Acids, Bases, Salts and pH – NDA Chemistry PYQs

Q1. The pH value of Milk of Magnesia is approximately:
(a) Zero (b) 7 (c) 10 (d) 14

Answer: (c) 10
Milk of Magnesia is Mg(OH)₂. It is a mild base. Its pH is approximately 10. Pure water is 7. Strong bases like NaOH are close to 14. pH 0 means strongly acidic.
[NDA 2021-II]

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Q2. Which one among the following is known as Milk of Magnesia?
(a) Magnesium bicarbonate (b) Magnesium carbonate (c) Magnesium sulphate (d) Magnesium hydroxide

Answer: (d) Magnesium hydroxide — Mg(OH)₂
Milk of Magnesia is an aqueous suspension of Mg(OH)₂. It looks white and milky. It is used as a mild antacid and laxative.
[NDA 2024-I]

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Q3. Which one among the following is present in the nettle leaf hairs that causes burning pain?
(a) Methanoic acid (b) Ethanoic acid (c) Benzoic acid (d) Acetic acid

Answer: (a) Methanoic acid
Nettle leaf hairs contain methanoic acid — also called formic acid (HCOOH). When you touch the plant, the hairs break and inject this acid into the skin. The same acid is found in ant stings.
[NDA 2024-I]

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Q4. Common salt (NaCl) is NOT used as a raw material for preparation of which one of the following?
(a) Bleaching powder (b) Baking soda (c) Plaster of Paris (d) Washing soda

Answer: (c) Plaster of Paris
POP is made from gypsum (CaSO₄·2H₂O). Gypsum is a calcium compound. It has nothing to do with NaCl. NaCl is used to make NaOH, Na₂CO₃, NaHCO₃ (all sodium compounds), and bleaching powder (through chlorine from NaCl electrolysis). POP is the odd one out.
[NDA 2021-I]

Q5. Number of molecules of water of crystallisation in copper sulphate, sodium carbonate and gypsum are:
(a) 5, 10 and 2 respectively (b) 10, 2 and 5 respectively (c) 5, 2 and 10 respectively (d) 2, 5 and 10 respectively

Answer: (a) 5, 10 and 2 respectively
CuSO₄·5H₂O = 5 | Na₂CO₃·10H₂O = 10 | CaSO₄·2H₂O (gypsum) = 2.
These three numbers are asked together in NDA. Memorise them as a set: 5, 10, 2.
[NDA 2020]

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Q6. Which one of the following is the correct sequence of colour change when a turmeric stain on white clothes is scrubbed by soap and then washed with water?
(a) Yellow – pink – blue (b) Yellow – reddish brown – yellow (c) Yellow – reddish brown – blue (d) Yellow – blue – pink

Answer: (b) Yellow – reddish brown – yellow
Turmeric stain = yellow. Soap is alkaline → turmeric turns reddish-brown. Washing with water (neutral) → turmeric returns to yellow. The sequence is YRY — Yellow, Reddish-brown, Yellow.
[NDA 2020]

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Q7. Which one of the following statements regarding Bleaching powder and DDT is correct?
(a) Both are inorganic compounds (b) Both are organic compounds (c) Both contain chlorine (d) Both contain calcium

Answer: (c) Both contain chlorine
CaOCl₂ is inorganic and contains chlorine. DDT is organic and also contains chlorine. Option (d) is wrong — DDT does not contain calcium.
[NDA 2020]

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Q8. At nearly 70°C, sodium bicarbonate decomposes. The product responsible for its use in bakery products is:
(a) Carbon dioxide (b) Hydrogen (c) Water vapour (d) Oxygen

Answer: (a) Carbon dioxide
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑. The CO₂ gas creates bubbles in the dough. This makes bread and cakes soft and fluffy.
[NDA 2020]

Q9. What is the number of water molecules present in a Ferrous Sulphate crystal?
(a) 1 (b) 3 (c) 5 (d) 7

Answer: (d) 7
Ferrous sulphate = FeSO₄·7H₂O. It is also called green vitriol. It has 7 water molecules.
[NDA 2021-II]

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Q10. What is the colour of the precipitate obtained by passing CO₂ gas through lime water?
(a) Green (b) Blue (c) White (d) Brown

Answer: (c) White
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. Calcium carbonate is a white insoluble solid. This white precipitate makes lime water look milky.
[NDA 2021-II]

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Q11. Which one of the following is the chemical formula of Plaster of Paris?
(a) CaSO₄·½H₂O (b) CaSO₄·2H₂O (c) CaSO₄·5H₂O (d) CaSO₄·4H₂O

Answer: (a) CaSO₄·½H₂O
POP is calcium sulphate hemihydrate. CaSO₄·2H₂O is gypsum — the starting material. POP is made by heating gypsum and losing water. When POP is mixed with water, it absorbs water and converts back to gypsum — and sets hard.
[NDA 2021-II]

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Q12. Sometimes indigestion is caused by secretion of too much HCl in the stomach. Which one among the following would be INAPPROPRIATE as a major reactant in an antacid tablet?
(a) CaCO₃ (b) MgCO₃ (c) NaOH (d) Mg(OH)₂

Answer: (c) NaOH
NaOH is a strong, corrosive base. It would burn and damage the stomach lining and oesophagus. Antacids must use only mild, safe bases. CaCO₃, MgCO₃, and Mg(OH)₂ are all mild and safe.
[NDA 2012-I]

Q13. A gas is evolved when zinc metal is placed in dilute sulphuric acid. What is the gas?
(a) Hydrogen (b) Oxygen (c) Water vapour (d) Sulphur dioxide

Answer: (a) Hydrogen
Zn + H₂SO₄ → ZnSO₄ + H₂↑. Zinc displaces hydrogen from the acid. Hydrogen gas is produced. This is a standard acid-metal displacement reaction.
[NDA 2013-II]

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Q14. The acid in gastric juice is:
(a) Acetic acid (b) Nitric acid (c) Hydrochloric acid (d) Sulphuric acid

Answer: (c) Hydrochloric acid
The stomach secretes HCl. It helps in digestion and kills harmful bacteria. Its pH is approximately 1 to 2.
[NDA 2013-I]

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Q15. Turmeric (Haldi) rapidly becomes colourless on addition of:
(a) Baking soda (b) Vinegar (c) Lemon juice (d) Alcohol

Answer: (a) Baking soda
Baking soda is NaHCO₃. It is alkaline. It turns turmeric from yellow to reddish-brown. Against white fabric, this reddish-brown appears colourless or faded. Vinegar and lemon juice are acidic — turmeric stays yellow with them.
[NDA 2013-II]

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Q16. The setting time of cement is lowered by adding:
(a) Oxides of aluminium (b) Gypsum (c) Oxides of magnesium (d) Silica

Answer: (b) Gypsum
Gypsum (CaSO₄·2H₂O) is added to cement clinker to slow down the setting time. Without gypsum, cement undergoes flash setting — it hardens too quickly before it can be shaped and placed properly.
[NDA 2016-II]

Q17. 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
(c) We can measure pH generally from 0 to 14 on a pH scale
(d) The ‘p’ in pH stands for ‘potenz’ in German meaning power

Answer: (b)
Higher H⁺ concentration means LOWER pH — not higher. pH and H⁺ concentration move in opposite directions. When acidity increases, pH decreases. Statement (b) says the opposite — it is the false statement.
[NDA 2025-I]

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Q18. Match List I (Chemical Name) with List II (Chemical Formula):
A. Washing soda — B. Baking soda — C. Bleaching powder — D. Gypsum

1. CaOCl₂ — 2. NaHCO₃ — 3. Na₂CO₃·10H₂O — 4. CaSO₄·2H₂O

Answer: A-3, B-2, C-1, D-4
Washing soda = Na₂CO₃·10H₂O (3). Baking soda = NaHCO₃ (2). Bleaching powder = CaOCl₂ (1). Gypsum = CaSO₄·2H₂O (4).
[NDA 2025-I]