Q35. Which chemical weathering process occurs when atmospheric or soil carbon dioxide dissolves in water?
[RRB NTPC Graduate Level CBT-II, 10 July 2026]
(A) Carburation
(B) Carbolation
(C) Carboxylation
(D) Carbonation
Answer: (D) Carbonation
Explanation: Carbon dioxide reacts with water to form weak carbonic acid. This acid attacks carbonate minerals, especially limestone, producing more soluble bicarbonates. The chemical weathering process involving carbonic acid formation and its attack on rocks is called carbonation.
Q34. What property of noble gases accounts for their very low chemical reactivity under standard conditions?
[RRB NTPC Undergraduate CBT-I, 20 June 2026, Shift 2]
(A) Completely filled valence shell
(B) High atomic mass
(C) High electronegativity
(D) Low ionization energy
Answer: (A) Completely filled valence shell
Explanation: Noble gases have stable outer electron shells. Helium contains two electrons in its first shell, while most other noble gases possess eight valence electrons. Their filled shells make electron gain, loss and sharing energetically unfavourable under ordinary conditions.
Q33. Match the element symbols with their classes: A. Sm; B. Cs; C. Os; D. Np. Classes: 1. Transition; 2. Alkali; 3. Actinoid; 4. Lanthanoid.
[UPSC CDS-I General Knowledge, 12 April 2026]
(A) A-4, B-2, C-1, D-3
(B) A-4, B-1, C-2, D-3
(C) A-3, B-1, C-2, D-4
(D) A-3, B-2, C-1, D-4
Answer: (A) A-4, B-2, C-1, D-3
Explanation: Samarium is a lanthanoid, caesium an alkali metal, osmium a transition metal and neptunium an actinoid. These classifications follow their electronic structures and periodic-table blocks. Thus the matching sequence is A-4, B-2, C-1 and D-3.
Q32. Which of the following elements is not classified as a metal?
[RRB NTPC Graduate CBT-I, 27 March 2026, Shift 3]
(A) Sulfur
(B) Potassium
(C) Calcium
(D) Sodium
Answer: (A) Sulfur
Explanation: Sulfur is a Group 16 non-metal, generally brittle and a poor conductor of electricity. Potassium and sodium are Group 1 alkali metals, while calcium belongs to Group 2. All three metallic options typically form positively charged ions.
Q31. Which gas in the options does not belong to the noble-gas family?
[RRB NTPC Graduate CBT-I, 19 March 2026, Shift 1]
(A) Helium
(B) Oxygen
(C) Argon
(D) Neon
Answer: (B) Oxygen
Explanation: Oxygen is in Group 16 and has six valence electrons, making it chemically reactive. Helium, argon and neon are Group 18 noble gases, characterized by complete outer shells and very low chemical reactivity under normal conditions.
Q30. Who defined an element as a substance that cannot be broken down by ordinary chemical reactions?
[RRB Group D, 21 December 2025, Shift 3]
(A) Antoine Laurent Lavoisier
(B) Robert Hooke
(C) Robert Boyle
(D) Charles Darwin
Answer: (A) Antoine Laurent Lavoisier
Explanation: Antoine Lavoisier used an operational definition of an element: a substance that contemporary chemical methods could not decompose into simpler substances. His 1789 chemistry treatise helped modernize the classification of substances and established more systematic chemical terminology.
Q29. Arrange these elements by increasing quantity in a typical 70-kg human body: 1. Sodium; 2. Potassium; 3. Copper; 4. Iron.
[UPPSC PCS Preliminary GS-I, 12 October 2025]
(A) 4, 3, 2, 1
(B) 3, 4, 1, 2
(C) 3, 4, 2, 1
(D) 4, 3, 1, 2
Answer: (B) 3, 4, 1, 2
Explanation: Copper occurs only in trace quantities in the human body, followed by iron at a few grams. Sodium and potassium are present in substantially larger amounts, with potassium generally exceeding sodium. The increasing sequence is copper, iron, sodium, potassium.
Q28. Place these major crustal elements in increasing order of mass percentage: 1. Aluminium; 2. Calcium; 3. Silicon; 4. Oxygen.
[UPSC CDS-II General Knowledge, 14 September 2025]
(A) 3, 4, 1, 2
(B) 4, 3, 1, 2
(C) 2, 1, 3, 4
(D) 1, 2, 3, 4
Answer: (C) 2, 1, 3, 4
Explanation: The approximate mass percentages in Earth’s crust are calcium 3.6%, aluminium 8.1%, silicon 27.7% and oxygen 46.6%. Therefore, the ascending sequence is calcium, aluminium, silicon and oxygen, corresponding to the code 2, 1, 3, 4.
Q27. Which electronic configuration represents transition-metal elements most generally?
[UPSC CDS-II General Knowledge, 14 September 2025]
(A) (n−2)d¹⁻¹⁰ ns²
(B) (n−2)f¹⁻¹⁴ (n−1)d⁰⁻¹ ns²
(C) ns² np⁶ nd¹⁻¹⁰
(D) (n−1)d¹⁻¹⁰ ns⁰⁻²
Answer: (D) (n−1)d¹⁻¹⁰ ns⁰⁻²
Explanation: Transition metals belong to the d-block, where electrons occupy the penultimate d subshell. Their general electronic configurations involve (n−1)d orbitals and outer ns orbitals, with exceptions caused by orbital-energy similarities. This accounts for variable oxidation states and coloured ions.
Q26. Which formula below is not a typical constituent of Portland cement?
[UPSC NDA-II General Ability Test, 14 September 2025]
(A) 2CaO·SiO₂
(B) 3CaO·SiO₂
(C) 4CaO·SiO₂
(D) 3CaO·Al₂O₃
Answer: (C) 4CaO·SiO₂
Explanation: Portland cement contains dicalcium silicate, tricalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite among its major clinker phases. The formula 4CaO·SiO₂ is not a standard cement clinker compound. Silicates principally contribute to the development of cement strength.
Q25. Why is an alloy classified as a mixture, despite being difficult to separate by simple physical methods?
[RRB NTPC Undergraduate CBT-I, 13 August 2025, Shift 3]
(A) It consists solely of non-metals
(B) Its composition can vary and it retains constituent-related properties
(C) It contains a single gaseous element
(D) All alloys have a fixed composition and are magnetic
Answer: (B) Its composition can vary and it retains constituent-related properties
Explanation: Alloys are mixtures of elements, ordinarily including at least one metal. Their constituent proportions may vary, unlike compounds with fixed chemical formulas. Intimate mixing at the atomic scale makes simple filtration ineffective, but it does not turn alloys into compounds.