JAMB Chemistry 2023 past questions

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

Ten questions from the 2023 JAMB Chemistry paper, with answers and explanations. We hold 81 questions from this paper, and the Skolra app has all of them.

JAMB Chemistry 2023 questions and answers

1. 1.

   Which separation technique is used to separate different pigments in a mixture based on their affinity for a stationary phase and a mobile phase?

   1. A
      Sublimation
   2. B
      Fractional distillation
   3. C
      Crystallization
   4. D
      Chromatography
      Correct
   Show answerHide answer

   Answer: D

   Chromatography is correct.

   Chromatography separates the components of a mixture (such as pigments) based on how each component distributes itself between a stationary phase (e.g. chromatography paper or the adsorbent in a column) and a mobile phase (the solvent that moves through or along the stationary phase).

   Only chromatography matches the description in the question, so it is the answer.

2. 2.

   What is the trend for ionization energy across a period in the periodic table?

   1. A
      It first decreases and then increases sharply across the period
   2. B
      It increases from left to right across the period
      Correct
   3. C
      It decreases steadily from left to right across the period
   4. D
      It remains constant across the period
   Show answerHide answer

   Answer: B

   Ionization energy increases across a period, from left to right.

   As you move across a period, electrons keep entering the same outer shell, so the shielding effect barely changes, but the number of protons in the nucleus keeps going up. The extra positive charge pulls the outer electrons in more tightly, so more energy is needed to remove one of them. That is why elements on the right side of a period (near the noble gases) have much higher ionization energies than the elements on the left (the reactive metals like sodium and magnesium).

3. 3.

   Which halogen is a gas at room temperature and is pale yellow in color?

   1. A
      Bromine
   2. B
      Iodine
   3. C
      Fluorine
      Correct
   4. D
      Chlorine
   Show answerHide answer

   Answer: C

   Fluorine is correct. Going down Group VII (the halogens), the elements change in state and colour: fluorine is a pale yellow gas, chlorine is a greenish-yellow (yellowish-green) gas, bromine is a reddish-brown liquid, and iodine is a purple-black solid that sublimes into a purple vapour.

   A candidate can easily mix up fluorine and chlorine because both are gases at room temperature, but the colour description "pale yellow" belongs specifically to fluorine — chlorine's colour is usually described as greenish-yellow, not plain pale yellow.

4. 4.

   Which of the following is a unique property of water compared to other liquids?

   1. A
      It contracts steadily as it cools, becoming denser right down to its freezing point
   2. B
      It expands on freezing, so solid ice is less dense than liquid water and floats on it
      Correct
   3. C
      It conducts electricity strongly even in its pure, unionised form
   4. D
      It has a low specific heat capacity, so it heats up and cools down quickly
   Show answerHide answer

   Answer: B

   Water's unique behaviour on freezing. Almost every liquid contracts steadily as it cools and becomes denser as it turns solid, so its solid form sinks in its own liquid. Water does the opposite between 4°C and 0°C: it reaches its maximum density at 4°C, then expands as it cools further and freezes. This is because water molecules arrange themselves into an open, hydrogen-bonded lattice (the ice structure), which takes up more space than the liquid. The result is that ice is less dense than liquid water, so it floats instead of sinking. This is why ponds and rivers freeze from the top down, letting aquatic life survive underneath — a behaviour almost no other common liquid shows.

5. 5.

   Which of the following reactions would be expected to have the highest entropy change?

   1. A
      2H₂(g) + O₂(g) → 2H₂O(l)
   2. B
      2KClO₃(s) → 2KCl(s) + 3O₂(g)
      Correct
   3. C
      N₂(g) + 3H₂(g) → 2NH₃(g)
   4. D
      CaCO₃(s) → CaO(s) + CO₂(g)
   Show answerHide answer

   Answer: B

   Entropy (disorder) rises sharply whenever a reaction produces more gas molecules than it started with, because gases are far more disordered than solids or liquids.

   Look at how each reaction changes the number of moles of gas:

   The combination of hydrogen and oxygen to form liquid water takes 3 moles of gas down to 0 moles of gas (liquid) — that is a big decrease in entropy, not an increase.

   The formation of ammonia from nitrogen and hydrogen takes 4 moles of gas down to 2 moles of gas — entropy also decreases here, which is why this reaction needs a favourable temperature/pressure balance industrially.

6. 6.

   Which trace gas in the atmosphere plays a significant role in the greenhouse effect?

   1. A
      Argon
   2. B
      Oxygen
   3. C
      Carbon(IV) oxide
      Correct
   4. D
      Nitrogen
   Show answerHide answer

   Answer: C

   Carbon(IV) oxide (carbon dioxide, CO₂) is the trace gas that plays a significant role in the greenhouse effect.

   Although carbon(IV) oxide makes up only a small fraction of the atmosphere (about 0.04%, which is why it is called a "trace gas"), its molecules absorb infrared radiation reflected from the Earth's surface and re-radiate it back down, trapping heat. This warming effect is the greenhouse effect, and carbon(IV) oxide from the burning of fossil fuels is the major gas driving it.

7. 7.

   What is the symbol used to represent an alpha particle?

   1. A
      β
   2. B
      n
   3. C
      γ
   4. D
      α
      Correct
   Show answerHide answer

   Answer: D

   α is the correct symbol. An alpha particle is made up of 2 protons and 2 neutrons, so it is really the nucleus of a helium atom (written in full as ⁴₂He). In radioactivity, it is shown using the Greek letter alpha, α.

   So when you see α in a nuclear equation, think: helium nucleus, 2 protons and 2 neutrons, charge of +2.

8. 8.

   What happens when alkanoic acids react with alcohols in the presence of an acid catalyst?

   1. A
      They undergo hydrolysis to produce an alkanol and an alkanoic acid
   2. B
      They react to form an alkanal and hydrogen gas
   3. C
      They react to form a salt and water, as in an acid–base neutralization
   4. D
      They combine to form an ester and water, a reaction known as esterification
      Correct
   Show answerHide answer

   Answer: D

   When an alkanoic acid (a carboxylic acid, R-COOH) reacts with an alcohol (R'-OH) in the presence of a small amount of concentrated acid catalyst (usually concentrated tetraoxosulphate(VI) acid), the -OH group of the acid combines with the -H of the alcohol to release water, while the remaining fragments join to form an ester.

   This is called esterification, a condensation reaction:

   R-COOH + R'-OH → R-COOR' + H₂O

   For example, ethanoic acid reacting with ethanol gives ethyl ethanoate and water. Esters are known for their pleasant, fruity smells and are used in perfumes and flavourings.

9. 9.

   Which of the following is an example of a primary cell?

   1. A
      Lithium-ion battery
   2. B
      Dry cell (Leclanché cell)
      Correct
   3. C
      Nickel–cadmium battery
   4. D
      Lead–acid accumulator
   Show answerHide answer

   Answer: B

   A primary cell is an electrochemical cell in which the chemical reaction that produces current is not reversible. Once the reactants inside are used up, the cell goes flat and cannot be recharged - it is simply discarded.

   The dry cell (Leclanché cell), the type used in torchlights and remote controls, is the classic example of a primary cell. Its zinc casing (anode) and manganese(IV) oxide/carbon rod (cathode) reaction cannot be reversed by recharging, so it is correct.

10. 10.

    What is the mass (in grams) of 500 mL of ethanol? (density of ethanol = 0.789 g/mL)

    1. A
       633.7 g
    2. B
       500 g
    3. C
       394.5 g
       Correct
    4. D
       789 g
    Show answerHide answer

    Answer: C

    Mass = Volume × Density

    Volume of ethanol = 500 mL
    Density of ethanol = 0.789 g/mL

    Mass = 500 mL × 0.789 g/mL = 394.5 g

    This is the only option that comes from multiplying the volume by the density correctly, which is the right formula for converting volume to mass.

    A candidate who divides volume by density instead of multiplying (500 ÷ 0.789) wrongly arrives at 633.7 g. A candidate who forgets to apply the density at all and just copies the volume gets 500 g. A candidate who multiplies by 1000 instead of using the density as given (mistaking g/mL for g/L conversion) gets 789 g. Always check that density is applied as mass per unit volume, and multiply, not divide, to get mass from volume.

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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Practise all 81 questions we hold from the 2023 paper, on a timed CBT screen

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