
Answer
(a) FeCl₃: reddish brown precipitate; FeSO₄: dirty green precipitate. (b) NH₄Cl on warming gives ammonia (pungent, turns moist red litmus blue); NaCl gives no gas.
Q7 · 3 marks · Short answerHarder board level
Write balanced equations for: (a) lead with NaOH; (b) aluminium hydroxide with NaOH; (c) ammonium sulphate with NaOH.
Hint
Plumbite, aluminate, ammonia.
Answer
(a) $P b + 2 N a O H \to N a_{2} P b O_{2} + H_{2}$ (b) $A l (O H)_{3} + N a O H \to N a A l O_{2} + 2 H_{2} O$ (c) $(N H_{4})_{2} S O_{4} + 2 N a O H \to N a_{2} S O_{4} + 2 H_{2} O + 2 N H_{3}$
Q8 · 3 marks · Short answerHarder board level
A blue solution X gives a pale blue precipitate with NaOH, insoluble in excess. With NH₄OH the precipitate dissolves in excess. (a) Identify the cation. (b) Name the coloured product in excess NH₄OH. (c) Write the equation for X (a sulphate) with NaOH.
Hint
Blue solution.
Answer
(a) Cu²⁺ (b) Tetraamminecopper(II) sulphate, inky blue (c) $C u S O_{4} + 2 N a O H \to C u (O H)_{2} + N a_{2} S O_{4}$
Q9 · 4 marks · Long answerHOTS / competency
Three white powders are zinc nitrate, lead nitrate and ammonium nitrate. Plan a two-reagent scheme using NaOH and NH₄OH to identify each, with the observation that decides each identity.
Hint
First NaOH, then NH₄OH on the two that precipitate.
Answer
Dissolve each in water and add NaOH: ammonium nitrate gives no precipitate but ammonia on warming (moist red litmus blue). The other two give white precipitates soluble in excess NaOH. To fresh samples of these two add excess NH₄OH: zinc nitrate's precipitate dissolves; lead nitrate's does not.
Q10 · 3 marks · Short answerHOTS / competency
A metal M reacts with both dilute HCl and hot concentrated NaOH, giving hydrogen in each case. Its oxide is M₂O₃. (a) Identify M. (b) Write both equations. (c) What does this show about M?
Hint
Valency 3, amphoteric.
Answer
(a) Aluminium. (b) $2 A l + 6 H C l \to 2 A l C l_{3} + 3 H_{2}$ ; $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ (c) It is amphoteric.
Q11 · 2 marks · Short answerHarder board level
Why is ammonium hydroxide, not sodium hydroxide, used to confirm copper(II) ions?
Hint
Unique colour in excess.
Answer
With excess NH₄OH, copper(II) hydroxide dissolves to give an inky blue solution, which no other ion in the syllabus gives; with NaOH the pale blue precipitate merely stays.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Colour of Fe(OH)₂?Show answer
Dirty green.
2.Colour of Fe(OH)₃?Show answer
Reddish brown.
3.Colour of Cu(OH)₂?Show answer
Pale blue.
4.Which hydroxides dissolve in excess NaOH?Show answer
Zn(OH)₂ and Pb(OH)₂.
5.Which hydroxides dissolve in excess NH₄OH?Show answer
Cu(OH)₂ (inky blue) and Zn(OH)₂ (colourless).
6.Name of Na₂ZnO₂?Show answer
Sodium zincate.
7.Name of Na₂PbO₂?Show answer
Sodium plumbite.
8.Gas from an ammonium salt with warm NaOH?Show answer
Ammonia.
9.Colour of an iron(III) chloride solution?Show answer
Yellowish brown.

Best order to study it

Learn the colours of the five ions' salts and solutions.
Learn the NaOH column (drops and excess) with equations.
Learn the NH₄OH column, especially copper and zinc in excess.
Learn the ammonium salt + NaOH test.
Finish with the amphoteric metal, oxide and hydroxide equations.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you fill in the master table from memory?
Can you distinguish Zn²⁺ from Pb²⁺, Fe²⁺ from Fe³⁺, and NH₄⁺ from Na⁺ with one reagent each?
Can you write both equations for copper(II) sulphate with excess NH₄OH?
Can you write zincate, plumbite and aluminate equations for metals, oxides and hydroxides?
Do you always answer with an observation, not just a product?

Analytical Chemistry: frequently asked questions

Are calcium salts tested with NaOH and NH₄OH in the ICSE 2027 Chemistry syllabus?

No. The 2027 syllabus lists salts of Fe, Cu, Zn and Pb only; calcium salts were in the 2026 syllabus and have been removed.

Do I need the names of the complex ions?

You should know that copper gives an inky blue solution of tetraamminecopper(II) sulphate in excess ammonium hydroxide, and be able to write its equation.

Does it matter whether I write NaOH or KOH for amphoteric reactions?

Either is accepted when the question says 'caustic alkali'. With KOH, write potassium zincate (K₂ZnO₂), plumbite (K₂PbO₂) or aluminate (KAlO₂).

Why does iron(II) hydroxide turn brown on standing?

Air oxidises it to iron(III) hydroxide, which is reddish brown.

Based on: CISCE ICSE 2027 Chemistry (Science Paper 2) syllabus; CISCE ICSE 2027 Chemistry (Science Paper 2) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2025; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2024