All 50 questions from the West African Examinations Council (WAEC) Chemistry 2025 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
Which of the following pairs of gases cannot be collected by downward delivery of air?
A. SO2 and NH3
B. CO and N2
C. Cl2 and N2
D. SO2 and N2Correct
Explanation
Only gases denser than air are collected by downward delivery (upward displacement of air). A gas is considered denser than air if its relative molecular mass is greater than 28.8. SO2=64, NH3=17, CO=28, N2=28, Cl2=71. SO2 and Cl2 can be collected this way, but NH3, CO and N2 cannot (they are less dense than or about the same as air), so SO2 and N2 is the pair that cannot both be collected by downward delivery.
Nickel is used in the hydrogenation of alkenes because it
A. increases the yield of the product
B. lowers the activation energy of the reactionCorrect
C. makes the reaction more exothermic
D. prevents a reverse reaction from occurring
Explanation
In hydrogenation reactions, nickel acts as a catalyst. Catalysts increase the rate of a chemical reaction by lowering the activation energy, providing an alternative pathway for the reaction to occur.
Aluminium is extracted from its ore, bauxite, by electrolysis (after purification by the Bayer process). The purified bauxite is dissolved in molten cryolite and then electrolyzed.
Which of the following gases is used in fire extinguishers?
A. Carbon (IV) oxideCorrect
B. Nitrogen (IV) oxide
C. Carbon (II) oxide
D. Sulphur (IV) oxide
Explanation
Carbon (IV) oxide (CO2) is commonly used in fire extinguishers because it displaces oxygen, thus suffocating the fire. CO2 is non-flammable and is heavier than air, so it effectively smothers flames by preventing oxygen from reaching them.
C. accept electrons to be reduced back to their neutral formsCorrect
D. have a negative oxidation state
Explanation
Both sodium (Na) and potassium (K) are alkali metals. These metals readily lose an electron to form cations (Na+, K+). As positively charged ions, Na+ and K+ have a tendency to accept electrons so they can be reduced back to their neutral forms.
If a gas occupies a volume of 400cm3 at a temperature of 400 K and a pressure of 1 atm, at what temperature and pressure would its volume be 200 cm3?
A. 800 K and 1 atm
B. 800 K and 0.5 atm
C. 200 K and 1 atmCorrect
D. 200 K and 0.5 atm
Explanation
Using the combined gas law, P1V1/T1 = P2V2/T2, with P1=1 atm, V1=400 cm3, T1=400 K, V2=200 cm3. Testing each option, only P2=1 atm and T2=200 K makes both sides equal: (1x400)/400 = (1x200)/200 = 1.
The most common hydrogen isotope, protium, has one proton, one electron and no neutron. Electrons are located outside the nucleus, so the nucleus of the most common hydrogen atom contains only one proton.
If acidified K2Cr2O7 acts as an oxidizing agent, the colour changes from
A. orange to red
B. yellow to green
C. orange to greenCorrect
D. yellow to red
Explanation
The orange colour of K2Cr2O7 is due to the dichromate ion (Cr2O7^2-). During redox, it is reduced to Cr3+, which is green. So the colour changes from orange to green: Cr2O7^2- + 14H+ + 6e- -> 2Cr3+ + 7H2O.
Metalloids show properties of both metals and non-metals. Common metalloids include boron, silicon, arsenic and selenium. Aluminium and sodium are metals while phosphorus is a non-metal.
Iodine could be separated from a mixture of iodine and calcium chloride by
A. evaporation
B. sublimationCorrect
C. fractional distillation
D. fractional crystallization
Explanation
Sublimation is the process where a solid changes directly to a gas without becoming liquid. Iodine can sublime, but calcium chloride cannot. So on heating the mixture, iodine sublimes and can be collected, leaving CaCl2 behind.
Sodium chloride is used in the following processes except
A. treatment of waterCorrect
B. manufacturing of chlorine
C. preservation of food
D. separation of soap from glycerol
Explanation
Sodium chloride (NaCl) is not a key agent used for water treatment. Water treatment typically uses chemicals like chlorine gas and alum for disinfection and coagulation, whereas NaCl is used in manufacturing chlorine gas (via electrolysis), food preservation, and salting-out during soap production.
Which of the following bonds is not an intermolecular force?
A. hydrogen bond
B. van der Waal's force
C. dipole-dipole force
D. metallic bondCorrect
Explanation
Intermolecular forces occur between molecules, e.g. hydrogen bonding, van der Waals forces and dipole-dipole forces. Metallic bonding, on the other hand, involves metal cations and delocalised electrons within a metal, so it is not an intermolecular force.
NO2 is a molecular (covalent) compound, formed by covalent bonds between nitrogen and oxygen sharing electrons; it does not form ions. AgCl, KNO3 and (NH4)2CO3 are all ionic compounds.
In the use of a universal indicator, red colour is an indication that the substance is a
A. concentrated acidCorrect
B. concentrated alkali
C. dilute acid
D. dilute base
Explanation
The universal indicator is a pH indicator composed of a mixture of several different indicators that show different colours at different pH levels. In an acidic solution, the pH is below 7, and the universal indicator will shift toward red or orange depending on the strength of the acid; a strong (concentrated) acid gives red.
Which of the following oxides would form two different types of acids when dissolved in water?
A. CO2
B. NO2Correct
C. SO2
D. SO3
Explanation
NO2 is a mixed anhydride. It dissolves in water to produce two different acids. When it dissolves in water, it undergoes a disproportionation reaction to produce HNO2 and HNO3: 2NO2 + H2O -> HNO2 + HNO3.
Ethanol is typically prepared in the laboratory by hydration of ethene. This reaction involves adding water to ethene (C2H4) in the presence of a catalyst (e.g. phosphoric acid) to produce ethanol (C2H5OH).
Alkali metals are good conductors of heat and electricity because they
A. contain delocalized electronsCorrect
B. are found in the d-block
C. have vibrating ions
D. are all metals
Explanation
Alkali metals (like sodium and potassium) are good conductors of heat and electricity because they have delocalized electrons that are free to move throughout the metallic structure, allowing them to transfer energy efficiently.
The change in enthalpy (delta H) is positive when the products have more enthalpy than the reactants. This indicates that the reaction is endothermic, meaning heat is absorbed during the reaction.
Propene (C3H6) has one pi-bond. In the double bond between two carbon atoms, one bond is a sigma bond (formed by overlap of sp2 hybridized orbitals), and the other is a pi bond (formed by sideways overlap of unhybridized p-orbitals).
Which of the following compounds would readily undergo addition reaction?
A. ethane
B. buteneCorrect
C. ethanol
D. benzene
Explanation
Butene is an alkene, and alkenes readily undergo addition reactions due to the presence of a double bond between carbon atoms. This double bond can be broken to add other atoms or groups, such as hydrogen (hydrogenation), halogens (halogenation), or hydrogen halides (hydrohalogenation).
A covalent hydride which does not exist as a gas at room temperature is
A. H2OCorrect
B. BH3
C. HCl
D. H2S
Explanation
Water is a covalent compound composed of hydrogen and oxygen. Despite being a small molecule, H2O is a liquid at room temperature due to the presence of strong hydrogen bonds between water molecules, which require significant energy to break, unlike BH3, HCl and H2S which are gases at room temperature.
D. hydrogen chloride gas is given off at the anode
Explanation
In the electrolysis of dilute hydrochloric acid, oxygen is produced at the anode while hydrogen is produced at the cathode. At the anode: 4OH- -> 2H2O + O2 + 4e-. At the cathode: 2H+ + 2e- -> H2.
In which of the following arrangements are the ions arranged in decreasing order of ionic size?
A. F-, Na+, Mg2+, Al3+Correct
B. Al3+, Na+, Mg2+, F-
C. Mg2+, F-, Al3+, Na+
D. Na+, F-, Mg2+, Al3+
Explanation
The ionic size decreases as the charge on the ion increases, because higher charges attract electrons more strongly, making the ions smaller. The order from largest to smallest ionic size is: F- > Na+ > Mg2+ > Al3+.
The filter in a cigarette reduces the amount of nicotine inhaled by
A. adsorptionCorrect
B. absorption
C. burning
D. evaporation
Explanation
Cigarette filters reduce the amount of harmful substances, including nicotine, by adsorbing (not absorbing) the particles onto their surface. This helps trap some of the toxic chemicals in smoke before they are inhaled. Adsorption involves the adhesion of molecules to the surface of a solid.
Biotechnology is applied in the following processes except
A. baking of bread
B. production of wine
C. production of antibiotics
D. manufacturing of soapCorrect
Explanation
Biotechnology is used in baking, wine production and the production of antibiotics, all of which involve living organisms/enzymes. However, soap manufacturing is a chemical process (saponification), not a biotechnological one.
Which of the following conditions is necessary for the preparation of alkanoates from alkanols and alkanoic acids?
A. Water and NaOH
B. Conc. H2SO4 and heatCorrect
C. NaOH and heat
D. Water and aqueous HCl
Explanation
Alkanoates (esters) are generally prepared through esterification, which involves the reaction of an alkanol and an alkanoic acid in the presence of concentrated H2SO4 (which serves as a catalyst) and heat.
A salt that could be prepared by double decomposition is
A. sodium trioxocarbonate (IV)
B. barium trioxonitrate (V)
C. lead (II) trioxonitrate (V)
D. silver chlorideCorrect
Explanation
Silver chloride (AgCl) can be prepared by double decomposition between silver nitrate (AgNO3) and sodium chloride (NaCl) solutions: AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq).
What is the IUPAC name of the compound CH3CHClCH2OH?
A. chloropropan-1-ol
B. 1-chloropropan-1-ol
C. 2-chloropropan-1-olCorrect
D. 2-chloropropan-3-ol
Explanation
The compound CH3CHClCH2OH has a three-carbon chain with a hydroxyl group (-OH) and a chlorine atom attached to the second carbon atom. Numbering starts from the end nearer to the hydroxyl group to give it the lowest possible number for the functional group, giving the name 2-chloropropan-1-ol.
A neutral atom of neon with atomic number 10 is isoelectronic with
A. K+
B. Mg2+Correct
C. Ca2+
D. Li+
Explanation
Isoelectronic species have the same number of electrons. Neon has an atomic number of 10, meaning it has 10 electrons. Mg originally has 12 electrons but loses 2 electrons to become Mg2+, so Mg2+ has 10 electrons, making it isoelectronic with neon.
Which of the following species is the most powerful reducing agent?
A. Bromine atom
B. Chlorine atom
C. Fluorine atom
D. Iodine atomCorrect
Explanation
The reducing power of a species is related to its ability to lose electrons and be oxidized. Among halogen atoms, reducing power increases down the group, with iodine being the most powerful because it has the lowest electronegativity, making it easier to lose electrons and be oxidized.
The electron configuration of element Y is 1s2 2s2 2p6 3s2 3p4; therefore Y
A. has a mass number of 16
B. has three unpaired electrons
C. belongs to period 4
D. belongs to group VICorrect
Explanation
The electron configuration 1s2 2s2 2p6 3s2 3p4 corresponds to sulphur (S), which is in group VI of the periodic table. The number of electrons in the orbitals with the highest principal quantum number (3s2 3p4) indicates it is in period 3 and group VI; the mass number is 32 (not 16) and it has two unpaired electrons, not three.
A tertiary alcohol/alkanol has the hydroxyl group (-OH) attached to a carbon atom that is bonded to three other carbon atoms. 2-methylpentan-2-ol is tertiary.
Which of the following elements is commercially obtained by the electrolysis of its fused salt?
A. silver
B. copper
C. helium
D. sodiumCorrect
Explanation
Sodium is obtained by the electrolysis of molten sodium chloride (NaCl) in a process known as the Downs process. Sodium metal is deposited at the cathode and chlorine gas is released at the anode.
The lustre of metals (their shiny appearance) is due to the free electrons in the metallic bond. These electrons can absorb and re-emit light, giving metals their characteristic shine.
Exothermic reactions take place when the energy given out in making bonds is
A. less than the energy taken in for breaking bonds
B. greater than the energy taken in for breaking bondsCorrect
C. equal to the energy taken in for breaking bonds
D. the same as the energy taken in
Explanation
In exothermic reactions, the energy released when new bonds are formed is greater than the energy required to break the initial bonds. The reverse is the case for endothermic reactions.
Which of the following metals would not release hydrogen when reacted with dilute HCl?
A. calcium
B. silverCorrect
C. iron
D. zinc
Explanation
Metals that are above hydrogen in the reactivity series will release hydrogen gas when reacted with dilute HCl. However, silver is below hydrogen in the reactivity series, so it does not react with dilute HCl to release hydrogen.
Five d-orbitals having the same energy level in an isolated transition metal atom are described as
A. degenerateCorrect
B. non-degenerate
C. high-level orbital
D. low-level orbital
Explanation
The five d-orbitals in a transition metal atom are described as degenerate because they all have the same energy level when the metal atom is isolated, meaning they are equal in energy.
The allotrope of carbon used in the decolorization of sugar is
A. charcoalCorrect
B. diamond
C. graphite
D. soot
Explanation
Charcoal is used in the decolorization of sugar, particularly in the refining process, because of its high surface area, which adsorbs impurities, including coloured compounds.
A few drops of sodium hydroxide are added to an aqueous solution, and a green precipitate is formed. The ion that may be present in the solution is
A. Cu2+
B. Fe2+Correct
C. NH4+
D. Fe3+
Explanation
When iron (II) ion (Fe2+) reacts with sodium hydroxide, it forms a green precipitate of iron (II) hydroxide, Fe(OH)2. NB: Cu2+ forms a blue precipitate with sodium hydroxide, NH4+ does not form a precipitate, and Fe3+ forms a brown precipitate of iron (III) hydroxide.
When a hypothesis is proven within the limit of available evidence, it becomes a
A. scope
B. theoryCorrect
C. principle
D. law
Explanation
When a hypothesis is supported by significant evidence and experimental results, it becomes a theory. A theory is a well-substantiated explanation of some aspect of the natural world that is based on a body of evidence and has been repeatedly tested.
The empirical formula is the simplest formula of a compound. It represents the whole number ratio of the atoms of each element present in the compound, without indicating the actual number of atoms present.
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