Acids, Bases, Salts and pH – NDA Chemistry PYQs

Solve NDA Chemistry previous-year questions on Acids, Bases, Salts and pH with detailed solutions and explanations.

Chapter-wise PYQs • Concept-based explanations • Exam insights

NDA 2026-II

Q. 1. In a dry test tube, 2 mL of HCl (0·1 N) is taken. To this, 4 mL of NaOH (0·1 N) is added. The solution is stirred well, followed by the addition of 2 drops of phenolphthalein. What will be the colour of the solution ?

(a) Colourless

(b) Red

(d) Blue

Answer: (c) Pink

Explanation: Moles of HCl = 0.1 N × 2 mL = 0.2 meq. Moles of NaOH = 0.1 N × 4 mL = 0.4 meq. After neutralisation, excess NaOH = 0.2 meq. The solution is alkaline (excess base). Phenolphthalein turns pink in alkaline solution. Concept Tested: Acid-base titration – phenolphthalein colour with excess NaOH

NDA 2026-I

Q. 2. Consider the following statement : “Acids are substances that dissociate in water to give hydrogen ions H⁺ (aq), and bases are substances that produce hydroxyl ions OH⁻ (aq).” The above statement is based on which one of the following concepts?

(a) Brønsted-Lowry concept

(b) Lewis acid-base concept

(d) Conjugate acid-base concept

Answer: (c) Arrhenius concept

Explanation: The Arrhenius concept defines acids as substances that produce H⁺ (aq) and bases as substances that produce OH⁻ (aq) in water. The Brønsted-Lowry concept defines acids as proton donors and bases as proton acceptors. The Lewis concept involves electron pair donors/acceptors. Concept Tested: Arrhenius definition of acids and bases

Q. 3. Consider the following : I. AlCl₃ II. NH₃ III. BF₃ IV. FeCl₃ How many of the above are Lewis acids?

(a) One

(b) Two

(d) All the four

Answer: (c) Three

Explanation: Lewis acids accept electron pairs (they have incomplete octets or empty orbitals). AlCl₃: Al has only 6 electrons around it – Lewis acid (Correct) . BF₃: B has only 6 electrons – Lewis acid (Correct) . FeCl₃: Fe³⁺ has empty d-orbitals – Lewis acid (Correct) . NH₃: nitrogen has a lone pair and is a Lewis BASE (electron pair donor) – not a Lewis acid (Incorrect). Three of the four are Lewis acids. Concept Tested: Lewis acids – identifying species with incomplete octets

NDA 2025-II

Q. 4. Which one of the following type of medicines is used for treating indigestion?

(a) Analgesic

(b) Antiseptic

(d) Antacid

Answer: (d) Antacid

Explanation: Indigestion caused by excess stomach acid is treated with antacids – substances that neutralise the acid. Analgesics relieve pain. Antiseptics prevent infection. Antibiotics treat bacterial infections. Concept Tested: Antacids – treatment for indigestion

NDA 2025-I

Q. 5. Which one of the following statements is NOT correct?

(a) A scale measuring hydrogen ion concentration in a solution is called pH scale

(b) The higher the hydrogen ion concentration in a solution, higher is its pH

(d) The ‘p’ in pH stands for ‘potenz’ in German meaning power

Answer: (b) The higher the hydrogen ion concentration in a solution, higher is its pH

Explanation: Higher H⁺ concentration = lower pH (pH = −log[H⁺]). So (b) is NOT correct – it states the opposite. The other three statements are correct. Concept Tested: pH definition – inverse relationship between H⁺ concentration and pH

NDA 2024-I

Q. 6. Human body works in the pH range of:

(a) 6·8 – 7·2

(b) 7·0 – 7·8

(d) 7·5 – 8·0

Answer: (a) 6·8 – 7·2

Explanation: Human blood pH is maintained in the narrow range of 7.35–7.45, and the body’s overall working pH is approximately 7.0–7.4. Among the options, (a) 6.8–7.2 most closely matches the physiological pH range for blood and cellular function. Concept Tested: Physiological pH – human body pH range

Q. 7. Which one among the following is present in the nettle leaf hairs that causes burning pain?

(a) Methanoic acid

(b) Ethanoic acid

(d) Acetic acid

Answer: (a) Methanoic acid

Explanation: Nettle leaf hairs contain methanoic acid (formic acid), which is injected when the leaf is touched. Note: ethanoic acid and acetic acid are the same substance (CH₃COOH), just different names. Concept Tested: Nettle sting – methanoic acid (repeated concept)

Q. 8. Which one among the following is known as Milk of Magnesia?

(a) Magnesium bicarbonate

(b) Magnesium carbonate

(d) Magnesium hydroxide

Answer: (d) Magnesium hydroxide

Explanation: Milk of Magnesia is an aqueous suspension of magnesium hydroxide (Mg(OH)₂). It is used as an antacid and laxative. Magnesium sulphate (Epsom salt) is different. Magnesium carbonate and bicarbonate are different compounds. Concept Tested: Milk of Magnesia – magnesium hydroxide Mg(OH)₂

NDA 2023-II

Q. 9. Which one of the following acid is used by goldsmith for cleaning of gold and silver articles ?

(a) Sulphuric acid

(b) Nitric acid

(d) Phosphoric acid

Answer: (b) Nitric acid

Explanation: Goldsmiths use dilute nitric acid (aqua fortis) to clean gold and silver articles. Nitric acid removes tarnish and oxide layers without dissolving pure gold (gold only dissolves in aqua regia – a mixture of HCl and HNO₃). Concept Tested: Use of nitric acid – goldsmith cleaning

NDA 2022-II

Q. 10. Bee sting leaves an acid which causes pain and irritation. The acid released is

(a) tartaric acid

(b) citric acid

(d) methanoic acid

Answer: (d) methanoic acid

Explanation: Bee sting injects methanoic acid (formic acid). This is the same concept as NDA 2011-I Q81 – both confirm methanoic acid as the bee sting acid. Concept Tested: Bee sting acid – methanoic acid (repeated)

NDA 2021-II

Q. 11. The pH value of Milk of Magnesia is approximately

(a) Zero

(b) 7

(d) 14

Answer: (c) 10

Explanation: Milk of Magnesia is an aqueous suspension of magnesium hydroxide (Mg(OH)₂), which is a mild alkali. Its pH is approximately 10 – alkaline but not strongly so. pH 14 would be a very strong alkali like NaOH. pH 7 is neutral. pH 0 is a very strong acid. Concept Tested: pH of Milk of Magnesia – mild alkaline ~10

NDA 2021-I

Q. 12. Which one of the following acids is predominantly found in tomatoes?

(a) Acetic acid

(b) Tartaric acid

(d) Lactic acid

Answer: (c) Oxalic acid

Explanation: Tomatoes predominantly contain oxalic acid and malic acid; citric acid is also present. Among the options, oxalic acid is associated with tomatoes. Note: some sources also cite citric acid. Acetic acid is in vinegar. Tartaric acid is in grapes/tamarind. Lactic acid is from fermentation. Concept Tested: Acids in common foods – oxalic acid in tomatoes

Q. 13. Which one of the following was the first mineral acid discovered?

(a) Sulphuric acid

(b) Hydrochloric acid

(d) Phosphoric acid

Answer: (b) Hydrochloric acid

Explanation: Same question as NDA 2020 Q148. Hydrochloric acid is the standard answer given in NDA context. This repeated question confirms it is a high-frequency concept. Concept Tested: First mineral acid discovered – HCl (repeated question)

★ JOVIK Exam Insight: This question appeared in NDA 2020 and NDA 2021-I with the same answer. A repeated question is a strong revision signal.

NDA 2020

Q. 14. Which one of the following was the first mineral acid discovered?

(a) Sulphuric acid

(b) Hydrochloric acid

(d) Phosphoric acid

Answer: (b) Hydrochloric acid

Explanation: Hydrochloric acid (HCl), historically known as muriatic acid or spirits of salt, is generally considered to be the first mineral acid discovered – known to alchemists as early as the 8th–9th century. However, this is a historically contested question. Concept Tested: History of mineral acids – first mineral acid discovered

NDA 2019-I

Q. 15. Which one of the following is an organic acid?

(a) Hydrochloric acid

(b) Nitric acid

(d) Sulphuric acid

Answer: (c) Acetic acid

Explanation: Acetic acid (CH₃COOH) is an organic acid – it contains carbon and is derived from living organisms (fermentation). HCl, HNO₃ and H₂SO₄ are all inorganic mineral acids. Concept Tested: Organic vs inorganic (mineral) acids – acetic acid

NDA 2018-II

Q. 16. Which one of the following is a tribasic acid?

(a) Hydrochloric acid

(b) Nitric acid

(d) Phosphoric acid

Answer: (d) Phosphoric acid

Explanation: Phosphoric acid (H₃PO₄) has three ionisable hydrogen atoms and can donate three protons – it is tribasic (triprotic). HCl is monobasic (one H⁺). HNO₃ is monobasic. H₂SO₄ is dibasic (two H⁺). Concept Tested: Basicity of acids – H₃PO₄ as tribasic acid

Q. 17. The solution of which one of the following will have pH less than 7?

(a) NaOH

(b) KCl

(d) NaCl

Answer: (c) FeCl₃

Explanation: FeCl₃ is a salt of a strong acid (HCl) and a weak base (Fe(OH)₃). Its aqueous solution is acidic due to hydrolysis of Fe³⁺ ions: Fe³⁺ + 3H₂O → Fe(OH)₃ + 3H⁺. pH < 7. NaOH is alkaline. KCl and NaCl are neutral salts (strong acid + strong base). Concept Tested: Salt hydrolysis – FeCl₃ gives acidic solution

NDA 2018-I

Q. 18. The accidental touch of Nettle leaves creates a burning sensation, which is due to inject of

(a) Hydrochloric acid

(b) Methanoic acid

(d) Sulphuric acid

Answer: (b) Methanoic acid

Explanation: Nettle leaves have hollow stinging hairs that inject methanoic acid (formic acid, HCOOH) into the skin, causing the burning sensation. The same acid is present in ant and bee stings. Concept Tested: Nettle sting acid – methanoic (formic) acid

Q. 19. Which one of the following gives the highest amount of hydrogen ions (H⁺)?

(a) Sodium hydroxide solution

(b) Milk of magnesia

(d) Gastric juice

Answer: (d) Gastric juice

Explanation: The highest H⁺ concentration corresponds to the lowest pH. Gastric juice has pH ~1–2, making it the most acidic among these options. Lemon juice is acidic (pH ~2–3) but less so than gastric juice. Sodium hydroxide and milk of magnesia are alkaline (low H⁺). Concept Tested: H⁺ concentration and pH – comparative acids and bases

NDA 2016-II

Q. 20. Which one of the following gases gives acidic solution on dissolving in water?

(a) Hydrogen

(b) Carbondioxide

(d) Oxygen

Answer: (b) Carbondioxide

Explanation: CO₂ dissolves in water to form carbonic acid: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. This makes the solution acidic. H₂, N₂ and O₂ do not react with water to form acidic solutions. Concept Tested: Acidic gas – CO₂ forming carbonic acid in water

NDA 2016-I

Q. 21. Suppose you have four test tubes labelled as ‘A’, ‘B’, ‘C’ and ‘D’. ‘A’ contains plain water, ‘B’ contains solution of an alkali, ‘C’ contains solution of an acid, and ‘D’ contains solution of sodium chloride. Which one of these solutions will turn phenolphthalein solution pink ?

(a) Solution ‘A’

(b) Solution ‘B’

(d) Solution ‘D’

Answer: (b) Solution ‘B’

Explanation: Phenolphthalein is colourless in acidic and neutral solutions and turns pink/red in alkaline (basic) solutions. Solution B contains an alkali, so phenolphthalein turns pink in B. Water (A) and NaCl solution (D) are neutral – no colour change. Acid solution (C) – no colour change. Concept Tested: Phenolphthalein indicator – colour in alkaline solution

NDA 2015-II

Q. 22. Boric acid is an acid because its molecule:

(a) accepts OH⁻ from water releasing proton

(b) combines with proton from water molecule

(d) gives up a proton

Answer: (a) accepts OH⁻ from water releasing proton

Explanation: Boric acid (H₃BO₃) is a Lewis acid – it accepts a hydroxide ion (OH⁻) from water: B(OH)₃ + H₂O → B(OH)₄⁻ + H⁺. This releases a proton, making the solution acidic. It is not a Brønsted acid that directly donates a proton; instead it accepts OH⁻. Concept Tested: Boric acid as a Lewis acid – mechanism of acidic behaviour

NDA 2015-I

Q. 23. Match List I with List II and select the correct answer using the code given below the Lists :

List I (Compound)

A. Sodium hydroxide

B. Calcium oxide

C. Acetic acid

D. Hydrochloric acid

List II (Nature)

1. Strong acid

2. Alkali

3. Weak acid

4. Base

Code :

A B C D

(a) 2 3 4 1

(b) 2 4 3 1

(d) 1 3 4 2

(a) 2 3 4 1

(b) 2 4 3 1

(d) 1 3 4 2

Answer: (b) A-2, B-4, C-3, D-1

Explanation: A. NaOH → soluble strong base = 2. Alkali. B. CaO → base but insoluble in water (when dissolved it forms Ca(OH)₂ which is a base, not an alkali as it is sparingly soluble) = 4. Base. C. CH₃COOH → acetic acid, weak acid = 3. Weak acid. D. HCl → strong acid = 1. Strong acid. Code (b): A-2, B-4, C-3, D-1. Concept Tested: Classification of compounds as alkali, base, weak acid, strong acid

NDA 2014-I

Q. 24. Which of the following acids is a mineral acid?

(a) Citric acid

(b) Hydrochloric acid

(d) Tartaric acid

Answer: (b) Hydrochloric acid

Explanation: Mineral acids are inorganic acids derived from minerals – they include HCl, H₂SO₄, HNO₃, H₃PO₄. Citric acid, ascorbic acid (Vitamin C) and tartaric acid are all organic acids found in fruits and foods. Concept Tested: Mineral acids vs organic acids – HCl as mineral acid

Q. 25. Which of the following is/are amphoteric?

(a) Al(OH)₃(s) and Fe(OH)₃(s)

(b) Al(OH)₃(s) and HCO₃⁻(aq)

(d) Al(OH)₃(s) only

Answer: (b) Al(OH)₃(s) and HCO₃⁻(aq)

Explanation: Amphoteric substances can act as both acids and bases. Al(OH)₃ reacts with acids (acting as base) and with NaOH (acting as acid) → Al(OH)₄⁻. HCO₃⁻ (bicarbonate ion) can accept a proton (act as base) or donate one (act as acid). Fe(OH)₃ is a base only. Ba(OH)₂ and NaOH are strong bases only. Concept Tested: Amphoteric substances – Al(OH)₃ and HCO₃⁻

NDA 2013-II

Q. 26. The pH of the solution obtained by dissolving pure sodium chloride in water is

(a) acidic

(b) basic

(d) dependent on the amount of sodium chloride dissolved in water

Answer: (c) neutral

Explanation: NaCl is a salt formed from a strong acid (HCl) and a strong base (NaOH). In water, NaCl does not hydrolyse – Na⁺ and Cl⁻ do not react with water to produce H⁺ or OH⁻ in excess. The solution is neutral (pH = 7). Concept Tested: Hydrolysis of salts – NaCl forms a neutral solution

Q. 27. Turmeric (Haldi) rapidly becomes colourless on addition of

(a) Baking soda

(b) Vinegar

(d) Alcohol

Answer: (a) Baking soda

Explanation: Turmeric is a natural indicator that is yellow in acidic and neutral conditions, but turns red/brown in alkaline conditions. Baking soda (NaHCO₃) is alkaline – when added to turmeric, it turns it red/brown and eventually appears to bleach or alter the colour. The question states it becomes colourless, which aligns with the alkaline effect of baking soda on turmeric’s curcumin pigment. Concept Tested: Turmeric as a natural indicator – colour change in alkali

NDA 2013-I

Q. 28. The pH of fresh milk is 6. When it turns sour, the pH

(a) becomes < 6

(b) remains the same, i.e., 6

(d) becomes neutral, i.e., 7

Answer: (a) becomes < 6

Explanation: When milk turns sour, bacteria convert lactose to lactic acid. This increases the concentration of H⁺ ions, lowering the pH below 6. Sour milk is more acidic than fresh milk. Concept Tested: pH change – milk souring due to lactic acid production

Q. 29. The acid in gastric juice is

(a) acetic acid

(b) nitric acid

(d) sulphuric acid

Answer: (c) hydrochloric acid

Explanation: The acid in gastric juice is hydrochloric acid (HCl). It is produced by parietal cells in the stomach lining and maintains the stomach pH around 1.5–3.5. Concept Tested: Gastric acid – HCl in stomach

NDA 2012-II

Q. 30. Which one among the following is a double salt?

(a) K₄[Fe(CN)₆]

(b) K₂SO₄·Al₂(SO₄)₃·24H₂O

(d) NaCl

Answer: (b) K₂SO₄·Al₂(SO₄)₃·24H₂O

Explanation: A double salt is formed by crystallisation of two salts together. K₂SO₄·Al₂(SO₄)₃·24H₂O is alum – a double salt of potassium sulphate and aluminium sulphate. K₄[Fe(CN)₆] is a complex salt (not a double salt). CuSO₄·5H₂O is a hydrated salt. NaCl is a simple salt. Concept Tested: Double salt vs complex salt – alum as example

Q. 31. Which one among the following is the correct order of strength of acids?

(a) H₂SO₄ > H₃PO₃ > CH₃COOH

(b) H₃PO₃ > H₂SO₄ > CH₃COOH

(d) CH₃COOH > H₂SO₄ > H₃PO₃

Answer: (a) H₂SO₄ > H₃PO₃ > CH₃COOH

Explanation: H₂SO₄ is a strong mineral acid (fully ionises in water). H₃PO₃ (phosphorous acid) is a moderate acid. CH₃COOH (acetic acid) is a weak organic acid. Strength order: H₂SO₄ > H₃PO₃ > CH₃COOH. Concept Tested: Relative acid strength – mineral vs organic acids

NDA 2012-I

Q. 32. Sometimes, indigestion is caused by the secretion of too much hydrochloric acid in the stomach. To ease the pain caused, a tablet can be taken that reacts to reduce the amount of acid present. Which one among the following would be inappropriate for a manufacturer to include as a major reactant in the tablet?

(a) CaCO₃

(b) MgCO₃

(d) Mg(OH)₂

Answer: (c) NaOH

Explanation: NaOH (sodium hydroxide, caustic soda) is a strong alkali and is corrosive – it would be harmful to ingest. It is inappropriate for antacid tablets. CaCO₃ (chalk), MgCO₃, and Mg(OH)₂ are mild, safe antacids that neutralise excess HCl without harmful side effects. Concept Tested: Antacids – safe vs unsafe alkalis for indigestion treatment

★ JOVIK Exam Insight: NaOH is too corrosive for antacid use. NDA tests this distinction. Common safe antacids: Mg(OH)₂ (milk of magnesia), CaCO₃, MgCO₃, NaHCO₃.

NDA 2011-II

Q. 33. Bases turn red litmus blue and acids turn blue litmus red. A student tested a liquid with a red litmus paper and it stayed red with no change. This shows that the liquid

(a) must be pure water.

(b) must be an acid.

(d) is neither a base nor an acid.

Answer: (c) is not a base.

Explanation: Red litmus stays red in the absence of a base. So if red litmus does not turn blue, the liquid is not a base. It could be an acid or a neutral substance – we cannot conclude it must be pure water or must be an acid. Concept Tested: Litmus test interpretation – not a base

NDA 2011-I

Q. 34. Solutions in test tubes containing H₂O and aqueous NaOH can be differentiated with the help of :

(a) red litmus.

(b) blue litmus

(d) HCl (aqueous)

Answer: (a) red litmus.

Explanation: NaOH is alkaline. Red litmus turns blue in the presence of a base (NaOH), but stays red in neutral water. Blue litmus stays blue in both water and NaOH (it only turns red in acid). So red litmus is the correct differentiator. Concept Tested: Using red litmus to detect a base – NaOH vs water

Q. 35. Human stomach produces acid ‘X’ which helps in digestion of food. Acid ‘X’ is :

(a) acetic acid.

(b) methanoic acid.

(d) citric acid.

Answer: (c) hydrochloric acid.

Explanation: The human stomach produces hydrochloric acid (HCl) as gastric acid. It helps in digestion by denaturing proteins and activating the enzyme pepsinogen to pepsin. Concept Tested: Gastric acid – hydrochloric acid in the stomach

★ JOVIK Exam Insight: HCl in the stomach is tested regularly in NDA. Also remember: bee sting = methanoic/formic acid; ant sting = formic acid; souring of milk = lactic acid; food souring = acetic acid.

Q. 36. When concentrated H₂SO₄ spilts on the surface, it should be immediately cleaned :

(a) with a piece of cloth.

(b) by adding cold water.

(d) by adding solid BaCl₂.

Answer: (c) by adding solid Na₂CO₃.

Explanation: Solid sodium carbonate (Na₂CO₃) neutralises the concentrated sulphuric acid: Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂. Adding water to concentrated H₂SO₄ is dangerous as it generates intense heat. Cloth would be destroyed. BaCl₂ does not neutralise the acid effectively. Concept Tested: Neutralisation of concentrated H₂SO₄ spill – using Na₂CO₃

Q. 37. A bee-sting leaves an acid which causes pain and irritation. The injected acid is :

(a) acetic acid.

(b) sulphuric acid.

(d) methanoic acid.

Answer: (d) methanoic acid.

Explanation: Bee stings inject methanoic acid (formic acid, HCOOH), which causes pain and irritation by lowering the local pH. Acetic acid is in vinegar. Citric acid is in citrus fruits. Sulphuric acid is an industrial mineral acid. Concept Tested: Bee sting acid – methanoic (formic) acid

★ JOVIK Exam Insight: NDA repeatedly tests bee sting = methanoic acid (formic acid, HCOOH). Ant sting = formic acid too. Nettle sting = also methanoic acid.

NDA 2010-II

Q. 38. Which one among the following is not a property of salt?

(a) Salts have ordered packing arrangements called lattices

(b) Salts have low melting points but high boiling points

(d) Salts conduct electricity when dissolved in water or even in the molten state

Answer: (b) Salts have low melting points but high boiling points

Explanation: Ionic salts have HIGH melting points (not low), because strong electrostatic forces hold the lattice together. They also have high boiling points. Options (a), (c), and (d) are all correct properties of ionic salts. Concept Tested: Properties of ionic salts – high melting points

Q. 39. Which among the following statements with regard to pH scale is/are correct?

1. It is a logarithmic scale

2. The scale is limited to 0—14 because the ionic product of water is about 10⁻¹⁴

3. The lower the value of pH the greater is the acidity of the solution

Select the correct answer using the code given below:

(a) 1 and 2 only

(b) 1, 2 and 3

(d) 2 only

Answer: (b) 1, 2 and 3

Explanation: All three statements are correct. The pH scale is logarithmic (pH = −log[H⁺]). It runs 0–14 because the ionic product of water Kw = 10⁻¹⁴. Lower pH means higher acidity (more H⁺ ions). Concept Tested: pH scale – logarithmic nature, 0–14 range, and acidity

Q. 40. Neutral water with pH about 7 becomes slightly acidic when aerated. This is because :

(a) oxygen from air is dissolved in the water which makes the water acidic

(b) dirt, which get contaminated with the water during aeration makes the water acidic

(d) carbon-dioxide from air is dissolved

Answer: (d) carbon-dioxide from air is dissolved

Explanation: CO₂ from air dissolves in water to form carbonic acid (CO₂ + H₂O ⇌ H₂CO₃), which then partially ionises to give H⁺ ions, lowering the pH below 7. Oxygen does not make water acidic. This is also the reason rain water is naturally slightly acidic. Concept Tested: CO₂ dissolving in water to form carbonic acid – pH of aerated water

Q. 41. Which one among the following combinations of solutions will produce a solid precipitate?

(a) Sodium sulphate and barium chloride

(b) Magnesium sulphate and barium bicarbonate

(d) Ammonium sulphate and potassium bromide

Answer: (a) Sodium sulphate and barium chloride

Explanation: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. Barium sulphate (BaSO₄) is insoluble (from the given rules: all sulphates are soluble EXCEPT Ba, Ag, Ca, Pb sulphates). So Ba²⁺ + SO₄²⁻ → BaSO₄ precipitate. Option (b): Ba(HCO₃)₂ is soluble; MgSO₄ is soluble – no precipitate. Option (c): LiI and BaCl₂ – LiI is soluble (all halides except Ag, Hg, Pb), BaCl₂ is soluble – no precipitate. Option (d): all four ions in solution are fully soluble. Concept Tested: Precipitation reactions – applying solubility rules

NDA 2009-II

Q. 42. Acid turns blue litmus red and base turns red litmus blue. A student tested a liquid with a red litmus paper which remained red with no change. This shows that the liquid

(a) is not a base

(b) is not an acid

(d) must be pure water

Answer: (a) is not a base

Explanation: Red litmus turns blue only in the presence of a base. If red litmus stays red, the liquid is not a base. However, it could still be an acid (acids keep red litmus red) or a neutral substance. We cannot conclude it is not an acid or that it must be pure water. Concept Tested: Litmus indicator – interpretation of colour-change results

★ JOVIK Exam Insight: If red litmus stays red → not a base (could be acid or neutral). If blue litmus stays blue → not an acid (could be base or neutral). This distinction is frequently tested.

Q. 43. Arrange the following bases in increasing order of their basic strength:

1. Sodium hydroxide

2. Magnesium hydroxide

3. Aluminium hydroxide

4. Ammonium hydroxide

Select the correct answer using the code given below:

(a) 4–2–1–3

(b) 4–1–2–3

(d) 1–2–3–4

Answer: (c) 4–3–2–1

Explanation: Increasing basic strength (weakest to strongest): NH₄OH (weakest, weak base) < Al(OH)₃ < Mg(OH)₂ < NaOH (strongest, strong base). NaOH is a strong base. Mg(OH)₂ is moderately strong. Al(OH)₃ is amphoteric and relatively weak. NH₄OH is a weak base. Order 4→3→2→1 means NH₄OH < Al(OH)₃ < Mg(OH)₂ < NaOH. Concept Tested: Relative basic strength of common hydroxide bases

Q. 44. The concentration of hydrochloric acid in a given solution is 10⁻⁸ M. What is the value of pH for this solution?

(a) 7

(b) > 7 but not 14

(d) 14

Answer: (c) < 7

Explanation: HCl is a strong acid, so even at 10⁻⁸ M, it adds H⁺ ions to water. Pure water has [H⁺] = 10⁻⁷ M (pH 7). Adding HCl increases [H⁺] slightly above 10⁻⁷ M (accounting for water’s own ionisation), making the pH slightly below 7 but close to 7. The solution is still acidic. pH < 7. Concept Tested: pH of very dilute strong acid – contribution of water auto-ionisation

★ JOVIK Exam Insight: This is an NDA-level trap. Even 10⁻⁸ M HCl gives pH < 7 (not exactly 8) because HCl still adds H⁺, shifting the equilibrium.

NDA 2009-I

Q. 45. When applied to the affected area, which one of the following will relieve the pain due to ant-bite or bee-sting?

(a) Lemon juice

(b) Vinegar

(d) Caustic soda

Answer: (c) Baking soda

Explanation: Ant and bee stings inject formic acid (methanoic acid), making the affected area acidic. Baking soda (sodium bicarbonate, NaHCO₃) is a mild alkali that neutralises the acid and relieves pain. Lemon juice and vinegar are acidic and would worsen the pain. Caustic soda (NaOH) is too strongly alkaline and would cause burns. Concept Tested: Acid-base neutralisation – treatment of insect stings

★ JOVIK Exam Insight: Ant and bee stings are acidic (formic/methanoic acid). Treat with a mild alkali like baking soda. This is a classic NDA acid-base application.