JAMB Chemistry 2020 past questions

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

Ten questions from the 2020 JAMB Chemistry paper, with answers and explanations. We hold 38 questions from this paper, and the Skolra app has all of them.

JAMB Chemistry 2020 questions and answers

1. 1.

   What process would coal undergo to give coal gas, coal tar, ammoniacal liquor and coke?

   1. A
      Cracking
   2. B
      Combustion
   3. C
      Destructive distillation
      Correct
   4. D
      Fractional distillation
   Show answerHide answer

   Answer: C

   Destructive distillation is correct. When coal is strongly heated in the absence of air (carbonized in a closed retort), it breaks down into several useful products: coal gas (the volatile gaseous mixture), coal tar (a thick liquid of organic compounds), ammoniacal liquor (containing ammonium salts), and coke (the solid carbon residue left behind).

   Fractional distillation only separates a liquid mixture into components by differences in boiling point; it does not chemically break down a solid like coal, so it cannot be responsible for producing coke from coal.

2. 2.

   The shape of ammonia molecule is?

   1. A
      Linear
   2. B
      Trigonal pyramidal
      Correct
   3. C
      Trigonal planar
   4. D
      Tetrahedral
   Show answerHide answer

   Answer: B

   In ammonia, NH₃, the nitrogen atom has 3 bonding pairs (to the three hydrogen atoms) and 1 lone pair, giving 4 electron domains around it.

   The electron domains arrange tetrahedrally, but the lone pair is not counted when naming the molecular shape. Since one of the four positions is a lone pair rather than an atom, the actual shape traced out by the atoms is trigonal pyramidal (a pyramid with the nitrogen at the apex and the three hydrogens forming the base).

3. 3.

   The substance often used for vulcanization of rubber is?

   1. A
      Sulphur
      Correct
   2. B
      Silica
   3. C
      Graphite
   4. D
      Carbon black
   Show answerHide answer

   Answer: A

   Sulphur is the substance used for vulcanization of rubber. Vulcanization is the process of heating natural rubber with sulphur, which forms cross-links (sulphur bridges) between the long polymer chains of the rubber. This makes the rubber stronger, more elastic, and more resistant to heat and abrasion, which is why vulcanized rubber is used for tyres and other durable rubber products.

   Carbon black is a common distractor because it is also added to rubber, but only as a reinforcing filler to improve strength and wear resistance, not to bring about the actual cross-linking (vulcanization) reaction.

4. 4.

   Which of the following substances is a mixture?

   1. A
      Sodium chloride
   2. B
      Distilled water
   3. C
      Brine
      Correct
   4. D
      Carbon(IV) oxide
   Show answerHide answer

   Answer: C

   Brine is a mixture. It is simply common salt (sodium chloride) dissolved in water, so it contains two or more substances (Na⁺, Cl⁻ ions and water molecules) that are not chemically combined and can be separated again by simple physical means such as evaporation or distillation.

5. 5.

   Which of the following can be obtained by fractional distillation?

   1. A
      Petroleum into petrol, kerosene and diesel fractions
      Correct
   2. B
      Iron filings from a mixture with sand
   3. C
      Pure water from ink
   4. D
      Common salt from sea water
   Show answerHide answer

   Answer: A

   Fractional distillation is used to separate a mixture of two or more miscible liquids whose boiling points are close to one another, using a fractionating column. The classic Nigerian-syllabus example is crude oil (petroleum) being separated into fractions such as petrol, kerosene and diesel at the refinery, based on their different boiling points.

   Common salt from sea water is obtained by evaporation and crystallization, not fractional distillation, since salt is a dissolved solid, not a liquid with its own boiling point close to water's.

   Iron filings are separated from sand by a magnet (magnetic separation), a purely physical sorting method with no boiling involved.

6. 6.

   Proteins in acid solution undergo?

   1. A
      Oxidation to form carboxylic acids
   2. B
      Hydrolysis, breaking down into their constituent amino acids
      Correct
   3. C
      Denaturation only, with no breaking of peptide bonds
   4. D
      Polymerisation into longer polypeptide chains
   Show answerHide answer

   Answer: B

   Proteins are made up of amino acids joined together by peptide bonds. When a protein is treated with an acid (acid hydrolysis), water molecules are used to break these peptide bonds, and the protein is broken down step by step into its individual constituent amino acids.

7. 7.

   The most important element on the earth's crust is?

   1. A
      Oxygen
      Correct
   2. B
      Aluminium
   3. C
      Silicon
   4. D
      Iron
   Show answerHide answer

   Answer: A

   Oxygen is the most abundant element in the earth's crust, making up roughly 46% of its mass. It occurs mainly combined with other elements in oxides and silicate minerals such as quartz, feldspar and clay.

   Silicon is often confused for the answer because it is the second most abundant element (about 27%), and the two are usually found together in silicate rocks. Aluminium is a common distractor because it is the most abundant metal in the crust, not the most abundant element overall. Iron, though very important industrially, ranks even lower in abundance (about 5%).

   So when the question asks for the element that is most important/abundant in the earth's crust, oxygen is correct.

8. 8.

   Aluminum hydroxide is used in the dyeing industry as a?

   1. A
      Pigment
   2. B
      Catalyst
   3. C
      Mordant
      Correct
   4. D
      Bleaching agent
   Show answerHide answer

   Answer: C

   Aluminium hydroxide, Al(OH)₃, is used in the dyeing industry as a mordant.

   A mordant is a substance that helps a dye bind firmly and permanently to a fabric. Many dyes cannot attach directly to fibres in a lasting way, so the fabric is first treated with a mordant such as aluminium hydroxide. The mordant forms an insoluble compound (called a "lake") with the dye molecules right inside the fibre, locking the colour in place so it does not wash or fade out easily. Aluminium hydroxide and aluminium salts like alum have long been used this way, especially with natural dyes.

9. 9.

   What are the possible oxidation numbers of an element if its atomic number is 17?

   1. A
      +1, +3, +5 and +7
   2. B
      -1, +1, +3, +5 and +7
      Correct
   3. C
      -1, +1, +5 and +7
   4. D
      -2, +2, +4 and +6
   Show answerHide answer

   Answer: B

   An element with atomic number 17 is chlorine, electronic configuration 2,8,7 — it belongs to Group 17 (VII) with 7 electrons in its outer shell.

   Chlorine can gain one electron to complete its octet, giving an oxidation number of -1 (as in NaCl). It can also form compounds where it shows positive oxidation states, using its outer 3s and 3p electrons: +1 (as in HOCl), +3 (as in HClO2), +5 (as in HClO3), and +7 (as in HClO4, the highest, using all 7 valence electrons).

   So the full set of possible oxidation numbers is -1, +1, +3, +5 and +7 — this is correct.

10. 10.

    When cathode rays are deflected onto the electrode of an electrometer, the instrument becomes?

    1. A
       Positively charged
    2. B
       Negatively charged
       Correct
    3. C
       Neutral
    4. D
       Magnetized
    Show answerHide answer

    Answer: B

    Cathode rays are streams of electrons, which carry a negative charge. This was shown by J.J. Thomson's experiments, where cathode rays were deflected towards the positive plate in an electric field, confirming they are negatively charged particles.

    When these electrons strike the electrode of an electrometer, they deposit their negative charge onto it. The electrometer therefore becomes negatively charged, and this charge can be measured, which is how early scientists confirmed the nature of cathode rays.

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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