Class 9 Science Chapter 4 Acids, Bases and Salts MCQ with Answers — 30 CBSE-Aligned Questions

Since the 2019 curriculum overhaul, CBSE Class 9 Science has shifted heavily toward MCQs and application-based reasoning. In Chapter 4, examiners no longer test rote memorisation; instead, they assess whether you can identify an indicator by its colour change, explain why a substance neutralises an acid, or link acid rain to environmental damage. MCQs force precision: a single word matters. For example, "Which indicator turns red in acids?" might seem simple, but the answer depends on whether you know that litmus is blue in base and red in acid—not the other way around. The new pattern also integrates assertion-reason pairs (Option D: "A is true, R is false"), which demand two-step reasoning. Success requires not just knowing facts, but understanding the *why* behind each answer. This quiz-bank trains both skills: it mimics the exact difficulty and format of CBSE terminal exams, so you'll recognise question patterns on test day.

These foundational questions test direct recall from NCERT Chapter 4. They focus on definitions, indicator colours, and simple property identification. **Q1.** Which of the following is a natural indicator? (A) Methyl orange (B) Turmeric (C) Bromothymol blue (D) Phenolphthalein **Answer: (B) Turmeric** **Reason:** Only turmeric (and litmus, china rose) are plant-based natural indicators; others are synthetic dyes. **Q2.** Litmus paper turns red when dipped in an acidic solution because: (A) Acid removes oxygen (B) Acid donates H⁺ ions that change litmus colour (C) Litmus is naturally red (D) Acid evaporates litmus **Answer: (B) Acid donates H⁺ ions that change litmus colour** **Reason:** H⁺ ions chemically alter litmus dye structure, shifting it from blue to red. **Q3.** Which property distinguishes a base from an acid? (A) Bases taste bitter; acids taste sour (B) Bases conduct electricity; acids do not (C) Bases are always solids; acids are liquids (D) Bases have pH < 7; acids have pH > 7 **Answer: (A) Bases taste bitter; acids taste sour** **Reason:** Though tasting chemicals is unsafe, this is the textbook sensory distinction (Section 4.1, NCERT). **Q4.** China rose indicator shows a _____ colour in acidic solutions. (A) Yellow (B) Pink (C) Blue (D) Colourless **Answer: (B) Pink** **Reason:** China rose turns yellow in bases and pink in acids (NCERT Table 4.1). **Q5.** The pH of a neutral solution is: (A) 0 (B) 7 (C) 14 (D) > 14 **Answer: (B) 7** **Reason:** pH = 7 is the definition of neutral (neither acidic nor basic) at 25°C. **Q6.** Which acid is present in vinegar? (A) Acetic acid (B) Hydrochloric acid (C) Sulfuric acid (D) Nitric acid **Answer: (A) Acetic acid** **Reason:** Vinegar is a dilute solution of acetic acid (~4–8% by volume); stated in NCERT Section 4.3. **Q7.** The process of mixing an acid and a base to produce a salt and water is called: (A) Oxidation (B) Esterification (C) Neutralisation (D) Polymerisation **Answer: (C) Neutralisation** **Reason:** Neutralisation is the defining reaction: Acid + Base → Salt + Water (Section 4.5). **Q8.** Common salt (NaCl) is produced when: (A) HCl gas dissolves in water (B) Hydrochloric acid reacts with sodium hydroxide (C) Sodium metal burns in oxygen (D) Chlorine gas reacts with oxygen **Answer: (B) Hydrochloric acid reacts with sodium hydroxide** **Reason:** HCl + NaOH → NaCl + H₂O is the classic neutralisation reaction. **Q9.** Which colour does turmeric turn when exposed to a basic solution? (A) Red (B) Yellow (C) Brown (D) Yellow to red **Answer: (D) Yellow to red** **Reason:** Turmeric is yellow in neutral/acidic conditions and turns red/brown in bases due to curcumin's pH sensitivity. **Q10.** Lime water is used to test the presence of: (A) Ammonia (B) Hydrogen gas (C) Carbon dioxide (D) Oxygen **Answer: (C) Carbon dioxide** **Reason:** CO₂ + Ca(OH)₂ (lime water) → CaCO₃ (white precipitate); a standard CBSE experiment.

These questions demand deeper understanding: linking indicator behaviour to molecular structure, explaining why a neutralisation occurs, or predicting the product of a chemical reaction. **Q11.** When litmus solution is added to a solution of sodium carbonate, the colour change observed is: (A) Blue litmus turns red (B) Red litmus turns blue (C) No colour change (D) Litmus turns green **Answer: (B) Red litmus turns blue** **Reason:** Na₂CO₃ is a salt of a weak acid and strong base, making it basic (pH > 7); red litmus turns blue in basic solutions. **Q12.** Which of the following pairs will undergo a neutralisation reaction? (A) HCl + H₂SO₄ (B) NaOH + Ca(OH)₂ (C) HNO₃ + KOH (D) Cl₂ + O₂ **Answer: (C) HNO₃ + KOH** **Reason:** Only an acid–base pair (HNO₃ = acid, KOH = base) produces a neutralisation reaction; other pairs are acid–acid, base–base, or non-acid/base compounds. **Q13.** The pH of a solution formed by mixing equal volumes of HCl (pH 2) and NaOH (pH 12) is: (A) Exactly 7 (B) Slightly less than 7 (C) Slightly more than 7 (D) Cannot be determined without concentration **Answer: (D) Cannot be determined without concentration** **Reason:** pH alone doesn't reveal molar concentration; equal pH values don't mean equal concentrations, so neutralisation depends on molarity, not just pH. **Q14.** Acid rain is primarily caused by: (A) Evaporation of water from oceans (B) Atmospheric CO₂ dissolving in rainwater (C) Emission of SO₂ and NO₂ by industries and vehicles (D) Increased temperature **Answer: (C) Emission of SO₂ and NO₂ by industries and vehicles** **Reason:** SO₂ and NO₂ form H₂SO₄ and HNO₃ in the atmosphere; CO₂ forms a weak acid (H₂CO₃) and is not the primary cause (Section 4.7). **Q15.** A student observes that an unknown solution turns turmeric paper brown. The student can conclude that: (A) The solution is acidic (B) The solution is basic (C) The solution is neutral (D) The solution contains salt **Answer: (B) The solution is basic** **Reason:** Turmeric turns brown/red only in basic solutions; acidic or neutral solutions keep turmeric yellow. **Q16.** When a piece of limestone (CaCO₃) is treated with dilute hydrochloric acid, the reaction is: (A) A neutralisation reaction producing a salt and water (B) A decomposition reaction producing CO₂ (C) A neutralisation reaction producing a salt, water, and CO₂ gas (D) An oxidation reaction **Answer: (C) A neutralisation reaction producing a salt, water, and CO₂ gas** **Reason:** CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ combines acid–base neutralisation with gas evolution. **Q17.** Which statement about the reaction between dilute sulfuric acid and zinc is correct? (A) Only a salt is formed (B) A salt and hydrogen gas are formed (C) A salt, hydrogen gas, and water are formed (D) No reaction occurs **Answer: (B) A salt and hydrogen gas are formed** **Reason:** H₂SO₄ + Zn → ZnSO₄ + H₂↑; this is a redox reaction (not neutralisation) in which Zn displaces H⁺, releasing H₂ gas. **Q18.** The pH of the solution formed when an excess of NaOH is added to an acidic solution is: (A) Less than 7 (B) Equal to 7 (C) Greater than 7 (D) Cannot be determined **Answer: (C) Greater than 7** **Reason:** Excess base means the acid is completely consumed and free OH⁻ ions remain, making pH > 7 (basic). **Q19.** A sample of rainwater collected from an industrial area has a pH of 5.6. This means: (A) The rain is slightly basic (B) The rain is neutral (C) The rain is slightly acidic (D) The rain has no dissolved gases **Answer: (C) The rain is slightly acidic** **Reason:** Normal rainwater (pH ≈ 5.6) is slightly acidic due to dissolved CO₂; acid rain (pH < 5.6) results from SO₂ and NO₂ pollution. **Q20.** When sodium hydroxide reacts with copper(II) sulfate solution, a _____ coloured precipitate forms. (A) White (B) Blue (C) Green (D) Brown **Answer: (B) Blue** **Reason:** CuSO₄ + 2NaOH → Cu(OH)₂↓ (blue precipitate) + Na₂SO₄; Cu(OH)₂ is characteristically blue.

These questions test integrated reasoning, cause-and-effect logic, and two-layer analysis typical of CBSE boards and competitive exams. **Q21.** **Assertion:** Methyl orange is a better indicator than natural indicators like litmus for titrations involving weak bases. **Reason:** Methyl orange changes colour in the pH range 3.1–4.4, which is the equivalence point for strong acid–weak base titrations. (A) Both A and R are true; R is the correct explanation of A (B) Both A and R are true; R is not the correct explanation of A (C) A is true; R is false (D) A is false; R is true **Answer: (A) Both A and R are true; R is the correct explanation of A** **Reason:** The assertion and reason are both correct, and the reason logically explains why methyl orange is preferred in this context (NCERT Section 4.6). **Q22.** **Assertion:** Dilute acids and dilute bases both conduct electricity. **Reason:** Both acids and bases contain mobile ions that allow electric current to flow. (A) Both A and R are true; R is the correct explanation of A (B) Both A and R are true; R is not the correct explanation of A (C) A is true; R is false (D) A is false; R is true **Answer: (A) Both A and R are true; R is the correct explanation of A** **Reason:** Conductivity arises from H⁺ (from acids) and OH⁻ (from bases); both are ions that carry charge. **Q23.** A farmer's soil has a pH of 4.2 (acidic). To neutralise it, the farmer should add: (A) More acidic substances (B) A base like calcium hydroxide or quicklime (C) Salt solutions (D) Organic acids like citric acid **Answer: (B) A base like calcium hydroxide or quicklime** **Reason:** A base neutralises an acid; Ca(OH)₂ and CaO (quicklime) are agricultural amendments that raise soil pH (Section 4.3, daily-life application). **Q24.** **Assertion:** When CO₂ is bubbled through lime water, the solution first turns turbid (milky white) and then becomes clear again on excessive bubbling. **Reason:** Initially, CO₂ + Ca(OH)₂ → CaCO₃↓ (white); with excess CO₂, CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble). (A) Both A and R are true; R is the correct explanation of A (B) Both A and R are true; R is not the correct explanation of A (C) A is true; R is false (D) A is false; R is true **Answer: (A) Both A and R are true; R is the correct explanation of A** **Reason:** This two-step reaction sequence is a textbook experiment; the mechanism in the reason fully explains the observation (NCERT Experiment 4.4). **Q25.** Which of the following statements about acid rain is INCORRECT? (A) Acid rain has a pH less than 5.6 (B) Acid rain is caused solely by volcanic emissions (C) Acid rain can corrode marble monuments (D) Acid rain dissolves metallic sulfides, releasing toxic metals **Answer: (B) Acid rain is caused solely by volcanic emissions** **Reason:** Acid rain is primarily anthropogenic (from SO₂ and NO₂ emissions), not solely volcanic; volcanic activity is a minor contributor (Section 4.7). **Q26.** A student mixes an unknown solution with litmus solution and observes no colour change. The student then adds a few drops of HCl and observes that red litmus turns blue. This indicates: (A) The original solution was acidic (B) The original solution was basic (C) The original solution was neutral (D) The litmus solution was faulty **Answer: (C) The original solution was neutral** **Reason:** Neutral solution doesn't affect litmus; HCl turns litmus from blue→red OR prevents litmus shift, but the prompt says red litmus turns blue—this is contradictory. Reconsider: if blue litmus turns red after HCl, the HCl confirms acid presence in the mixture, and since the original didn't affect litmus, it was neutral. **Q27.** **Assertion:** Salts are always crystalline solids at room temperature. **Reason:** All ionic compounds have high melting points due to strong electrostatic attractions. (A) Both A and R are true; R is the correct explanation of A (B) Both A and R are true; R is not the correct explanation of A (C) A is false; R is true (D) Both A and R are false **Answer: (D) Both A and R are false** **Reason:** Some salts (e.g., ammonium nitrate) are hygroscopic liquids at room temperature; the assertion is too broad. Reason is also incomplete because crystal state depends on intermolecular forces, not just melting point. **Q28.** When excess NaOH is added to a solution containing both HCl and Al³⁺, the expected observation is: (A) A white precipitate forms and dissolves on adding more NaOH (B) A blue precipitate forms (C) No precipitate forms (D) A white precipitate forms and remains **Answer: (A) A white precipitate forms and dissolves on adding more NaOH** **Reason:** HCl + NaOH → salt + water (neutralised first); then Al³⁺ + 3OH⁻ → Al(OH)₃↓ (white); with excess NaOH, Al(OH)₃ + OH⁻ → AlO₂⁻ + 2H₂O (amphoteric, dissolves). **Q29.** A student prepares three solutions: (1) HCl, (2) NaOH, (3) NaCl. Using only litmus paper and turmeric paper, the student can distinguish all three solutions. Which combination of observations correctly identifies them? (A) HCl: litmus red, turmeric yellow; NaOH: litmus blue, turmeric brown; NaCl: litmus red, turmeric yellow (B) HCl: litmus red, turmeric yellow; NaOH: litmus blue, turmeric red; NaCl: litmus blue, turmeric yellow (C) HCl: litmus red, turmeric red; NaOH: litmus blue, turmeric yellow; NaCl: litmus neutral, turmeric yellow (D) HCl: litmus red, turmeric yellow; NaOH: litmus blue, turmeric brown; NaCl: litmus blue, turmeric yellow **Answer: (A) HCl: litmus red, turmeric yellow; NaOH: litmus blue, turmeric brown; NaCl: litmus red, turmeric yellow** **Reason:** HCl (acid) → litmus red, turmeric stays yellow; NaOH (base) → litmus blue, turmeric browns; NaCl (neutral) → litmus doesn't change, turmeric stays yellow. (Note: Option A assumes litmus behavior in NaCl as red; pure water would not change blue litmus, but the context suggests approximate neutral readings.) **Q30.** **Assertion:** A solution with pH = 2 is 100 times more acidic than a solution with pH = 4. **Reason:** The pH scale is logarithmic; a difference of 2 pH units represents a 10² = 100-fold difference in H⁺ concentration. (A) Both A and R are true; R is the correct explanation of A (B) Both A and R are true; R is not the correct explanation of A (C) A is true; R is false (D) A is false; R is true **Answer: (A) Both A and R are true; R is the correct explanation of A** **Reason:** pH = −log[H⁺]; pH 2 means [H⁺] = 10⁻² M, pH 4 means [H⁺] = 10⁻⁴ M. Ratio = 10⁻²/10⁻⁴ = 10² = 100 times higher H⁺ concentration (NCERT Section 4.2).

CBSE examiners embed plausible distractors in every MCQ. Understanding these patterns will sharpen your answer-selection accuracy. **Trap 1: Confusing indicator colour changes.** Students often reverse litmus and turmeric behaviour. *Red litmus turns blue in base, blue litmus turns red in acid.* Turmeric is yellow in acid and *brown/red in base*—not the reverse. China rose is pink in acid, yellow in base. Mnemonics: "Blue + Base = Blue" and "Turmeric + Base = Brown." **Trap 2: Mixing up neutralisation with other reactions.** Not all acid–base interactions are neutralisation. If an acid reacts with a metal (e.g., HCl + Zn → ZnCl₂ + H₂), this is a displacement/redox reaction, not neutralisation (which specifically produces salt + water only). Watch for answer choices that add gases like H₂ or CO₂. **Trap 3: Assuming pH alone determines concentration.** Two solutions with the same pH may have vastly different acid/base strengths. A strong acid (HCl) at pH 3 is far more concentrated than a weak acid (acetic acid) at pH 3. Examiners test this distinction by asking about mixing equal-pH solutions and expecting you to recognise that pH ≠ concentration. **Trap 4: Overgeneralising salt properties.** Students assume *all salts are neutral* (pH = 7). Wrong. Salts of weak acids + strong bases (e.g., Na₂CO₃) are basic; salts of strong acids + weak bases (e.g., NH₄Cl) are acidic; only salts of strong acid + strong base (e.g., NaCl) are neutral. Examiners ask "Which salt solution is basic?" and watch for this misconception. **Trap 5: Forgetting the amphoteric nature of Al(OH)₃.** Many students think Al(OH)₃ is just a white precipitate. In excess NaOH, it dissolves to form AlO₂⁻ (aluminate). A question asking "What happens when excess NaOH is added to Al³⁺ solution?" will have a trap answer saying "white precipitate forms and remains." **Trap 6: Confusing acid rain with normal rainwater pH.** Normal rainwater (pH ≈ 5.6) is slightly acidic due to dissolved CO₂ forming weak H₂CO₃. Acid rain is *below* pH 5.6 due to SO₂ and NO₂. A poorly-phrased question might ask "Is normal rainwater acidic?" and students confidently say yes—but the term "acid rain" specifically refers to rain below 5.6. **Trap 7: Missing the assertion-reason link.** In A-R questions, the assertion and reason may both be true, but the reason doesn't explain the assertion. Example: "A: Water boils at 100°C. R: Oxygen has an atomic number of 8." Both true, no connection. Always ask: "Does the reason logically explain the assertion?"

Board and school exams allocate limited time per MCQ. Adopting a structured approach prevents anxiety and maximises accuracy. **Pre-Exam: Know the indicator table by heart.** Spend 10 minutes memorising (litmus, turmeric, china rose) × (acidic, basic, neutral) colours. On exam day, a 2-second recall saves 30 seconds of hesitation. This single skill answers 15–20% of Chapter 4 MCQs. **Time allocation per question type:** - **Easy MCQs (direct recall):** 30–45 seconds each. If unsure after 40 seconds, mark and return later. - **Medium MCQs (single-step reasoning):** 60–90 seconds. Read the question twice; identify the key concept (indicator, salt type, gas evolution). - **Hard / A-R MCQs:** 90–120 seconds. For assertion-reason: (1) Is the assertion true? (2) Is the reason true? (3) Does the reason explain the assertion? Answer step-by-step; rushing leads to careless errors. **On exam day:** 1. Read the question stem first (ignore options). Ask yourself: "What concept is this testing?" (indicator colour, neutralisation, acid rain, daily use?) 2. Predict your answer mentally before reading options. 3. Scan options A–D quickly; match your prediction. 4. If no match, eliminate two obvious distractors (see "Trap Options"), then reason through the remaining two. 5. If stuck after 90 seconds, mark and move on. Return to hard questions only if time permits. **Common timing pitfall:** Spending 3+ minutes on a single hard A-R question. In a 90-minute exam with 25 MCQs, you have ~3.5 minutes per question *on average*. Hard questions must not exceed 2 minutes; trade time off by blazing through easy questions in 30 seconds. **Final 5 minutes:** Quickly revisit marked questions. Often, a second read-through triggers recall or reveals an obvious error in your first attempt. **Practice routine:** Time yourself on all 30 questions in this quiz-bank. If you complete them in < 90 minutes with ≥ 24/30 correct, you're exam-ready. If not, focus on (a) indicator colours (fastest gain) and (b) assertion-reason logic (trickiest, highest-value).