Question 1 · MCQ · Analyse · 1 mark
Comparing soil samples
pH in everyday life
Soil extracts P, Q, R and S have pH 4, 6, 7 and 9 respectively. Which is most acidic?
(A) P
(B) Q
(C) R
(D) S
Show answer and solution
Answer: (A) P
Worked solution: A smaller pH indicates a more acidic aqueous sample. Among these values, pH 4 is the lowest.
Question 2 · MCQ · Analyse · 1 mark
A gas from a kitchen ingredient
Acid and base properties
Dilute ethanoic acid is added to sodium hydrogen carbonate. Which gas makes limewater milky?
(A) Hydrogen
(B) Carbon dioxide
(C) Oxygen
(D) Nitrogen
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Answer: (B) Carbon dioxide
Worked solution: An acid reacts with a hydrogen carbonate to release CO₂. Carbon dioxide forms insoluble calcium carbonate when passed through limewater.
Question 3 · MCQ · Analyse · 1 mark
Dry gas and wet litmus
Indicators and ions
Dry hydrogen chloride gas does not change dry blue litmus, but turns moist blue litmus red. Why?
(A) Water neutralises the gas completely.
(B) Dry litmus is always basic.
(C) Water allows HCl to furnish hydrogen ions.
(D) Moisture produces sodium hydroxide.
Show answer and solution
Answer: (C) Water allows HCl to furnish hydrogen ions.
Worked solution: HCl exhibits acidic behaviour in water because it ionises to produce hydrated hydrogen ions. Without water, dry litmus does not show the usual acidic colour change.
Question 4 · MCQ · Analyse · 1 mark
Choosing a casting material
Salts and their uses
A material used for making casts hardens after water is added. Which substance is initially used?
(A) Sodium chloride
(B) Sodium hydroxide
(C) Washing soda
(D) Plaster of Paris
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Answer: (D) Plaster of Paris
Worked solution: Plaster of Paris, CaSO₄·½H₂O, combines with water to form gypsum, CaSO₄·2H₂O, and sets into a hard mass.
Question 5 · A-R · Analyse · 1 mark
Acidic soil treatment
pH in everyday life
Assertion (A): Adding an appropriate amount of slaked lime can reduce the acidity of acidic soil.
Reason (R): Slaked lime is a base that can neutralise acid.
(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: Calcium hydroxide neutralises acidity, so the reason explains the treatment. The amount should be based on the soil requirement; excess base is not implied.
Question 6 · A-R · Analyse · 1 mark
Universal indicator interpretation
Indicators and ions
Assertion (A): A solution of pH 10 is acidic.
Reason (R): Aqueous solutions with pH greater than 7 are basic at ordinary room temperature.
(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: (D) A is false, but R is true.
Worked solution: pH 10 is basic, so the assertion is false. The reason states the correct interpretation of the pH scale.
Question 7 · VSA · Apply · 2 marks
Choosing a metal container
Acid and base properties
Why is a copper container unsuitable for storing acidic lemon pickle for a long time? Give a chemistry-based explanation.
Show answer and solution
Answer: Acidic contents can react with the metal surface or its compounds and contaminate the pickle with metal salts.
Worked solution: Acids in the food can corrode the metal or dissolve surface compounds over time. A suitable non-reactive storage material avoids this contamination; not every metal reacts with every acid at the same rate.
Question 8 · VSA · Apply · 2 marks
Why soda crystals change mass
Salts and their uses
Washing soda crystals are left in dry air and lose some water of crystallisation. Has the sodium carbonate necessarily evaporated? Explain.
Show answer and solution
Answer: No. Water of crystallisation can be lost while sodium carbonate remains.
Worked solution: Washing soda is Na₂CO₃·10H₂O. The fall in mass can be due to loss of bound water rather than evaporation of the salt itself.
Question 9 · SA · Analyse · 3 marks
Dilution and pH
Neutralisation and dilution
An acidic solution of pH 2 is diluted with pure water at room temperature. Predict the direction of pH change, explain it, and state whether dilution alone makes it strongly basic.
Show answer and solution
Answer: The pH rises towards 7; dilution lowers hydrogen-ion concentration. It does not become strongly basic by dilution alone.
Worked solution: Adding water decreases the concentration of hydrogen ions in the acid. Its pH therefore increases towards the neutral value. Dilution does not add a base, so it cannot turn this acid into a strongly basic solution.
Question 10 · SA · Analyse · 3 marks
Products of the chlor-alkali process
Chemical reactions of salts
Brine is electrolysed. Identify the gas at the anode, the gas at the cathode, and the important substance remaining in solution. Give the overall equation.
Show answer and solution
Answer: Chlorine at the anode, hydrogen at the cathode and sodium hydroxide in solution.
Worked solution: The chlor-alkali process produces chlorine and an alkali: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Chloride ions supply chlorine at the anode; hydrogen forms at the cathode.
Question 11 · LA · Analyse · 5 marks
Comparing three useful salts
Salts and their uses
A school labels three samples: baking soda, washing soda and Plaster of Paris. For each, give its formula and one practical use. Explain why baking soda can help a cake rise and why Plaster of Paris hardens with water.
Show answer and solution
Answer: NaHCO₃: baking; Na₂CO₃·10H₂O: cleaning or water softening; CaSO₄·½H₂O: casts. CO₂ expands the cake; hydration sets the plaster.
Worked solution: Baking soda releases CO₂ on heating or reaction with an acid; gas bubbles help the mixture rise. Washing soda is hydrated sodium carbonate used for cleaning and removing permanent hardness. Plaster of Paris absorbs water to form hard gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
Question 12 · Case Study · Analyse · 4 marks
Interpreting four pH readings
Indicators and ions
A lab records pH values at room temperature: lemon juice 2, pure water 7, soap solution 10 and dilute sodium hydroxide 12.
Answer the following:
(i) Which sample is neutral? [1 mark]
(ii) Which listed sample is most basic? [1 mark]
(iii) Predict the direction of pH change when a little lemon juice is added to the soap solution, and explain. [2 marks]
Show answer and solution
Answer: (i) Pure water. (ii) Dilute sodium hydroxide. (iii) The pH decreases as the acid neutralises some of the base.
Worked solution: Neutral water has pH 7. The largest pH, 12, indicates the most basic listed sample. Adding acid to a basic solution consumes hydroxide ions, lowering the pH; the final pH depends on the quantities and concentrations.
