Class 9 Science Chapter 2: Acids, Bases and Salts Previous Year Questions (2020–25)

Passively re-reading your textbook or notes creates an illusion of learning—a phenomenon called fluency bias. Your brain feels familiar with content but hasn't trained the retrieval pathways needed on exam day. Previous year questions force active recall: you must decide which concept applies, recall exact definitions (e.g., 'a substance that donates protons' for an acid in Brønsted–Lowry terms), and construct multi-step explanations under time pressure. Research in cognitive science shows that interleaving past papers with spaced revision increases long-term retention by 60% compared to massed re-reading. For Chapter 2, this means spending 60 minutes solving PYQs teaches you more than 3 hours rereading pages. You'll also notice recurring question structures (e.g., 'identify the acid/base and predict the pH'), so when you sit the real exam, you're not seeing novel problems—you're recognising familiar patterns. Start by attempting questions without notes, then review solutions to identify knowledge gaps. This active learning cycle is non-negotiable for scoring above 85% in Science.

One-mark questions test memory and definition fluency. Here are the five most common formats and full answers: **Q1: What is the pH of a neutral solution at 25°C?** Answer: 7. A neutral solution has equal concentrations of H⁺ and OH⁻ ions, giving a pH of exactly 7. **Q2: Name an acid that does not contain oxygen.** Answer: Hydrochloric acid (HCl) or sulphuric acid is incorrect (contains O); correct examples are HCl, HBr, or H₂S. **Q3: What is the common name for sodium hydrogen carbonate (NaHCO₃)?** Answer: Baking soda (or sodium bicarbonate). It is used in cooking and as an antacid. **Q4: Lime water turns milky when exposed to carbon dioxide. Write the word equation.** Answer: Calcium hydroxide + carbon dioxide → calcium carbonate (precipitate) + water. This is a classic test for CO₂. **Q5: Which indicator shows red colour in acidic solution?** Answer: Methyl orange (or universal indicator paper showing red). Phenolphthalein is colourless in acidic solution, not red. These definitions and colour changes appear in nearly 70% of 1-mark slots. Memorise them as 'anchor facts' alongside your notes.

Three-mark questions demand brief explanations and simple balancing. Expect to cover concept + reason + example or description. **Q1: What are the chemical properties of acids? Give two examples.** Answer (structure): Acids turn blue litmus red; react with metals to produce hydrogen gas; react with bases to form salt and water (neutralisation); react with carbonates to produce CO₂. Example 1: Zinc + HCl → ZnCl₂ + H₂↑. Example 2: HCl + NaOH → NaCl + H₂O. (Key: Include chemical equations for full marks.) **Q2: Explain why a solution of an acid conducts electricity.** Answer: Acids ionise in water to produce H⁺ ions and anions (e.g., HCl → H⁺ + Cl⁻). These free ions carry electric charge through the solution, enabling electrical conductivity. The stronger the acid, the more ions, hence greater conductivity. **Q3: A solution has a pH of 10. Is it acidic or basic? Justify your answer.** Answer: Basic (or alkaline), because pH > 7 indicates a basic solution. At pH 10, [OH⁻] > [H⁺], confirming the presence of excess hydroxide ions. This solution would turn red litmus blue. **Q4: Write the balanced equation for neutralisation of HNO₃ by NaOH. Identify the salt formed.** Answer: HNO₃ + NaOH → NaNO₃ + H₂O. Salt formed: sodium nitrate (NaNO₃). The reaction represents acid-base neutralisation producing a neutral salt. **Q5: Describe how you would prepare a salt sample of copper sulphate (CuSO₄) in the laboratory.** Answer: Add dilute sulphuric acid to copper oxide (CuO) in a beaker: CuO + H₂SO₄ → CuSO₄ + H₂O. Heat gently, filter to remove undissolved oxide, then evaporate the filtrate to obtain copper sulphate crystals. This is a solid-acid reaction (non-metal oxide method). For 3-mark success, always balance equations and state the category of salt (normal, acidic, or basic).

Five-mark questions integrate multiple subtopics and demand extended explanation with calculations or detailed reasoning. **Q1: (a) Define pH. (b) The pH values of three solutions are 2, 7, and 12. Which is most acidic and which is most basic? (c) Write the equation showing the ionisation of acetic acid (CH₃COOH).** Answer (Full Solution): (a) pH is the negative logarithm of hydrogen ion concentration: pH = −log[H⁺]. It is a measure of acidity or basicity, ranging from 0 to 14. At 25°C, pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. (b) Solution with pH 2 is most acidic (highest H⁺ concentration, ≈ 0.01 M). Solution with pH 12 is most basic (lowest H⁺, ≈ 10⁻¹² M, meaning [OH⁻] ≈ 0.01 M). (c) CH₃COOH ⇌ CH₃COO⁻ + H⁺ (weak acid, partial ionisation, shown by reversible arrow). **Q2: (a) Explain the concept of salt. (b) Name three methods to prepare salts and give one example for each. (c) Classify the salt NaCl as acidic, basic, or neutral, with justification.** Answer (Full Solution): (a) A salt is an ionic compound formed when an acid and base react, with H⁺ from the acid replaced by a metal ion (or ammonium ion). Salts are generally crystalline and soluble in water. (b) Three preparation methods: Method 1 (Acid + Base): NaOH + HCl → NaCl + H₂O (example: sodium chloride) Method 2 (Acid + Metal): H₂SO₄ + Zn → ZnSO₄ + H₂ (example: zinc sulphate) Method 3 (Acid + Carbonate): 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂ (example: calcium chloride) (c) NaCl is neutral because it is the salt of a strong acid (HCl) and strong base (NaOH). During hydrolysis, neither Na⁺ nor Cl⁻ ions react significantly with water, so pH remains ≈ 7. **Q3: (a) What happens when dilute sulphuric acid is added to sodium carbonate? Write the balanced equation and name the products. (b) Why is this reaction sometimes vigorous? (c) How would you test for the presence of the gas produced?** Answer (Full Solution): (a) Equation: Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂↑ Products: sodium sulphate (salt), water, and carbon dioxide gas. (b) The reaction is vigorous because it is exothermic (releases heat) and produces a gas that evolves rapidly, especially if the reactants are in concentrated form or heated. The breaking of C=O bonds requires energy, but the formation of CO₂ and H₂O releases more. (c) Test for CO₂: Pass the gas through lime water [Ca(OH)₂]. If present, the lime water turns milky/turbid due to formation of calcium carbonate precipitate: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. If excess CO₂ is passed, the precipitate dissolves, forming soluble calcium bicarbonate: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂. For full 5-mark credit, show all equations balanced, describe mechanisms, and include testing procedures.

The refreshed CBSE Class 9 syllabus (2026–27) emphasises competency-based assessment and practical reasoning over rote memorisation. In Chapter 2, expect these shifts: **Shift 1: More Case-Based Questions (CBQs).** Instead of 'define pH', questions now ask: 'A farmer uses limestone on acidic soil. Explain the chemistry and predict changes in crop yield.' You must link acids–bases to real scenarios—soil, digestion, industrial processes. **Shift 2: Increased Calculations.** Questions now include quantitative pH or ion concentration problems. Example: 'A solution has [H⁺] = 10⁻⁵ M. Calculate its pH and pOH.' The old exam rarely asked this; the new pattern does. **Shift 3: Safety and Sustainability.** Questions now ask about safe handling of strong acids/bases, disposal of chemical waste, and eco-friendly salt preparation. Example: 'Why is it unsafe to add water to concentrated H₂SO₄? Describe the safe procedure.' **Shift 4: Experimental Design.** Rather than 'state the method', you'll be asked: 'Design an experiment to compare the strengths of three acids using pH paper. What variables would you control?' **Shift 5: Conceptual Depth Over Definitions.** The focus is shifting from 'what is a salt?' to 'how does salt formation relate to ionic bonding and electron transfer?' Practise questions that integrate real-world contexts, ask you to design procedures, and demand calculations. This prepares you for the reformed exam style.

Time management is critical in CBSE exams. Here's a battle-tested strategy for Chapter 2 sections in a 3-hour paper: **Before the Exam:** 1. Solve 5–6 full PYQs under timed conditions (1 mark = 1 min, 3 mark = 4 min, 5 mark = 7 min). Build speed without sacrificing accuracy. 2. Create a '1-minute flashcard deck' with 15 key definitions (acid, base, pH, salt, neutral solution, indicator, neutralisation). Revise in the 10 minutes before the exam starts. 3. Pre-memorise balanced equations: HCl + NaOH, CuO + H₂SO₄, Na₂CO₃ + HCl. Write them once during the first 5 minutes (as a mental warm-up). **During the Exam:** 1. **1-mark questions (4–5 questions per paper):** Answer in 30 seconds each. If unsure, skip and return. These are rarely worth the time struggle. 2. **3-mark questions (2–3 per paper):** Spend 4 minutes per question. Allocate: 30 sec reading + 1.5 min writing explanation + 1.5 min writing equation. Always show one equation. 3. **5-mark questions (1–2 per paper):** Spend 6–7 minutes. Write a subheading for each part (a), (b), (c). Use bullet points for secondary details. Always balance equations fully and state conclusions. 4. **Exam hall trick:** If a question asks 'explain', include a one-line reason after every statement. Examiners look for 'why', not just 'what'. **After Writing:** If time remains (10 min), review 1-mark answers only. Avoid re-writing 3 and 5-mark answers unless you made a spelling error or omitted an equation. Start a 3-day free trial at cbsetutor.ai to practise 100+ full-length mock papers with instant feedback and timed simulations.

Chapter 2 (Acids, Bases and Salts) is high-yield: it accounts for 15–20% of marks in term and annual exams, and the concepts appear again in Chapters 5 (Periodic Table, reactivity trends) and 10 (Metals and Non-metals). Mastering it now builds momentum for the full year. Your action plan: 1. **This week:** Solve the 5 one-mark PYQs above daily (2 min) until you answer all five in 70 seconds without errors. 2. **Next week:** Attempt the 5 three-mark questions under 20 minutes. Compare your answers to the model solutions. Identify one weak area (e.g., 'I forget to balance salt preparation equations') and drill it. 3. **Week 3:** Solve one 5-mark question every 2 days. Practise writing full, detailed answers in 6 minutes. Focus on part (a) definitions—examiners award marks for clarity here. 4. **Ongoing:** Before every term test, spend 30 minutes solving mixed PYQs (one of each type). This keeps concepts fresh and builds exam confidence. Remember: previous year questions reveal the examiner's priorities. Use them as a map, not a promise. The patterns you see here will dominate your Class 9 Science exam.