JAMB Chemistry 2025 past questions

Checked by the Skolra question team · Last updated 23 October 2026

Ten questions from the 2025 JAMB Chemistry paper, with answers and explanations. We hold 106 questions from this paper, and the Skolra app has all of them.

JAMB Chemistry 2025 questions and answers

1. 1.

   Which of the following statements is false about hard water?

   1. A
      Hard water contains dissolved calcium and magnesium salts
   2. B
      Hard water leaves a scaly deposit (fur) in kettles and pipes when heated
   3. C
      Hard water forms a thick, curdy scum instead of lather when soap is added
   4. D
      Hard water lathers freely and easily with ordinary soap
      Correct
   Show answerHide answer

   Answer: D

   Hard water is water that contains dissolved calcium and magnesium salts (mainly bicarbonates, sulphates and chlorides), picked up as the water passes through rocks such as limestone and gypsum. This makes the statement that hard water contains dissolved calcium and magnesium salts a true fact about hard water, not the false one being asked for.

2. 2.

   The products of the thermal decomposition of ammonium trioxonitrate(v) are

   1. A
      Dinitrogen oxide (nitrogen(I) oxide) and water
      Correct
   2. B
      Nitrogen(IV) oxide and oxygen
   3. C
      Ammonia and trioxonitrate(V) acid
   4. D
      Nitrogen and oxygen
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   Answer: A

   Ammonium trioxonitrate(V), NH₄NO₃ (ammonium nitrate), is unstable to heat. On gentle heating it undergoes thermal decomposition:

   NH₄NO₃(s) → N₂O(g) + 2H₂O(g)

   So the products are dinitrogen oxide (nitrogen(I) oxide, also called nitrous oxide) and water. This is a standard reaction in the SS chemistry curriculum and a favourite JAMB fact.

   Now look at why the other options are traps:

   Nitrogen(IV) oxide and oxygen is what you get when you heat the nitrates of less reactive (heavy/transition) metals, such as lead(II) trioxonitrate(V) or copper(II) trioxonitrate(V) — not ammonium trioxonitrate(V), because ammonium salts contain no metal to leave behind as an oxide.

3. 3.

   Mg$_{(s)}$  +   2HCl$_{(aq)}$ → MgCl$_2$$_{(aq)}$ +  H$_2$$_{(g)}$

   In the reaction above, magnesium is a reducing agent because

   1. A
      it loses electrons, so its oxidation number rises from 0 in the metal to +2 in MgCl2
      Correct
   2. B
      its oxidation number decreases from +2 to 0 as it is itself reduced by the chloride ions
   3. C
      it gains electrons from the hydrogen ions, so its oxidation number falls from 0 to −2
   4. D
      it keeps a constant oxidation number throughout the reaction while only hydrogen changes
   Show answerHide answer

   Answer: A

   In Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g), track the oxidation numbers of the atoms that actually change.

   Magnesium starts as the free element, oxidation number 0, and ends up as Mg²⁺ inside MgCl₂, oxidation number +2. Going from 0 to +2 means magnesium has lost two electrons — it has been oxidized.

   Hydrogen, meanwhile, starts as H⁺ in HCl (oxidation number +1) and ends up as H₂ gas (oxidation number 0). Hydrogen has gained electrons — it has been reduced.

4. 4.

   CH$_3$ - CH$_2$ - COOCH$_2$ - CH$_3$

   From the condensed structure above, the reactants are

   1. A
      Butanoic acid and ethanol
   2. B
      Propanoic acid and methanol
   3. C
      Propanoic acid and ethanol
      Correct
   4. D
      Ethanoic acid and ethanol
   Show answerHide answer

   Answer: C

   The structure CH₃-CH₂-COOCH₂-CH₃ is an ester, formed by condensation (esterification) of a carboxylic acid with an alcohol, releasing water.

   Split the ester at the -COO- link. The acid portion, CH₃-CH₂-COOH, has three carbons in its chain (counting the carbonyl carbon), which is propanoic acid. The alcohol portion, -CH₂-CH₃ attached to oxygen, becomes CH₃-CH₂-OH on hydrolysis, which is ethanol.

   So the reactants are propanoic acid and ethanol, joined with loss of water: CH₃CH₂COOH + CH₃CH₂OH → CH₃CH₂COOCH₂CH₃ + H₂O.

5. 5.

   What is the function of concentrated H$_2$SO$_4$?

   1. A
      It acts as an alkali, neutralising acidic substances in solution
   2. B
      It acts as a dehydrating agent, removing hydrogen and oxygen from compounds in the ratio of water
      Correct
   3. C
      It acts as a reducing agent, donating electrons to substances it reacts with
   4. D
      It acts as a catalyst, speeding up reactions without being consumed
   Show answerHide answer

   Answer: B

   Concentrated tetraoxosulphate(VI) acid, H₂SO₄, is best known in the Nigerian chemistry syllabus for its dehydrating property — it strips water, or the elements hydrogen and oxygen in the same 2:1 ratio as water, out of compounds.

   A classic demonstration is pouring concentrated H₂SO₄ on sugar (C₁₂H₂₂O₁₁): the acid removes the water, leaving a black spongy mass of carbon behind. It is used the same way to convert ethanol to ethene by removing a water molecule.

6. 6.

   An example of an alkaline gas is

   1. A
      Ammonia
      Correct
   2. B
      Hydrogen chloride
   3. C
      Sulphur(IV) oxide
   4. D
      Carbon(IV) oxide
   Show answerHide answer

   Answer: A

   Ammonia (NH₃) is the correct answer. Ammonia gas dissolves in water to form ammonium hydroxide (NH₄OH), a weak alkali (base), and it turns moist red litmus paper blue — the standard test for an alkaline gas.

   The other gases listed are all acidic, not alkaline, so they are the wrong picks. Hydrogen chloride dissolves in water to form hydrochloric acid, a strong acid, and turns moist blue litmus red. Sulphur(IV) oxide (sulphur dioxide) dissolves in water to form sulphurous acid, another acidic gas. Carbon(IV) oxide (carbon dioxide) dissolves in water to form the weak acid, carbonic acid, and also turns moist blue litmus red, not blue.

7. 7.

   A molecule with a polar covalent bond between atoms exhibits

   1. A
      Zero dipole moment due to equal sharing of electrons
   2. B
      Delocalization of electrons over the whole molecule
   3. C
      Complete transfer of electrons from one atom to the other
   4. D
      A permanent dipole moment
      Correct
   Show answerHide answer

   Answer: D

   A polar covalent bond forms when two bonded atoms have different electronegativities. The shared electron pair is not shared equally — it lies closer to the more electronegative atom, giving that atom a slight negative charge (δ-) and the other atom a slight positive charge (δ+).

   This unequal, but partial, separation of charge across the bond is called a permanent dipole moment, and it is the defining property of a polar covalent bond.

   Zero dipole moment would only occur if the electrons were shared perfectly equally, which is the case for a nonpolar covalent bond (e.g. between two identical atoms), not a polar one.

8. 8.

   C$_2$H$_5$OH   +  CH$_3$COOH  ⇌  CH$_3$COOC$_2$H$_5$  +  H$_2$O

   The reaction above is

   1. A
      A neutralization reaction
   2. B
      A reversible reaction
      Correct
   3. C
      An irreversible reaction
   4. D
      An addition reaction
   Show answerHide answer

   Answer: B

   The correct answer is that this is a reversible reaction.

   Look closely at the symbol joining the two sides of the equation: it is a double half-arrow (⇌), not a single forward arrow (→). In chemistry, ⇌ is used specifically to show that a reaction can proceed in both directions at once — the products can react back to reform the reactants.

9. 9.

   The source of carbon(II)oxide that acts as air pollutant is

   1. A
      Exhaust fumes from incomplete combustion of petrol in automobile engines
      Correct
   2. B
      Respiration in living organisms
   3. C
      Decomposition of limestone by heat
   4. D
      Complete combustion of fossil fuels
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   Answer: A

   Carbon(II)oxide (CO) is formed when carbon-containing fuels burn with an insufficient supply of oxygen — that is, incomplete combustion. This happens a lot in automobile (car) engines, where petrol does not burn completely, releasing CO gas in the exhaust fumes. This is why exhaust fumes from vehicles are a major source of carbon(II)oxide pollution in cities.

   The other options are all sources of carbon(IV)oxide (CO2), not carbon(II)oxide, so they are incorrect for this question:

   Complete combustion of fossil fuels (with enough oxygen) produces CO2, since every carbon atom bonds with two oxygen atoms.
   Respiration in living organisms releases CO2 as a waste product of cellular metabolism.
   Heating (decomposing) limestone, CaCO3 → CaO + CO2, also produces CO2, not CO.

   So the key distinction a candidate must remember: incomplete combustion (not enough oxygen) → CO; complete combustion (enough oxygen) → CO2.

10. 10.

    The metal that will liberate H$_2$ gas from dilute HNO$_3$ is

    1. A
       Zinc
    2. B
       Magnesium
       Correct
    3. C
       Iron
    4. D
       Copper
    Show answerHide answer

    Answer: B

    Dilute trioxonitrate(V) acid, HNO₃, is an oxidizing acid. Unlike dilute HCl or H₂SO₄, when it reacts with most metals the nitrate ion is reduced instead of the hydrogen ion, so the gas given off is usually nitrogen(II) oxide, NO, rather than hydrogen.

    Zinc, copper and iron all follow this normal behaviour with dilute HNO₃: they react (or in copper's case, react only because HNO₃ is oxidizing, since copper is below hydrogen in the reactivity series) and release oxides of nitrogen, not hydrogen gas.

    So the metal that liberates H₂ gas from dilute HNO₃ is magnesium.

The questions come from past JAMB, WAEC and NECO papers. We use software to help write the answer options and explanations, and every answer is checked twice before it goes on the site. How we check

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