ChemistryICSE Class 10 · Analytical Chemistry

Amphoteric Oxides and Hydroxides: Reactions with NaOH and KOH (ICSE Class 10)

Updated 17 October 2026 · Fact-checked

An amphoteric substance reacts both with acids and with alkalis to give a salt and water. Aluminium, zinc and lead, their oxides (Al₂O₃, ZnO, PbO) and hydroxides (Al(OH)₃, Zn(OH)₂, Pb(OH)₂) are amphoteric. With hot concentrated NaOH or KOH the metals give hydrogen and a soluble aluminate, zincate or plumbite, while the oxides and hydroxides give the same salts with water. This is why white precipitates of Zn(OH)₂ and Pb(OH)₂ dissolve in excess caustic alkali.

Notice that ZnO behaves as a base with an acid and as an acid with NaOH, always forming a salt and water; Al₂O₃ and PbO do the same.

Illustration drawn for QuestionBuddy.

Understand Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis

1. The idea of amphoteric behaviour. Most metal oxides are basic (they react only with acids) and most non-metal oxides are acidic (they react only with alkalis). An amphoteric oxide or hydroxide shows both behaviours: it behaves as a base towards an acid and as an acid towards a strong alkali. The syllabus names aluminium, zinc and lead.

2. Reactions with an acid (basic behaviour).

$Z n O + 2 H C l \to Z n C l_{2} + H_{2} O$
$A l_{2} O_{3} + 6 H C l \to 2 A l C l_{3} + 3 H_{2} O$
$P b O + 2 H N O_{3} \to P b (N O_{3})_{2} + H_{2} O$
$Z n (O H)_{2} + H_{2} S O_{4} \to Z n S O_{4} + 2 H_{2} O$
$A l (O H)_{3} + 3 H C l \to A l C l_{3} + 3 H_{2} O$

3. Reactions with a caustic alkali (acidic behaviour). Caustic alkalis are NaOH and KOH. The product is a salt called an aluminate, zincate or plumbite, plus water.

Oxides:

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

Hydroxides:

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

Metals (also release hydrogen; use hot concentrated alkali):

$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$
$P b + 2 N a O H \to N a_{2} P b O_{2} + H_{2}$
With KOH, the products are potassium zincate, aluminate and plumbite (K₂ZnO₂, KAlO₂, K₂PbO₂).

4. Link with analytical chemistry. On adding NaOH to a solution of a zinc or lead salt, the white precipitate of hydroxide forms first and then dissolves in excess because it is amphoteric. This is why sodium hydroxide cannot separate zinc from lead (see the ammonium hydroxide page).

5. Amphoteric versus other oxides.

Type	Reaction with acid	Reaction with alkali	Example
Basic oxide	yes	no	CuO, CaO
Acidic oxide	no	yes	CO₂, SO₂
Amphoteric oxide	yes	yes	ZnO, Al₂O₃, PbO
Neutral oxide	no	no	CO, H₂O

Copper(II) oxide and copper(II) hydroxide are basic, not amphoteric; iron hydroxides do not dissolve in NaOH.

Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis: formulas and key facts

Oxides with NaOH: $Z n O + 2 N a O H \to N a_{2} Z n O_{2} + H_{2} O$ ; $A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$ ; $P b O + 2 N a O H \to N a_{2} P b O_{2} + H_{2} O$
Hydroxides with NaOH: $Z n (O H)_{2} + 2 N a O H \to N a_{2} Z n O_{2} + 2 H_{2} O$ ; $A l (O H)_{3} + N a O H \to N a A l O_{2} + 2 H_{2} O$ ; $P b (O H)_{2} + 2 N a O H \to N a_{2} P b O_{2} + 2 H_{2} O$
Metals with NaOH: $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ ; $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ ; $P b + 2 N a O H \to N a_{2} P b O_{2} + H_{2}$
Oxides with an acid: $Z n O + 2 H C l \to Z n C l_{2} + H_{2} O$ ; $A l_{2} O_{3} + 6 H C l \to 2 A l C l_{3} + 3 H_{2} O$
Salt names: Na₂ZnO₂ sodium zincate
NaAlO₂ sodium aluminate
Na₂PbO₂ sodium plumbite

Na₂ZnO₂	sodium zincate
NaAlO₂	sodium aluminate
Na₂PbO₂	sodium plumbite

How to solve Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis questions

Write amphoteric equations in a fixed order.

1Identify the type: metal, oxide or hydroxide of Al, Zn or Pb.
2Choose the reagent: a mineral acid gives the ordinary salt; NaOH/KOH gives the aluminate, zincate or plumbite.
3Write the product names: zinc gives zincate (ZnO₂), aluminium gives aluminate (AlO₂), lead gives plumbite (PbO₂).
4Add water or hydrogen: oxides and hydroxides give water; metals give hydrogen.
5Balance metals, then radicals, then hydrogen and oxygen.
6For mole numericals, use the balanced mole ratio.

Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis in real life

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.

Why aluminium cookware should not be cleaned with a strong alkaline drain cleaner

Suppose an aluminium vessel is left overnight with a strongly alkaline (caustic soda) drain cleaner in it. Next morning the surface is pitted and bubbles of gas were seen rising from it.

Explain, using chemical equations, what has happened.

Try it first, then show the solutionSolution

Aluminium oxide on the surface first reacts with the alkali: $A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$ .
The bare aluminium then reacts, releasing hydrogen bubbles: $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ .

Answer: Aluminium and its oxide are amphoteric and dissolve in caustic alkali to give sodium aluminate with hydrogen.

Memory hook: Alkali eats aluminium.

Tricks to remember Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis

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

AZP are amphoteric
Aluminium, Zinc, Lead: the A-Z-P metals whose oxides and hydroxides react with both acids and alkalis.
Use it when: Recalling the amphoteric set.
Name the product by the metal
Aluminium - aluminate, zinc - zincate, lead - plumbite (from Latin plumbum).
Use it when: Writing products with NaOH.
Metal gives hydrogen
Metal + alkali gives hydrogen; oxide or hydroxide + alkali gives water.
Use it when: Completing the equation.

Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define · 2 marks · Short answerBoard level

What is meant by an amphoteric oxide? Give two examples.

Try it first, then show the solutionSolution

An amphoteric oxide reacts with both acids and alkalis to form salt and water.
Examples: zinc oxide and aluminium oxide (also lead(II) oxide).

Answer: An oxide that reacts with both acids and alkalis, e.g. ZnO and Al₂O₃.

Example 2: Two equations for one hydroxide · 2 marks · Short answerBoard level

Aluminium hydroxide dissolves in both dilute hydrochloric acid and sodium hydroxide solution. Write the balanced equation for each reaction.

Try it first, then show the solutionSolution

With an acid: $A l (O H)_{3} + 3 H C l \to A l C l_{3} + 3 H_{2} O$ .
With an alkali: $A l (O H)_{3} + N a O H \to N a A l O_{2} + 2 H_{2} O$ .

Answer: Both equations as above.

Example 3: Hydrogen from aluminium · 3 marks · Short answerHarder board level

Find the volume of hydrogen at S.T.P. obtained when 2.7 g of aluminium reacts with excess hot concentrated sodium hydroxide. (Al = 27; 1 mole of gas = 22.4 dm³) $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$

Try it first, then show the solutionSolution

Moles of Al = 2.7 / 27 = 0.1.
From the equation, 2 mol Al gives 3 mol H₂, so 0.1 mol Al gives 0.15 mol H₂.
Volume = 0.15 x 22.4 = 3.36 dm³.

Answer: 3.36 dm³.

Example 4: Mass of alkali for the oxide · 3 marks · Short answerHarder board level

What mass of sodium hydroxide is needed to dissolve 8.1 g of zinc oxide completely? $Z n O + 2 N a O H \to N a_{2} Z n O_{2} + H_{2} O$ (Zn = 65, O = 16, Na = 23, H = 1)

Try it first, then show the solutionSolution

Molar mass of ZnO = 65 + 16 = 81 g/mol; moles = 8.1 / 81 = 0.1 mol.
ZnO : NaOH = 1 : 2, so NaOH needed = 0.2 mol.
Mass = 0.2 x 40 = 8.0 g.

Answer: 8.0 g.

Example 5: Deduce the oxide · 5 marks · Long answerHOTS / competency

A white solid X dissolves in dilute hydrochloric acid and also in sodium hydroxide solution, giving colourless solutions. When X is heated with a strong alkali, no gas is evolved. (a) What kind of oxide is X likely to be? (b) Name two possible compounds. (c) Write the equation for X = Al₂O₃ with the alkali. (d) A second white solid Y dissolves in the acid but not in the alkali. Name one such oxide and say what type it is.

Try it first, then show the solutionSolution

(a) An amphoteric oxide (it reacts with both).
(b) Aluminium oxide or zinc oxide.
(c) $A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$ .
(d) Calcium oxide (a basic oxide); a basic oxide reacts only with acids.

Answer: (a) Amphoteric. (b) Al₂O₃ or ZnO. (c) as above. (d) CaO, basic.

Where students lose marks in Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis

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

Mistake: Writing H₂O as a product in the metal-alkali reaction
Metals give hydrogen, not water.
Wrong working
Zn + 2NaOH -> Na₂ZnO₂ + H₂O
Right working
Zn + 2NaOH -> Na₂ZnO₂ + H₂
Do this: Metal gives hydrogen gas; oxide or hydroxide gives water.
Usually costs: 1 mark.
Mistake: Naming the wrong salt
Aluminate, zincate and plumbite are interchanged.
Do this: Product name comes from the metal: Al - aluminate, Zn - zincate, Pb - plumbite.
Usually costs: 1 mark.
Mistake: Calling CuO or Fe(OH)₃ amphoteric
These do not react with excess NaOH.
Do this: Amphoteric set: Al, Zn, Pb only.
Usually costs: 1 mark.
Mistake: Not balancing 2Al + 2NaOH + 2H2O
The water in the aluminium equation is left out.
Do this: Include water as a reactant and balance H and O last.
Usually costs: 1 mark.

Exam tips

Give both equations (with acid and with alkali) to justify "amphoteric".
Use the formulae Na₂ZnO₂, NaAlO₂ and Na₂PbO₂ and name the salts.
Say "hot concentrated" alkali for the metals.
In identification questions, a white precipitate soluble in excess NaOH points to Zn or Pb.

Practice questions on Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis

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
Which of the following oxides is amphoteric?
(a) Na₂O
(b) CO₂
(c) ZnO
(d) CuO
Hint
It reacts with both acid and alkali.
Answer
(c) ZnO.
Q2 · 1 mark · MCQBoard level
The product obtained when aluminium hydroxide dissolves in sodium hydroxide is:
(a) Na₂ZnO₂
(b) NaAlO₂
(c) Na₂PbO₂
(d) Al₂O₃
Hint
Aluminate.
Answer
(b) NaAlO₂.
Q3 · 2 marks · Short answerBoard level
Write the equation of the reaction between zinc and sodium hydroxide, and name the gas evolved.
Hint
Metal with alkali gives hydrogen.
Answer
$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ ; hydrogen.
Q4 · 2 marks · Short answerHarder board level
Write balanced equations for the reaction of lead(II) oxide with (a) dilute nitric acid, (b) sodium hydroxide solution.
Hint
Basic and acidic behaviour.
Answer
(a) $P b O + 2 H N O_{3} \to P b (N O_{3})_{2} + H_{2} O$ (b) $P b O + 2 N a O H \to N a_{2} P b O_{2} + H_{2} O$ .
Q5 · 3 marks · Short answerHarder board level
A white precipitate formed on adding a little sodium hydroxide to a solution of zinc sulphate dissolves in excess alkali. Explain with two equations.
Hint
Precipitate first, then amphoteric dissolving.
Answer
$Z n S O_{4} + 2 N a O H \to Z n (O H)_{2} + N a_{2} S O_{4}$ ; the hydroxide is amphoteric and dissolves: $Z n (O H)_{2} + 2 N a O H \to N a_{2} Z n O_{2} + 2 H_{2} O$ .
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Aluminium hydroxide dissolves in both hydrochloric acid and sodium hydroxide solution.
Reason (R): Aluminium hydroxide is amphoteric.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Why does it dissolve in both?
Answer
(a).
Q7 · 4 marks · Long answerHarder board level
Calculate the volume of hydrogen at S.T.P. liberated when 13 g of zinc reacts with excess sodium hydroxide, and the mass of sodium zincate formed. (Zn = 65, Na = 23, O = 16, H = 1)
Hint
Zn : H₂ : Na₂ZnO₂ = 1 : 1 : 1.
Answer
Moles of Zn = 13 / 65 = 0.2. H₂ = 0.2 x 22.4 = 4.48 dm³. Na₂ZnO₂ (M = 46 + 65 + 32 = 143 g/mol) = 0.2 x 143 = 28.6 g.
Q8 · 5 marks · Long answerHOTS / competency
A metal M forms an oxide that dissolves in both HCl and NaOH. Its hydroxide is white and gelatinous and dissolves in excess NH₄OH. M reacts with NaOH on heating to give hydrogen. (a) Suggest M. (b) Write the equations for M with NaOH and for the hydroxide with HCl. (c) Why is magnesium oxide unlike M, although both are white solids?
Hint
Choose the metal from Al, Zn, Pb.
Answer
(a) Zinc (aluminium hydroxide does not dissolve in excess NH₄OH, but zinc hydroxide does). (b) $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ ; $Z n (O H)_{2} + 2 H C l \to Z n C l_{2} + 2 H_{2} O$ . (c) Magnesium oxide is basic and does not react with alkalis.

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.Name the three amphoteric metals in the syllabus.Show answer
Aluminium, zinc and lead.
2.Formula of sodium zincate?Show answer
Na₂ZnO₂.
3.Gas given by aluminium with NaOH?Show answer
Hydrogen.
4.Give an oxide that is basic only.Show answer
Copper(II) oxide or calcium oxide.
5.Name the product of Al(OH)₃ with NaOH.Show answer
Sodium aluminate.

Amphoteric Metals, Oxides and Hydroxides with Caustic Alkalis: frequently asked questions

What does amphoteric mean?

A substance that reacts with both acids and alkalis to form salts.

Why does zinc hydroxide dissolve in excess NaOH?

Because it is amphoteric and forms soluble sodium zincate.

Is iron(III) hydroxide amphoteric?

No, it does not dissolve in excess sodium hydroxide.

Can KOH be used instead of NaOH?

Yes; the products are the potassium aluminate, zincate or plumbite.