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In ICSE Class 10 analytical chemistry, every answer rests on one observation: a colour, or whether a precipitate dissolves. This post sets out what the syllabus asks, what each reagent does to iron, copper, zinc and lead salts, the equations, and a three-step way to name an unknown ion.
What does the ICSE syllabus ask for in analytical chemistry?
The ICSE 2027 Chemistry syllabus lists Analytical Chemistry as a Class X unit with two parts. The first is the action of ammonium hydroxide and sodium hydroxide on salt solutions: the colour of the salt and its solution, and the formation and colour of the hydroxide precipitated for salts of Fe, Cu, Zn and Pb, added drop by drop and then in excess, with equations. It names two special cases: ammonium hydroxide on copper salts, and sodium hydroxide on ammonium salts. The second part is the action of caustic alkalis on aluminium, zinc and lead, their oxides and hydroxides, to show they are amphoteric. The practical list uses the same tests on unknowns.
What happens when you add sodium hydroxide drop by drop?
Hydroxide ions pull the metal out as an insoluble hydroxide. Note the colour, then keep adding.
Iron(II), e.g. FeSO4: green precipitate of Fe(OH)2, insoluble in excess. FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4. AQA's GCSE chemistry specification gives the same colours: blue for copper(II), green for iron(II), brown for iron(III). The green darkens on standing because, as Chemguide's iron page explains, air oxidises iron(II) hydroxide to iron(III) hydroxide.
Iron(III), e.g. FeCl3: reddish-brown precipitate of Fe(OH)3, insoluble in excess. FeCl3 + 3NaOH → Fe(OH)3 + 3NaCl. The ICSE 2027 specimen paper and marking scheme use exactly this: an iron chloride giving a reddish-brown precipitate is ferric chloride, and the precipitate "remains insoluble in excess of NaOH".
Copper(II), e.g. CuSO4: blue precipitate of Cu(OH)2, insoluble in excess. CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4.
Zinc, e.g. ZnSO4: white precipitate of Zn(OH)2 that dissolves in excess to a colourless solution of sodium zincate. Zn(OH)2 + 2NaOH → Na2ZnO2 + 2H2O.
Lead, e.g. Pb(NO3)2: white precipitate of Pb(OH)2 that also dissolves in excess. Chemguide's lead page describes it redissolving to a colourless solution; ICSE names the product sodium plumbite. Pb(OH)2 + 2NaOH → Na2PbO2 + 2H2O.
How is ammonium hydroxide different?
Ammonia can also bind to some metal ions. With both iron ions, ammonium hydroxide gives the same precipitates as sodium hydroxide; Chemguide says the iron(III) reaction "looks just the same". Three results differ.
Zinc: white precipitate that dissolves in excess, forming a colourless zinc-ammonia complex, as Chemguide's ammonia page shows.
Lead: white precipitate that does not dissolve in excess. The 2027 marking scheme tells zinc nitrate from lead nitrate on this point alone.
Copper, the special case: a blue precipitate first, which dissolves in excess to a deep blue solution. Cu(OH)2 + 4NH3 → [Cu(NH3)4]2+ + 2OH−. Chemguide's copper page explains that ammonia replaces water around the copper ion, and an answer in the ICSE 2026 specimen marking scheme describes the blue precipitate dissolving "to form an inky blue solution".
What does sodium hydroxide do to ammonium salts?
Warm an ammonium salt with sodium hydroxide and ammonia gas is given off: NH4Cl + NaOH → NaCl + NH3 + H2O, the equation in the 2027 marking scheme. No precipitate forms, so this is a test for a gas, not a colour.
Why do zinc, aluminium and lead react with caustic alkalis?
Their oxides and hydroxides are amphoteric: they react with acids and with bases. NCERT's Class 10 chapter Metals and Non-metals defines amphoteric oxides this way and gives Al2O3 + 2NaOH → 2NaAlO2 + H2O. The ICSE marking scheme gives ZnO + 2NaOH → Na2ZnO2 + H2O. The metals react too: NCERT's Acids, Bases and Salts chapter shows 2NaOH + Zn → Na2ZnO2 + H2. One specimen question turns this into a trap: zinc and aluminium powders cannot be told apart with concentrated sodium hydroxide, because both give hydrogen.
Worked example: name the unknown ion
A colourless solution X gives a white precipitate with sodium hydroxide that dissolves in excess. With ammonium hydroxide it gives a white precipitate that stays, even in excess. What is the cation?
A colourless solution and a white precipitate rule out copper and iron.
Dissolving in excess sodium hydroxide fits both zinc and lead.
Staying in excess ammonium hydroxide rules out zinc.
So X contains Pb2+. Write the evidence in that order, with one equation.
Common mistakes and how to avoid them
Swapping the iron colours. Iron(II) is green, iron(III) reddish brown; rust is an iron(III) compound.
Saying copper(II) hydroxide dissolves in excess sodium hydroxide. Only excess ammonium hydroxide dissolves it.
Writing "the precipitate dissolves" without saying in excess of which reagent.
Describing a precipitate in the ammonium salt test, where only a gas forms.
A practice routine for this unit
Make one card per ion with three lines: a few drops of NaOH, excess NaOH, excess NH4OH. Cover the ion name and say it from the observations. Then practise "distinguish between" pairs, such as zinc nitrate and lead nitrate, writing the reagent, both observations and the conclusion. Our guide to the ICSE Class 10 chemistry paper shows where this unit sits in the paper.
If an idea behind the tests feels shaky, such as how a hydroxide can react with an alkali, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can read that idea on its own and then return to the tests.
Tonight, write the four-ion card set from memory and check each line against this post.
