160. Which air pollutant is most closely linked to the formation of acid rain?
[RRB NTPC — 2026]
(A) Ammonia
(B) Sulphur dioxide
(C) Ozone
(D) Methane
Answer: (B) Sulphur dioxide
Explanation: Sulphur dioxide released by burning sulphur-bearing fuels undergoes oxidation in air and can form sulphuric acid in precipitation. Nitrogen oxides provide another major route to acid deposition. Both damage sensitive freshwater systems. Rainwater acidity is monitored because deposition changes both soil chemistry and lake water quality.
159. Which set consists entirely of Lewis acids?
[UPSC CDS — 2025]
(A) AlCl3, Co³+, BF3
(B) BCl3, NH3, OH−
(C) Mg²+, H2O, F−
(D) BF3, NH3, H2O
Answer: (A) AlCl3, Co³+, BF3
Explanation: Lewis acids accept electron pairs. Aluminium chloride, boron trifluoride and highly charged metal ions can accept electron density; ammonia and hydroxide commonly donate electron pairs. Both oxidation states and chemical availability determine the agricultural and industrial roles of nitrogen compounds.
158. An aqueous solution with pH below 7 is classified as what?
[UPSC CDS — 2025]
(A) Neutral
(B) Basic
(C) Acidic
(D) Hypotonic
Answer: (C) Acidic
Explanation: A pH below seven indicates acid character at approximately 25°C. Lower pH generally means greater hydrogen-ion activity; pH seven represents neutral water at that temperature. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
157. Which oxide is classified as neutral?
[UPSC NDA — 2025]
(A) CO
(B) CO2
(C) Na2O
(D) MgO
Answer: (A) CO
Explanation: Carbon monoxide is a neutral oxide in ordinary acid-base classification. Carbon dioxide is acidic; sodium and magnesium oxides are typically basic. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
156. Which statement about pH is incorrect?
[UPSC NDA — 2025]
(A) The pH scale reflects hydrogen-ion activity
(B) Higher hydrogen-ion concentration means higher pH
(C) The common teaching scale is about 0 to 14
(D) The p relates historically to power
Answer: (B) Higher hydrogen-ion concentration means higher pH
Explanation: pH decreases when hydrogen-ion concentration increases because it is defined by a negative logarithm. More acidic solutions therefore have smaller pH values. The concentration and mobility of ions help explain the observed chemical behaviour in solution.
155. Rain is termed acid rain when its pH falls below roughly what value?
[UPSC NDA — 2025]
(A) 7.0
(B) 6.6
(C) 5.6
(D) 8.0
Answer: (C) 5.6
Explanation: Natural rainfall is mildly acidic through dissolved carbon dioxide. A pH below approximately 5.6 usually indicates additional acid deposition from pollutants. Rainwater acidity is monitored because deposition changes both soil chemistry and lake water quality.
154. Match washing soda, baking soda, bleaching powder and gypsum to their formulas.
[UPSC NDA — 2025]
(A) CaSO4·2H2O, NaHCO3, CaOCl2, Na2CO3·10H2O
(B) CaSO4·2H2O, CaOCl2, NaHCO3, Na2CO3·10H2O
(C) Na2CO3·10H2O, CaOCl2, NaHCO3, CaSO4·2H2O
(D) Na2CO3·10H2O, NaHCO3, CaOCl2, CaSO4·2H2O
Answer: (D) Na2CO3·10H2O, NaHCO3, CaOCl2, CaSO4·2H2O
Explanation: The corresponding formulas are washing soda Na2CO3·10H2O, baking soda NaHCO3, bleaching powder conventionally CaOCl2 and gypsum CaSO4·2H2O. The formulas distinguish these common salts and minerals, even when their trade names sound similar.
153. Which of these mixtures is called water gas?
[UPSC NDA — 2025]
(A) H2O + CO2
(B) NH3
(C) H2 + N2
(D) CO + H2
Answer: (D) CO + H2
Explanation: The combination CO and H2 is known as water gas. It differs from producer gas, which contains appreciable nitrogen. Nitrogen compounds are particularly important for agriculture because plants need nitrogen for protein synthesis.
152. Which statement does NOT describe graphene?
[UPSC CDS — 2025]
(A) It is extremely thin
(B) It is nearly transparent
(C) It is an excellent electrical conductor
(D) It is a wide-band-gap semiconductor
Answer: (D) It is a wide-band-gap semiconductor
Explanation: Graphene consists of a single atomic layer of carbon in a hexagonal arrangement. It is transparent and highly conductive, but pristine graphene has essentially zero electronic band gap. Changes in carbon bonding and crystal arrangement can transform electrical and mechanical behaviour dramatically.
151. Arrange HF, HCl, HBr and HI in increasing acid strength.
[RPSC Senior Teacher (Science) — 2025]
(A) HBr < HI < HCl < HF
(B) HF < HCl < HBr < HI
(C) HI < HBr < HCl < HF
(D) HF < HBr < HCl < HI
Answer: (B) HF < HCl < HBr < HI
Explanation: For hydrogen halides in water, acidity increases down the group because the hydrogen-halogen bond weakens. Hydrogen fluoride’s particularly strong bond makes it the weakest acid of these four. The chemical properties of group-17 elements follow from their seven outer-shell electrons.
