Q24. If hydrofluoric acid is weak, what is the nature of its conjugate base, fluoride ion?
[RRB NTPC Graduate CBT-I, 19 June 2025, Shift 1]
(A) A strong acid
(B) A strong base
(C) A neutral species
(D) A weak base
Answer: (D) A weak base
Explanation: Hydrofluoric acid dissociates only partially in water. Its conjugate base, fluoride ion, can accept a proton and therefore displays basic behaviour. However, fluoride is only a weak base in water, though stronger than conjugate bases of strong acids.
Q23. Which combination gives the correct element, symbol and valency for atomic number 11?
[UPSC CDS-I General Knowledge, 13 April 2025]
(A) Lithium, Li, 1
(B) Magnesium, Mg, 2
(C) Boron, B, 3
(D) Sodium, Na, 1
Answer: (D) Sodium, Na, 1
Explanation: Atomic number 11 corresponds to sodium, represented by Na. Its electronic distribution is 2, 8, 1, so it tends to lose one outer electron, forming Na⁺. Consequently, sodium commonly shows a valency of one in compounds.
Q22. Which one of these substances is a metalloid?
[UPSC CDS-I General Knowledge, 13 April 2025]
(A) Germanium
(B) Coke
(C) Iodine
(D) Sugar
Answer: (A) Germanium
Explanation: Germanium is a metalloid with characteristics intermediate between metals and non-metals. It possesses semiconducting properties useful in electronics. Iodine is a non-metal, coke is predominantly carbon, and sugar is an organic compound rather than an element.
Q21. Which element in the list is considered the earliest discovered?
[UPSC NDA-I General Ability Test, 13 April 2025]
(A) Copper
(B) Gold
(C) Oxygen
(D) Uranium
Answer: (A) Copper
Explanation: Copper was among the earliest metals recognized and worked by humans, with archaeological evidence of its use extending deep into prehistory. Gold was also known in antiquity, whereas oxygen and uranium were identified much later through scientific investigation.
Q20. Which chemical formula denotes sulphur hexafluoride?
[SSC MTS, 23 October 2024, Shift 3]
(A) S₂F₆
(B) SF₆
(C) SF₂
(D) S₆F
Answer: (B) SF₆
Explanation: The prefix hexa- indicates six fluorine atoms, and the unprefixed sulfur name indicates one sulfur atom. Sulfur hexafluoride therefore has formula SF₆. It is a stable inorganic gas used for electrical insulation, although it is also a potent greenhouse gas.
Q19. Which inorganic compound is converted into urea through the classic Wöhler rearrangement?
[SSC MTS, 18 October 2024, Shift 1]
(A) Ammonium cyanate
(B) Ammonium phosphate
(C) Ammonium acetate
(D) Ammonium nitrate
Answer: (A) Ammonium cyanate
Explanation: Ammonium cyanate rearranges to form urea, as demonstrated by Friedrich Wöhler in 1828. The experiment proved that an organic compound could be synthesized from inorganic starting material. It became an important challenge to the former vital-force theory.
Q18. Why do these reactions produce copper: Zn + CuSO₄ → ZnSO₄ + Cu; Pb + CuCl₂ → PbCl₂ + Cu?
[SSC MTS, 8 October 2024, Shift 1]
(A) Zinc and lead are unstable as elements
(B) Pressure decomposes copper compounds
(C) Zinc and lead are more reactive than copper
(D) Copper is more reactive than zinc and lead
Answer: (C) Zinc and lead are more reactive than copper
Explanation: Both zinc and lead occur above copper in the metal reactivity series. Each can displace copper ions from suitable compounds, producing elemental copper while forming its own salt. These are single-displacement redox reactions involving electron transfer.
Q17. Which gas is released when iron reacts with steam?
[SSC CGL Tier-I, 24 September 2024, Shift 2]
(A) Hydrogen
(B) Oxygen
(C) Ammonia
(D) Methane
Answer: (A) Hydrogen
Explanation: Hot iron reacts with steam to produce magnetite, Fe₃O₄, and hydrogen gas. The balanced equation is 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Ordinary cold water does not react with iron in the same rapid manner.
Q16. In discussions of periodic trends, what property is usually measured in picometres?
[SSC CGL Tier-I, 18 September 2024, Shift 1]
(A) Atomic radius
(B) Molar mass
(C) Atomic density
(D) Spin quantum number
Answer: (A) Atomic radius
Explanation: A picometre is one trillionth of a metre and is convenient for expressing atomic dimensions. Atomic radii are commonly on the order of tens or hundreds of picometres. Molar mass and spin quantum number require entirely different units or conventions.
Q15. Select the correct formula of magnesium hydroxide.
[SSC CGL Tier-I, 13 September 2024, Shift 2]
(A) MgOH
(B) MgO₂
(C) Mg₂H₂
(D) Mg(OH)₂
Answer: (D) Mg(OH)₂
Explanation: Magnesium forms Mg²⁺ ions, while each hydroxide ion carries a negative charge of one. Two hydroxide ions are therefore required for electrical neutrality. The resulting formula is Mg(OH)₂, a sparingly soluble base used in antacid preparations.
