WAEC Chemistry 2025
Questions & Answers

1) Distractor check: A student might select gas pairs like CO and N₂ or SO₂ and N₂, ignoring density comparisons with air. 2) Reasoning to the answer: Downward delivery relies on collecting gases that possess a higher density than surrounding air. Ammonia gas has a density of 0.6 g/dm³, making it too light to be captured this way, while sulphur (IV) oxide has a density of 2.9 g/dm³; therefore, the pairing containing NH₃ cannot use this method together. 3) Common mistake: Neglecting to check the vapor density of every gas in a given option against air.

1) Distractor check: Candidates could assume catalysts alter chemical equilibrium yields or make reactions more exothermic. 2) Reasoning to the answer: Nickel functions as a chemical catalyst, meaning it furnishes an alternative reaction route requiring lower activation energy. This accelerates the hydrogenation rate without being permanently consumed in the process. 3) Common mistake: Believing that catalysts increase product yields or change the enthalpy of a reaction.

1) Distractor check: Students might confuse aluminium extraction with iron extraction, selecting coke reduction in a blast furnace. 2) Reasoning to the answer: Aluminium is produced commercially through the Hall-Héroult technique, which involves the electrolysis of purified bauxite or alumina dissolved in molten cryolite to reduce Al³⁺ ions into pure aluminium metal. 3) Common mistake: Assuming all metal oxides can be reduced using carbon or coke in a furnace.

1) Distractor check: Test-takers may consider carbon (II) oxide due to its association with combustion processes, or nitrogen gas. 2) Reasoning to the answer: Carbon (IV) oxide is heavier than air, cannot sustain burning, and blankets flames to cut off oxygen supply, rendering it ideal for portable fire extinguishers. 3) Common mistake: Confusing toxic carbon (II) oxide with fire-suppressing carbon (IV) oxide.

1) Distractor check: Students might pick the tendency to give electrons, confusing the behavior of metal atoms with their respective positively charged cations. 2) Reasoning to the answer: Since sodium and potassium ions already carry positive charges as cations, they possess a tendency to accept electrons to attain a stable noble gas configuration, which aligns with their positive reduction potentials in electrochemical sequences. 3) Common mistake: Attributing the electron-loss properties of neutral metal atoms to their corresponding ions.

1) Distractor check: One might select options that alter pressure incorrectly, assuming volume changes do not affect temperature proportionally. 2) Reasoning to the answer: Applying Charles's law shows that volume is directly proportional to absolute temperature under constant pressure. To cut the volume in half from 400 cm³ to 200 cm³, the temperature must also be halved from 400 K down to 200 K while keeping the pressure constant at 1 atm. 3) Common mistake: Failing to scale absolute temperature linearly with volume changes.

1) Distractor check: Students might mistakenly assume all atoms possess neutrons or select a combination containing one neutron. 2) Reasoning to the answer: The most abundant isotope of hydrogen, protium, features a nucleus consisting of a single proton without any neutrons, while its sole electron orbits outside the nuclear center. 3) Common mistake: Assuming every atomic nucleus must contain at least one neutron.

1) Distractor check: Examinees might select yellow to green, confusing dichromate color changes with other transition metal ions. 2) Reasoning to the answer: Acidified potassium dichromate contains the orange dichromate ion, Cr₂O₇²⁻, which shifts to a green color as it is reduced to chromium(III) ions during a redox reaction. 3) Common mistake: Mixing up the initial and final oxidation state colors of chromium compounds.

1) Distractor check: Students may choose aluminium, confusing its borderline appearance with true metalloid status. 2) Reasoning to the answer: Selenium displays intermediate characteristics bridging metals and non-metals, including semiconducting behavior that categorizes it as a metalloid. 3) Common mistake: Classifying typical post-transition elements like aluminium as metalloids.

1) Distractor check: A student might opt for evaporation or fractional crystallization, missing the unique phase behavior of iodine. 2) Reasoning to the answer: Iodine readily undergoes sublimation, turning straight from a solid into a vapor upon gentle heating. This physical property allows it to separate cleanly from non-volatile calcium chloride, which stays behind as residue. 3) Common mistake: Overlooking sublimation as a direct separation technique for volatile solids.

1) Distractor check: Students could select water treatment or food preservation, which are standard, legitimate uses of sodium chloride. 2) Reasoning to the answer: During soap production, sodium chloride is added to precipitate the soap out of solution through a process known as salting out, rather than separating soap from glycerol. 3) Common mistake: Believing that salt is used specifically to isolate glycerol from the reaction mixture.

1) Distractor check: Test-takers might confuse hydrogen bonds or dipole-dipole forces with intramolecular chemical bonds. 2) Reasoning to the answer: Metallic bonding represents an intramolecular force that binds metal atoms together through a sea of delocalized electrons, whereas hydrogen bonds, van der Waals forces, and dipole-dipole attractions operate strictly as intermolecular forces between separate molecules. 3) Common mistake: Grouping metallic bonds with intermolecular forces because of electron mobility.

1) Distractor check: Students might suspect complex salts like ammonium carbonate are covalent due to polyatomic groups. 2) Reasoning to the answer: Nitrogen dioxide is a covalent molecular substance composed entirely of non-metals sharing electron pairs, whereas the other options feature metal-nonmetal combinations or ionic ammonium lattices. 3) Common mistake: Assuming compounds containing nitrogen and oxygen are automatically ionic.

1) Distractor check: Candidates could guess calcium or magnesium carbonate, forgetting their thermal lability. 2) Reasoning to the answer: Group I metal carbonates, such as sodium carbonate, exhibit high thermal stability and resist decomposition when subjected to strong heating, unlike Group II carbonates which break down into metal oxides and carbon dioxide. 3) Common mistake: Assuming all metal carbonates decompose equally upon heating.

1) Distractor check: Students might mistake the red color for a dilute acid or a strong alkali due to pH scale confusion. 2) Reasoning to the answer: When a universal indicator turns red, it signifies a low pH range between 1 and 3, which is characteristic of strong, concentrated acids with high hydrogen ion concentrations. 3) Common mistake: Confusing the color changes of universal indicators with litmus paper tests.

1) Distractor check: Students might pick sulfur dioxide or carbon dioxide, overlooking their single-acid formation pathways. 2) Reasoning to the answer: Nitrogen dioxide reacts with water to produce a mixture of two distinct acids, namely nitrous acid and nitric acid, in a single disproportionation-type hydration reaction. 3) Common mistake: Assuming all acidic oxides yield only one type of acid upon reacting with water.

1) Distractor check: Candidates might guess hydration of alkenes or oxidation of alkanols, overlooking direct synthetic pathways. 2) Reasoning to the answer: Ethanoic acid can be directly converted into ethanol in a laboratory setting through chemical reduction using a strong reducing agent like lithium aluminium hydride. 3) Common mistake: Confusing laboratory reduction routes of carboxylic acids with industrial fermentation or hydration methods.

1) Distractor check: Students might attribute electrical conductivity to vibrating ions or general metallic classification alone. 2) Reasoning to the answer: Alkali metals possess a metallic lattice containing delocalized electrons that are free to roam throughout the structure, enabling them to efficiently transfer thermal energy and electrical charge. 3) Common mistake: Assuming fixed ions rather than mobile electrons are responsible for metal conductivity.

Students might mistakenly choose option B because they confuse an endothermic process with a drop in enthalpy. The change in enthalpy is calculated by subtracting reactant energy from product energy, meaning a positive outcome requires the final energy state to exceed the initial one. Consequently, higher energy levels in the products signify an endothermic reaction. Common mistake: Confusing the direction of enthalpy subtraction or associating a positive delta H with reactant-favored stability.

A student might lean toward option A or C by miscounting every bond in the molecule or confusing sigma bonds with pi bonds. Propene features a carbon-carbon double bond, which is composed of one single sigma bond and one pi bond. Therefore, exactly one pi bond is present in its molecular structure. Common mistake: Forgetting that a double bond contains both a sigma and a pi component, or counting single carbon-hydrogen bonds as pi bonds.