Question 1 · MCQ · Analyse · 1 mark
Why soap forms scum
Soaps and detergents
A student gets little lather and a white scum when soap is shaken with hard water. What is the best explanation?
(A) The soap has become pure water.
(B) Calcium and magnesium ions form insoluble salts with soap.
(C) Hard water contains no dissolved ions.
(D) Soap produces oxygen gas.
Show answer and solution
Answer: (B) Calcium and magnesium ions form insoluble salts with soap.
Worked solution: Calcium and magnesium ions in hard water react with soap to form insoluble salts called scum, leaving less soap available for cleaning.
Question 2 · MCQ · Analyse · 1 mark
Identifying an unsaturated hydrocarbon
Saturated and unsaturated compounds
An unknown hydrocarbon rapidly decolourises bromine water in a standard addition test without sunlight. Which of these could it be?
(A) Methane
(B) Ethane
(C) Ethene
(D) Propane
Show answer and solution
Answer: (C) Ethene
Worked solution: Ethene contains a carbon-carbon double bond and undergoes addition with bromine. The listed alkanes are saturated. The stated conditions distinguish this from light-driven substitution.
Question 3 · MCQ · Analyse · 1 mark
Predicting the next alcohol
Homologous series
Methanol, CH₃OH, and ethanol, C₂H₅OH, are adjacent members of a homologous series. What is the molecular formula of the next member?
(A) C₃H₇OH
(B) C₂H₅COOH
(C) C₃H₆
(D) CH₄
Show answer and solution
Answer: (A) C₃H₇OH
Worked solution: Successive members differ by CH₂ and retain the alcohol functional group. Adding CH₂ to ethanol gives C₃H₇OH.
Question 4 · MCQ · Analyse · 1 mark
Distinguishing two colourless liquids
Ethanol and ethanoic acid
Which observation can distinguish aqueous ethanoic acid from ethanol in a school laboratory?
(A) Both are colourless.
(B) Both can mix with water.
(C) Both contain carbon.
(D) Only the ethanoic acid sample effervesces with sodium hydrogen carbonate.
Show answer and solution
Answer: (D) Only the ethanoic acid sample effervesces with sodium hydrogen carbonate.
Worked solution: Ethanoic acid reacts with sodium hydrogen carbonate to release CO₂; ethanol does not give this acid-carbonate reaction under these conditions.
Question 5 · A-R · Analyse · 1 mark
A weakly conducting molecular compound
Covalent bonding
Assertion (A): Pure methane is a poor conductor of electricity.
Reason (R): Methane contains covalent molecules and does not supply mobile ions under ordinary conditions.
(a) Both A and R are true, and R correctly explains A.
(b) Both A and R are true, but R does not correctly explain A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show answer and solution
Answer: (A) Both A and R are true, and R correctly explains A.
Worked solution: Methane shares electrons in covalent bonds and has no mobile charged particles available for ordinary electrical conduction.
Question 6 · A-R · Analyse · 1 mark
Hydrogenation changes bonding
Saturated and unsaturated compounds
Assertion (A): Adding hydrogen across the double bond of ethene produces ethane.
Reason (R): Ethane contains a carbon-carbon double bond.
(a) Both A and R are true, and R correctly explains A.
(b) Both A and R are true, but R does not correctly explain A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show answer and solution
Answer: (C) A is true, but R is false.
Worked solution: Hydrogenation converts C₂H₄ to C₂H₆ in the presence of a suitable catalyst. Ethane has a carbon-carbon single bond, so the reason is false.
Question 7 · VSA · Apply · 2 marks
Why carbon makes many structures
Versatile nature of carbon
Carbon forms long chains, branched chains and rings. Name the property responsible and explain why carbon can form four bonds.
Show answer and solution
Answer: Catenation; carbon is tetravalent.
Worked solution: Carbon can bond strongly to other carbon atoms, forming chains and rings. Its four valence electrons allow it to complete its outer shell by sharing four electron pairs.
Question 8 · VSA · Apply · 2 marks
An oxidation in the laboratory
Ethanol and ethanoic acid
Warm ethanol is treated with a suitable oxidising agent such as acidified potassium dichromate. Name the carboxylic acid product and give its formula.
Show answer and solution
Answer: Ethanoic acid, CH₃COOH.
Worked solution: Oxidation converts the two-carbon alcohol ethanol into the two-carbon carboxylic acid ethanoic acid under suitable conditions.
Question 9 · SA · Analyse · 3 marks
Explaining an oil-water demonstration
Soaps and detergents
Oil and water do not mix readily. After adding soap and shaking, fine oil droplets remain dispersed. Explain the roles of the two ends of a soap molecule and name the aggregate formed.
Show answer and solution
Answer: Hydrophobic tails associate with oil, hydrophilic heads face water, and micelles surround the oil.
Worked solution: Soap molecules have a long non-polar tail and an ionic head. The tails enter oil or grease, while the heads remain in water. Many molecules enclose a droplet in a micelle, helping disperse it so it can be rinsed away.
Question 10 · SA · Analyse · 3 marks
Comparing two three-carbon compounds
Nomenclature and functional groups
A contains CH₃CH₂CH₂OH and B contains CH₃CH₂CHO. Name each compound, identify its functional group, and explain why they are not members of the same homologous series.
Show answer and solution
Answer: A: propan-1-ol, alcohol; B: propanal, aldehyde. Their functional groups differ.
Worked solution: A has a hydroxyl group attached to a saturated carbon chain. B has a terminal aldehyde group, –CHO. Members of one homologous series have the same functional group and similar chemical properties, so equal carbon count alone is insufficient.
Question 11 · LA · Analyse · 5 marks
Four reactions of carbon compounds
Chemical reactions of carbon compounds
For each change, name the reaction type and give a suitable equation or description: (i) complete combustion of methane; (ii) ethene reacting with hydrogen using nickel; (iii) methane reacting with chlorine in sunlight to form chloromethane; (iv) ethanol oxidised to ethanoic acid. (v) State which change reduces carbon-carbon unsaturation.
Show answer and solution
Answer: (i) Combustion; (ii) addition; (iii) substitution; (iv) oxidation; (v) hydrogenation of ethene.
Worked solution: (i) CH₄ + 2O₂ → CO₂ + 2H₂O. (ii) C₂H₄ + H₂ → C₂H₆ using Ni. (iii) CH₄ + Cl₂ → CH₃Cl + HCl in sunlight. (iv) CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. (v) Hydrogen adds across the double bond of ethene and converts it to a single bond.
Question 12 · Case Study · Analyse · 4 marks
A fair cleaning comparison
Soaps and detergents
Equal volumes of soft water and hard water are placed in identical bottles. A student adds the same soap solution drop by drop and shakes each bottle in the same way. Persistent lather forms after 4 drops in soft water and 15 drops in hard water.
Answer the following:
(i) Which sample uses more soap before persistent lather forms? [1 mark]
(ii) Name the ions mainly responsible for hardness. [1 mark]
(iii) Explain the different readings and state one variable kept constant. [2 marks]
Show answer and solution
Answer: (i) Hard water. (ii) Calcium and magnesium ions. (iii) Some soap forms insoluble scum; water volume or shaking procedure is kept constant.
Worked solution: Soap is consumed in precipitating calcium and magnesium salts before enough remains to make persistent lather. Equal water volumes and equal shaking make the comparison fair. A suitable detergent generally performs better in hard water because it does not form the same insoluble salts.
