Analytical Chemistry

Q: Write balanced equations for the action of sodium hydroxide, first a few drops and then in excess, on zinc sulphate solution, naming the product formed in excess.

ZnSO4 + 2NaOH → Zn(OH)2↓ + Na2SO4; Zn(OH)2 + 2NaOH → Na2ZnO2 + 2H2O. The product in excess is sodium zincate, a colourless solution.

ICSE Class 10 · Chemistry · Chapter 4

What this chapter covers

What analytical chemistry does
Colour of salts and their solutions
Action of sodium hydroxide on salt solutions
Action of ammonium hydroxide on salt solutions
Special action of ammonium hydroxide on copper salts
Special action of sodium hydroxide on ammonium salts
Action of alkalis on certain metals
Action of alkalis on oxides and hydroxides: amphoteric nature
Putting the tests together

Key definitions

Analytical chemistry is the branch of chemistry that deals with finding out which substances are present in a sample and in what amounts.

Qualitative analysis is the part of analytical chemistry that identifies the elements, ions or compounds present in a sample, without measuring how much of each there is.

Quantitative analysis is the part of analytical chemistry that measures the amount of each substance present in a sample.

A reagent is a substance of known composition that is added to a sample to bring about a reaction which reveals what the sample contains.

A precipitate is an insoluble solid that separates out from a solution as the result of a chemical reaction taking place in that solution.

Precipitation is the process in which an insoluble solid is formed when two solutions are mixed and react.

A complex salt is a salt that contains a complex ion, in which a central metal ion is joined to a fixed number of neutral molecules or ions, and which does not give the separate simple ions of all its parts in solution.

Tetraamminecopper(II) sulphate is the deep blue complex salt, [Cu(NH₃)₄]SO₄, formed when excess ammonium hydroxide dissolves copper(II) hydroxide.

Caustic alkalis are the strong alkalis sodium hydroxide (caustic soda) and potassium hydroxide (caustic potash), so called because they burn and corrode skin and many materials.

An amphoteric oxide is a metallic oxide that reacts with both acids and alkalis to form a salt and water.

An amphoteric hydroxide is a metallic hydroxide that reacts with both acids and alkalis to form a salt and water.

Key points

Analytical chemistry finds what a sample contains; qualitative analysis identifies the ions, quantitative analysis measures amounts.
Salts of Na, K, Ca, Zn, Pb and NH₄⁺ are white with colourless solutions; copper(II) salts are blue, iron(II) salts pale green and iron(III) salts yellowish brown.
Sodium hydroxide and ammonium hydroxide precipitate metal hydroxides from salt solutions; the colour of the precipitate and its behaviour in excess identify the metal ion.
Fe(OH)₂ is dirty green and Fe(OH)₃ reddish brown; both are insoluble in excess of either alkali.
Cu(OH)₂ is pale blue; it is insoluble in excess NaOH but dissolves in excess NH₄OH to give deep blue tetraamminecopper(II) sulphate, [Cu(NH₃)₄]SO₄.
Zn(OH)₂ is white and gelatinous; it dissolves in excess NaOH (sodium zincate) and in excess NH₄OH (colourless tetraamminezinc(II) sulphate).
Pb(OH)₂ is white and chalky; it dissolves in excess NaOH (sodium plumbite) but not in excess NH₄OH.
Ammonium salts warmed with sodium hydroxide give ammonia, which smells pungent, turns moist red litmus blue and gives white fumes with HCl.
Zinc, aluminium and lead react with hot caustic alkalis to give hydrogen and sodium zincate, sodium meta-aluminate or sodium plumbite.
The oxides and hydroxides of zinc, aluminium and lead are amphoteric: they form a salt and water with both acids and alkalis.

Questions answered

Define a precipitate, and name the precipitate formed when sodium hydroxide is added to iron(III) chloride solution, giving its colour.

A precipitate is an insoluble solid that separates out from a solution as the result of a chemical reaction. Iron(III) chloride gives a reddish brown precipitate of iron(III) hydroxide, Fe(OH)₃.

State the colour of (a) copper(II) sulphate crystals, (b) iron(II) sulphate solution, (c) zinc sulphate solution, (d) potassium dichromate solution.

(a) Blue; (b) pale green; (c) colourless; (d) orange.

Why does an iron(II) sulphate solution turn yellowish brown when it is left exposed to air?

Oxygen of the air oxidises the pale green iron(II) ions to yellowish brown iron(III) ions.

Write balanced equations for the action of sodium hydroxide, first a few drops and then in excess, on zinc sulphate solution, naming the product formed in excess.

ZnSO₄ + 2NaOH → Zn(OH)₂↓ + Na₂SO₄; Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O. The product in excess is sodium zincate, a colourless solution.

Name a reagent that distinguishes zinc nitrate solution from lead(II) nitrate solution, and state the observation in each case.

Ammonium hydroxide. Both give a white precipitate, but the precipitate from zinc nitrate dissolves in excess to give a colourless solution, while that from lead nitrate stays undissolved.

Name a reagent that distinguishes copper(II) sulphate solution from zinc sulphate solution, and give the observation.

Sodium hydroxide (or ammonium hydroxide). Copper sulphate gives a pale blue precipitate; zinc sulphate gives a white precipitate (which dissolves in excess sodium hydroxide).

What do you see when ammonium hydroxide is added to copper(II) sulphate solution, first a little and then in excess? Name the final product and give its formula.

A pale blue precipitate of copper(II) hydroxide forms first, then dissolves in excess to give a deep blue solution of tetraamminecopper(II) sulphate, [Cu(NH₃)₄]SO₄.

Why does copper(II) hydroxide dissolve in excess ammonium hydroxide but not in excess sodium hydroxide?

Ammonium hydroxide supplies ammonia molecules, which join the copper(II) ion to form the soluble complex ion [Cu(NH₃)₄]²⁺. Sodium hydroxide contains no ammonia, and copper(II) hydroxide is not amphoteric, so it stays insoluble.

Past board exam questions

ICSE 2027 sample paper · 4 marks

A chloride of iron ‘R’, when treated with sodium hydroxide gives P and Q as a product. Q is a reddish-brown precipitate. (a) Identify R. (b) State the formula of the compounds P and Q formed after the reaction. (c) What do you observe when excess NaOH is added to Q?

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(a) R = Ferric chloride (iron(III) chloride), FeCl₃ (b) P = NaCl, Q = Fe(OH)₃ [FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl] (c) The reddish-brown precipitate remains insoluble in excess NaOH.

ICSE 2027 sample paper · 2 marks

State giving reasons if: (a) Zinc metal and aluminium metal can be distinguished by heating the metal powders separately in two different test tubes with concentrated sodium hydroxide solution. (b) Zinc nitrate and lead nitrate can be distinguished by adding ammonium hydroxide solution to the salt solution.

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(a) No. Both metals react with hot concentrated NaOH to liberate hydrogen gas, which burns with a pop sound (Zn + 2NaOH → Na₂ZnO₂ + H₂; 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂), so the observation is the same. (b) Yes. With zinc nitrate a white gelatinous precipitate of Zn(OH)₂ forms which dissolves in excess NH₄OH; with lead nitrate a chalky white precipitate of Pb(OH)₂ forms which remains insoluble in excess NH₄OH.

ICSE 2026 sample paper · 1 mark

Identify from the following metal oxide which can react with an acid as well as an alkali.

Silver oxide
Calcium oxide
Copper(II) oxide
Aluminium oxide

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Aluminium oxide

ICSE 2026 sample paper · 2 marks

A student was asked to perform two experiments in the laboratory based on the instructions given: Read the description given below and state one observation for each of the Experiments 1 and 2 that you would notice on mixing the given solutions. (a) Experiment 1: a solution of zinc sulphate is poured from one test tube into a test tube containing solution of barium chloride. (b) Experiment 2: ammonium hydroxide solution is poured from one test tube into a test tube containing precipitate of copper hydroxide.

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(a) A white precipitate (of barium sulphate) is formed: BaCl₂ + ZnSO₄ → BaSO₄ + ZnCl₂. (b) The pale blue precipitate dissolves in excess ammonium hydroxide to form a deep (inky) blue solution.

ICSE 2026 board paper · 3 marks

Give one relevant observation for the following: (a) Sodium hydroxide is added dropwise to Calcium nitrate solution. (b) Dilute Hydrochloric acid is added to Iron (II) sulphide. (c) An amphoteric metal is added to hot concentrated alkali.

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(a) A white precipitate of calcium hydroxide forms, which does not dissolve in excess sodium hydroxide (sparingly soluble). (b) A colourless gas with a rotten-egg smell (hydrogen sulphide) is evolved: FeS + 2HCl → FeCl₂ + H₂S (c) Brisk effervescence — hydrogen gas is evolved (e.g. Zn + 2NaOH → Na₂ZnO₂ + H₂) and the metal dissolves.

ICSE 2025 sample paper · 1 mark

A compound P is heated in a test tube with sodium hydroxide solution. A red litmus paper held at the mouth of the test tube turns blue. Which of the following could compound P be?

zinc sulphate
copper sulphate
ferrous sulphate
ammonium sulphate

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ammonium sulphate

ICSE 2025 compartment paper · 1 mark

A salt solution which gives a reddish-brown precipitate with NaOH and a white precipitate with BaCl₂ solution is:

CuSO₄
Ca(NO₃)₂
Fe₂(SO₄)₃
FeCl₃

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Fe₂(SO₄)₃

ICSE 2025 sample paper · 1 mark

A nitrate which forms a precipitate with ammonium hydroxide and is also soluble in excess of it:

ferrous nitrate
ferric nitrate
lead nitrate
copper nitrate

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copper nitrate