The 2025 Ordinary Level national exam in Chemistry I was sat on 14 July 2025, and it tested almost everything a Senior 3 student is expected to know: bonding, oxides, the periodic table, mole calculations, titration, organic chemistry, water pollution, and industrial processes. If you sat this paper, or you are using it to prepare for your own exams, this guide walks through every question with a full explanation, not just the final answer.
For related preparation material, you can also check our S3 Mathematics I 2025 worked solutions and our S3 Physics I 2025 worked solutions, since these three subjects are usually revised together for the O Level national exams.
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Section A: Attempt ALL Questions (55 marks)
Question 1: Chemistry Careers and Applications
i) Answer: B) Pharmacist.
A pharmacist works directly with drug composition, dosage, and chemical interactions, which makes chemistry the core subject behind the profession. Economics, financial analysis, and market research rely far more on statistics and business studies than chemistry.
ii) Answer: B) Making school desks.
Making desks is primarily woodworking and carpentry. Water treatment, paint manufacturing, and making soaps and detergents all depend on chemical processes such as purification, polymer formulation, and saponification.
iii) Answer: D) Accidental chemical reactions causing harm to individuals or communities.
Misuse of chemistry knowledge creates risk, not benefit. Options A, B, and C describe positive outcomes of chemistry, which is the opposite of what the question is asking for.
Question 2: Bonding
i) Answer: C) They involve the sharing of electrons between two non-metal atoms.
Covalent bonds form when non-metal atoms share electron pairs to complete their outer shells. Ionic bonds, not covalent bonds, involve transferring electrons and forming charged ions.
ii) Answer: D) The ability of metals to be hammered into thin sheets.
Malleability specifically refers to shaping metal into sheets under force. The ability to be drawn into wires is a separate property called ductility.
iii) Answer: B) Fe (Iron).
Iron is a pure metal, and metallic bonding exists between metal atoms sharing a "sea" of delocalized electrons. Graphite has covalent bonding, NaCl is ionic, and CO₂ is covalent.
Question 3: Waste Terminology (Fill in the Blanks)
i) waste materials: substances discarded after use, from manufacturing, consumption, or daily life.
ii) non-biodegradable wastes : plastics and metals that persist in the environment for a long time.
iii) biodegradable wastes: food scraps and paper that decompose naturally.
Question 4: True or False on Oxygen
i) False. Oxygen does not turn moist red litmus paper blue; it is a neutral gas and has no effect on litmus paper. Turning red litmus blue is a test for a basic/alkaline gas like ammonia.
ii) True. Oxygen gas is colorless, odorless, and tasteless.
iii) False. The equation as written is not correctly balanced; the coefficients and formula given do not represent a valid balanced decomposition of hydrogen peroxide.
Question 5: Matching Solution Types
i) Unsaturated solution → B (contains less solute than it can dissolve at that temperature)
ii) Supersaturated solution → A (contains the maximum solute at an elevated temperature, more than a normal saturated solution can hold once cooled)
iii) Saturated solution → C (contains as much solute as can dissolve at that temperature)
Question 6: Oxides
i) Answer: D) Ca₂O.
Calcium has a valency of 2 and oxygen has a valency of 2, so the correct formula is CaO, not Ca₂O. K₂O, SO₂, and P₂O₃ are all correctly balanced.
ii) Answer: A) Al₂O₃.
Aluminium oxide is amphoteric, meaning it reacts with both acids and bases. Na₂O is basic, MgO is basic, and SO₂ is acidic.
iii) Answer: C) acidic.
Non-metals react with oxygen to form acidic oxides (like SO₂ and CO₂), while metals form basic oxides.
Question 7: Matching Boiling Points
i) Pentane → D (309 K)
ii) Ethane → B (90 K)
iii) Propane → C (184 K)
iv) Methane → B...
Note: methane and ethane cannot both map to option B (90 K). Based on known boiling points, methane boils at roughly 109 K and ethane at roughly 184 K, while propane boils at roughly 231 K and pentane at roughly 309 K. Using this real ordering: Pentane → D (309 K), Propane → A (231 K), Ethane → C (184 K), Methane → B (90 K, the lowest value given, since methane has the smallest molecule and weakest intermolecular forces among the four). Always rank boiling points using chain length: longer carbon chains have stronger van der Waals forces and higher boiling points.
Question 8: Laboratory Apparatus and Safety
i) Answer: C) To allow easy observation of reactions and contents.
Transparent apparatus lets you watch color changes, gas evolution, and reaction progress directly.
ii) Answer: B) flammable, easily catches fire and burns.
The flame symbol is the standard hazard symbol for flammable substances.
iii) Answer: D) To avoid cross-contamination and maintain the purity of each chemical.
Reusing a spatula between different chemical containers risks mixing residues and contaminating stock chemicals.
iv) Answer: B) V has a necked line that gives more precise volumes than B.
Apparatus V is a volumetric flask, which has a single calibration mark for precise volume measurement. B, a beaker, only gives approximate volume readings.
Question 9: Periodic Table Properties (True or False)
i) True. Sodium is a metal and a good conductor; sulphur is a non-metal and a poor conductor, even though both sit in period 3.
ii) False. Graphite is actually a good conductor of electricity because of its layered structure with delocalized electrons, unlike diamond.
iii) False. Magnesium, a metal with strong metallic bonding, has a higher melting point than chlorine, which exists as simple diatomic molecules with weak intermolecular forces.
iv) True. Zinc reacts with dilute sulphuric acid to produce zinc sulphate and hydrogen gas:
Question 10: Nitrogen and Chemical Equations
i) Answer: C) Nitrogen molecules are held together by a strong triple bond.
The N≡N triple bond requires a very high amount of energy to break, making nitrogen gas largely unreactive at normal conditions.
ii) Answer: D) (NH₄)₂SO₄.
Ammonia reacts with sulphuric acid:
iii) Answer: A) 3Mg(s) + N₂(g) → Mg₃N₂(s).
This is the only option that is correctly balanced: 3 magnesium atoms combine with 1 nitrogen molecule to form magnesium nitride.
iv) Answer: B) Advise farmers to avoid cultivating near river banks.
Eutrophication is caused by excess nutrients (like fertilizers) entering water bodies. Keeping farming away from riverbanks reduces fertilizer runoff into water, unlike the other options which would make the problem worse.
Question 11: Balancing Equations (Fill in the Blanks)
i)
$$\text{Na}_2\text{O(s)} + \text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)}$$ii) $$\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)}$$
iii) $$\text{NaCl(s)} + \text{H}_2\text{SO}_4\text{(l)} \rightarrow \text{NaHSO}_4\text{(aq)} + \text{HCl(g)}$$
iv) $$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$$
Question 12: Formulas and Balanced Equations
i) Answer: A) K₃PO₄.
K⁺ has charge +1 and PO₄³⁻ has charge −3, so three potassium ions are needed to balance one phosphate ion.
ii) Answer: C) 2Na(s) + Cl₂(g) → 2NaCl(s).
This is the only version with whole-number coefficients that is fully balanced on both sides for sodium and chlorine atoms.
iii) Answer: C) 2Li(s) + O₂(g) → Li₂O(s)...
Checking the balance carefully:
$$4\text{Li(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Li}_2\text{O(s)}$$ is the fully balanced version, since 4 lithium atoms and 2 oxygen atoms on the left match 4 lithium atoms and 2 oxygen atoms on the right (2 × Li₂O = 4 Li + 2 O). Based on the answer options provided, this corresponds to D) 4Li(s) + O₂(g) → 2Li₂O(s).iv) Answer: D) x = 2, y = 2, z = 3.
The decomposition of potassium chlorate is:
Question 13: Percentage Composition and Moles in Glucose
Glucose has the molecular formula C₆H₁₂O₆ and a molar mass of 180 g/mol.
i) Percentage of carbon: Answer: B) 40%
ii) Percentage of oxygen: Answer: D) 53.33%
iii) Percentage of hydrogen: Answer: C) 6.66%
iv) Moles in 9.0 g of glucose: Answer: A) 0.05 moles
$$n = \frac{\text{mass}}{\text{molar mass}} = \frac{9.0}{180} = 0.05 \text{ mol}$$Question 14: Titration and Concentration
i) Answer: B) Test tube.
A test tube is not part of the standard titration setup. Titration uses a burette, pipette, and conical flask.
ii) Answer: C) Eye B.
The correct way to read a burette is at eye level with the bottom of the meniscus. Reading from above or below (Eye A or Eye C) causes parallax error.
iii) Molarity of NaOH: Answer: D) 0.05 mol/l
Using the neutralization relationship for a 1:1 reaction between NaOH and HCl:
iv) Concentration in g/l : Answer: A) 8.76 g/l
Question 15: Carboxylic Acids
The general formula for carboxylic acids is CₙH₂ₙ₊₁COOH.
i) a) Two-carbon carboxylic acid (n = 1): CH₃COOH (ethanoic acid / acetic acid)
b) Three-carbon carboxylic acid (n = 2): CH₃CH₂COOH (propanoic acid)
ii) Reaction of ethanoic acid with sodium metal:
iii) Molecular mass of propanoic acid (CH₃CH₂COOH, or C₃H₆O₂):
Section B: Attempt Any THREE Questions (30 marks)
Question 16: Laboratory Preparation of Hydrogen Chloride
i) Answer: A) Dropping funnel.
Apparatus X drips concentrated sulphuric acid onto the solid sodium or potassium chloride to generate HCl gas.
ii) Answer: D) Delivery tubes.
The Y apparatuses carry the gas from the flask through the washing bottle to the collection jar.
iii) Answer: C) to dry the gas being prepared.
Concentrated sulphuric acid is a strong drying agent that removes moisture from HCl gas as it passes through.
iv) Answer: b, d, e are correct physical properties of hydrogen chloride:
b) HCl is highly polar and very soluble in water.
d) HCl melts at −114°C, lower than water's melting point.
e) HCl has a density of 1.19 g/cm³, denser than air.
(Option a is incorrect because HCl actually has a lower boiling point than water, and option c is incorrect because HCl is a gas at room temperature.)
v) Table completion:
| HCl | Added reagent | Main product | Observable change |
|---|---|---|---|
| HCl(aq) | AgNO₃(aq) | AgCl (silver chloride) | A white precipitate forms |
| HCl(g) | NH₃(g) | NH₄Cl (ammonium chloride) | Dense white smoke/fumes form |
Question 17: Oxygen Properties and Gas Collection
i) Answer: B and C
B) Oxygen has no effect on litmus paper; it is a neutral gas.
C) The density of oxygen (1.43 g/cm³) is greater than the density of air (1.225 g/cm³), so oxygen is denser than air.
ii) Gas collection methods:
a) Suitable method for collecting oxygen: P (downward displacement of water, or upward delivery, since oxygen is only slightly soluble in water)
b) Upward delivery (downward displacement of air): M
c) Downward delivery (upward displacement of air): N
d) Method suitable for gases insoluble or slightly soluble in water that do not react with water: P (collection over water)
Question 18: Alkenes and Alcohols
i) Answer: C) CₙH₂ₙ₊₁OH.
This is the general formula for the alcohol homologous series.
ii) True or False:
a) True. Ethanol has hydrogen bonding between molecules, giving it a higher boiling point than ethene, which only has weak van der Waals forces.
b) False. Pent-1-ene and pent-2-ene are positional isomers, not chain isomers, since the carbon chain length is the same but the double bond position differs.
c) True. Alcohols are covalent molecular compounds and do not conduct electricity.
d) True. Alkenes such as ethene are used to manufacture plastics through polymerization.
iii) Propanol reaction chart:
a) IUPAC name for CH₃-CH₂-CH₂OH: propan-1-ol
b) Main products H and I: H is propene (CH₃CH=CH₂), formed by dehydration of propan-1-ol at 170°C with concentrated sulphuric acid. I is propanoic acid (CH₃CH₂COOH), formed by oxidation.
c) Reagents/conditions J and K: J is concentrated sulphuric acid catalyst, esterification with propanoic acid to form the ester shown. K is sodium metal, which reacts with the alcohol to release hydrogen gas.
Question 19: Water Pollution
i) Fill in the blanks:
a) point source
b) sewage
c) non-point source
ii) Matching dangers to causal agents:
A) Eutrophication → 2 (Phosphates)
B) Health Hazards → 3 (Bacteria)
C) Acidification of water bodies → 1 (Carbon dioxide)
iii) Matching pollutants to examples:
A) Pathogens → 2 (Escherichia coli)
B) Plastic → 4 (Polythene)
C) Chemical waste (Industrial waste) → 1 (Mercury)
D) Alien species → 3 (Water hyacinth)
Question 20: Rate of Reaction, HCl and Limestone
i) When did the reaction stop?
The volume of gas stays constant at 40 cm³ from 5.0 minutes onward, so the reaction stopped at 5.0 minutes.
ii) Gas released: Carbon dioxide (CO₂)
iii) Balanced equation:
iv) Effect of factors on rate of reaction:
a) Adding a catalyst speeds up the reaction by providing an alternative pathway with lower activation energy, so more particles can react successfully without needing extra heat.
b) Increasing the concentration of HCl increases the number of acid particles per unit volume, so collisions between HCl and CaCO₃ particles happen more frequently, increasing the reaction rate.
v) Answer: A) Powdered limestone at 40°C.
Powdered limestone has a much larger surface area than chips, allowing more collisions per second, and the higher temperature (40°C) gives particles more kinetic energy, so both factors combine to make this the fastest-reacting combination.
Section C: Attempt Any ONE Question (15 marks)
Most students attempt Question 21 (solubility curves) since it requires reading a graph rather than recalling an industrial process, but we've worked through both questions below for full preparation.
Question 21: Solubility Curves
i) Answer: C) increases with increase of temperature.
Most solid salts become more soluble as temperature rises, which is why solubility curves generally slope upward.
ii) Answer: A) KI.
Reading the graph at 20°C, potassium iodide (KI) has the highest solubility value among the options listed.
iii) Answer: B) 30g/100g of H₂O.
At 10°C, the NaNO₃ curve reads approximately 30 g per 100 g of water.
iv) Answer: A) 45g/100g of H₂O.
At 30°C, the KClO₃ curve reads approximately 45 g per 100 g of water...
Reading solubility curve questions accurately depends entirely on tracing the correct vertical and horizontal gridlines, and small misreads are one of the most common ways students lose marks in this section. This is exactly the kind of graph-reading skill our tutors drill during one-on-one Chemistry sessions.
v) True or False:
a) True. At 0°C, KNO₃ has lower solubility than NaCl on the graph.
b) True. Ammonia gas (NH₃) becomes less soluble as temperature increases, which is typical for dissolved gases.
c) True. At 70°C, the NH₄Cl curve reads approximately 60 g/100g of H₂O.
d) False. At 90°C, the KClO₃ curve reads higher than 20 g/100g of H₂O based on its steep upward trend.
e) True. At 0°C, KNO₃ is lower than NH₄Cl, and by 60°C the KNO₃ curve has risen above the NH₄Cl curve, consistent with KNO₃'s steeper solubility increase.
vi) Chemical tests to distinguish ions:
a) Fe²⁺(aq) and Fe³⁺(aq): Add a few drops of sodium hydroxide solution to each. Fe²⁺ forms a dirty green precipitate of iron(II) hydroxide, while Fe³⁺ forms a reddish-brown precipitate of iron(III) hydroxide.
b) SO₄²⁻(aq) and SO₃²⁻(aq): Add dilute hydrochloric acid followed by barium chloride solution. SO₄²⁻ gives a white precipitate that does not dissolve in the added acid, while SO₃²⁻ does not form a precipitate under these acidified conditions (or releases a sulphur dioxide smell when acidified alone).
Question 22: Industrial Manufacture of Sulphuric Acid
i) Process name: The Contact Process
ii) Two starting raw materials: Sulphur (or sulphur ore) and air (for oxygen)
iii) Oxidation of sulphur in air:
iv) a) Gas G: Sulphur trioxide (SO₃)
b) Liquid M: Concentrated sulphuric acid (H₂SO₄)
v) Catalyst and temperature for formation of Gas G:
Vanadium(V) oxide (V₂O₅) catalyst, at a temperature of approximately 450°C.
vi) Reaction in the absorption chamber:
vii) Two uses of sulphuric acid: Manufacture of fertilizers (such as ammonium sulphate) and use in car batteries as the electrolyte.
viii) Environmental problem: Sulphur dioxide released during the process contributes to acid rain, which damages plants, buildings, and water ecosystems.
ix) Measure to minimize the problem: Install scrubbers (using substances like limestone) in factory chimneys to absorb sulphur dioxide before it is released into the atmosphere.
How to Use This Guide Effectively
Reading worked solutions is only half of exam preparation. To actually improve your score, redo each question on paper first, without looking at the answer, then compare your working step by step against what's shown here. Pay close attention to Section A, since with 55 of the 100 marks concentrated there, small recurring mistakes (like sign errors in equations or misreading a graph) cost the most marks overall.
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The solutions on this page are prepared by the Mathrone Academy team for revision and learning purposes only. This is not an official NESA marking scheme or REB-approved answer guide. While every effort has been made to ensure accuracy, answers and explanations may differ from the official examiners' marking guide. Always refer to your school teacher or the official NESA publications for authoritative marking guidance. Mathrone Academy accepts no responsibility for any discrepancies between these solutions and official results.
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