190. Why do many molecular organic carbon compounds have lower melting points than ionic compounds?
[RRB Group D — 13 Sep 2022, Shift 1]
(A) Presence of ionic bonds
(B) Presence of covalent bonds alone
(C) Very strong intermolecular attractions
(D) Relatively weak intermolecular attractions
Answer: (D) Relatively weak intermolecular attractions
Explanation: Although covalent bonds within an organic molecule may be strong, the forces between separate molecules are often comparatively weak. Melting many such substances therefore requires less energy than disrupting the strong electrostatic attraction in a typical ionic crystal.
189. Hydrocarbons with a double bond between two carbon atoms are called:
[RRB Group D — 09 Sep 2022, Shift 2]
(A) Inorganic compounds
(B) Natural compounds
(C) Saturated compounds
(D) Unsaturated compounds
Answer: (D) Unsaturated compounds
Explanation: A carbon–carbon double bond indicates unsaturation because fewer hydrogen atoms are present than in the corresponding saturated alkane. Alkenes are the main open-chain class featuring such bonds. These molecules often undergo addition across the double bond.
188. Which of the following members of the alkane series has the highest melting point?
[RRB Group D — 08 Sep 2022, Shift 3]
(A) C₄H₁₀
(B) C₂H₆
(C) C₃H₈
(D) CH₄
Answer: (A) C₄H₁₀
Explanation: Among the listed small alkanes, butane has the highest melting point. Molecular packing and London dispersion interactions affect alkane melting behaviour. The general trend with increasing carbon number is useful, although melting points are not perfectly monotonic throughout homologous series.
187. Which gas evolves during complete burning of natural gas (CH₄)?
[RRB Group D — 06 Sep 2022, Shift 3]
(A) NH₃
(B) CO₂
(C) N₂
(D) H₂
Answer: (B) CO₂
Explanation: Methane reacts with sufficient oxygen to produce carbon dioxide and water while releasing heat. The balanced combustion equation is CH₄ + 2O₂ → CO₂ + 2H₂O. Natural gas is a major domestic and industrial hydrocarbon fuel.
186. Which of these statements is correct?
[RRB Group D — 06 Sep 2022, Shift 2]
(A) Ethanol has three carbon atoms
(B) Acetic acid has two carbon atoms
(C) Methane has four carbon atoms
(D) Chloroform has three carbon atoms
Answer: (B) Acetic acid has two carbon atoms
Explanation: Acetic acid has the structure CH₃COOH and therefore contains two carbon atoms. Ethanol also has two, methane has one, and chloroform has one. Comparing formulas instead of names helps avoid common errors in carbon counting.
185. Statement I: Most carbon compounds are good conductors of electricity.
Statement II: Intermolecular forces in many molecular carbon compounds are not very strong.
Choose the correct answer.
[RRB Group D — 02 Sep 2022, Shift 1]
(A) II correct; I incorrect
(B) I correct; II incorrect
(C) Both incorrect
(D) Both correct
Answer: (A) II correct; I incorrect
Explanation: Most common molecular organic compounds lack mobile charge carriers, so statement I is incorrect. Their intermolecular forces are frequently weaker than ionic-lattice forces, making statement II broadly correct. Some carbon allotropes and conducting polymers are exceptions.
184. Which element shows catenation property similar to carbon?
[RRB Group D — 01 Sep 2022, Shift 2]
(A) Ne
(B) Si
(C) O
(D) K
Answer: (B) Si
Explanation: Silicon, like carbon, is a group 14 element with four valence electrons. It can form silicon–silicon covalent bonds and linked structures, though its catenation tendency is generally less extensive than that of carbon. Neon and potassium are unsuitable examples here.
183. The catenation property shown by carbon refers to:
[RRB Group D — 01 Sep 2022, Shift 1]
(A) Self-combination to form long atomic chains
(B) Occurrence as different allotropes
(C) Self-degeneration
(D) Giving combustibility to all compounds
Answer: (A) Self-combination to form long atomic chains
Explanation: Catenation is an element’s ability to bond with atoms of its own kind. Carbon forms long chains, branched frameworks and rings because carbon–carbon bonds are strong and stable. This is distinct from allotropy, which describes different structural forms of an element.
182. Which chemicals are used in traditional gunpowder for fireworks?
[SSC CPO — 09 Nov 2022, Shift 2]
(A) Silver nitrate and charcoal
(B) Silver nitrate and sulphur
(C) Potassium sulphate and phosphorus
(D) Potassium nitrate, sulphur and charcoal
Answer: (D) Potassium nitrate, sulphur and charcoal
Explanation: The historical term black powder denotes a mixture involving potassium nitrate, sulphur and charcoal. In examinations it is identified by its constituent names and classified as an early explosive material. The question concerns factual recognition, not preparation.
181. Which of the following is not a biofuel?
[WBCS Mains 2021 — held May 2022, Paper IV]
(A) Vegetable oil
(B) Green diesel
(C) Biogas
(D) Gasoline
Answer: (D) Gasoline
Explanation: Vegetable oils, renewable diesel and biogas can be obtained from biomass or other renewable biological feedstocks. Conventional gasoline is primarily a petroleum-refining product containing a mixture of hydrocarbons. Therefore it is not ordinarily categorised as a biofuel.
