250. Which of the following has a branched-chain hydrocarbon?
[RRB Group D — 10 Dec 2025, Shift 1]
(A) Isobutane
(B) Butane
(C) Propane
(D) Ethane
Answer: (A) Isobutane
Explanation: Isobutane is 2-methylpropane and has a side branch in its carbon skeleton. The other three choices are straight-chain alkanes. Isobutane and normal butane are structural isomers, illustrating how hydrocarbons can share a molecular formula but have different structures.
249. Which property of carbon permits one carbon atom to bond with other carbon atoms?
[RRB Group D — 09 Dec 2025, Shift 2]
(A) Valency
(B) Polymerisation
(C) Catenation
(D) Isomerisation
Answer: (C) Catenation
Explanation: Catenation allows carbon atoms to form chains, branched structures and rings by bonding to other carbon atoms. The comparatively strong carbon–carbon covalent bond gives these structures stability. Polymerisation is a chemical process, whereas catenation is an underlying bonding capability.
248. An organic compound always contains which of the following?
[RRB Group D — 09 Dec 2025, Shift 1]
(A) Silicon and phosphorus
(B) Halogens and noble gases
(C) Hydrogen, oxygen and nitrogen
(D) At least one carbon atom
Answer: (D) At least one carbon atom
Explanation: Organic compounds contain carbon, although not every carbon compound is classed as organic. Many organic molecules also contain hydrogen, oxygen, nitrogen or halogens, but these additional elements are not universal requirements. Carbon-based bonding provides the essential chemical framework.
247. What type of bonding exists in most organic compounds?
[RRB Group D — 08 Dec 2025, Shift 1]
(A) Coordinate
(B) Covalent
(C) Ionic
(D) Metallic
Answer: (B) Covalent
Explanation: Covalent bonding involves the sharing of electron pairs between atoms. It predominates in the carbon skeletons of organic molecules because carbon bonds readily with nonmetals. This is why many molecular organic compounds lack the mobile charged particles needed for electrical conduction.
246. What is the IUPAC name of an organic compound having two carbon atoms and a hydroxyl (–OH) group?
[RRB Group D — 05 Dec 2025, Shift 3]
(A) Methanal
(B) Ethanol
(C) Propanone
(D) Ethanoic acid
Answer: (B) Ethanol
Explanation: The prefix eth- identifies two carbon atoms, while the suffix -ol identifies an alcohol containing a hydroxyl functional group. The compound is ethanol, C₂H₅OH. Methanal is an aldehyde, propanone a ketone, and ethanoic acid a carboxylic acid.
245. Detergents can work in which of the following?
[RRB Group D — 05 Dec 2025, Shift 3]
(A) Only distilled water
(B) Only hot water
(C) Only soft water
(D) Both hard and soft water
Answer: (D) Both hard and soft water
Explanation: Many synthetic detergents remain effective in hard water because their calcium and magnesium salts generally do not produce the same insoluble scum as ordinary soaps. This makes detergents useful for washing where dissolved mineral ions are abundant.
244. Which of the following products is formed on complete combustion of hydrocarbons?
[RRB Group D — 05 Dec 2025, Shift 2]
(A) CO and water
(B) CO₂ and water
(C) Methane and hydrogen
(D) Methane and oxygen
Answer: (B) CO₂ and water
Explanation: Hydrocarbons contain carbon and hydrogen. When oxygen is sufficient, carbon is fully oxidised to carbon dioxide and hydrogen to water. Carbon monoxide may arise under oxygen-limited combustion and should not be confused with the normal products of complete burning.
243. Which compound is classified as a saturated hydrocarbon?
[RRB Group D — 03 Dec 2025, Shift 3]
(A) Benzene
(B) Ethane
(C) Ethyne
(D) Ethene
Answer: (B) Ethane
Explanation: Ethane is an alkane with only single covalent bonds and the molecular formula C₂H₆. Ethene contains a double bond, ethyne a triple bond, and benzene is aromatic. Thus ethane represents a fully saturated open-chain hydrocarbon.
242. Which structural formula represents the branched-chain compound neopentane?
[RRB Group D — 03 Dec 2025, Shift 2]
(A) CH₃–CH(CH₃)–CH₂–CH₂–CH₃
(B) CH₃–CH(CH₃)–CH₃
(C) CH₃–CH(CH₃)–CH₂–CH₃
(D) C(CH₃)₄
Answer: (D) C(CH₃)₄
Explanation: Neopentane is 2,2-dimethylpropane, one of the three structural isomers of pentane. A central carbon is bonded to four methyl groups, yielding the condensed formula C(CH₃)₄. Its compact branching gives distinctive physical properties.
241. When a hydrocarbon is burnt in excess air, the final products are:
[RRB Group D — 01 Dec 2025, Shift 2]
(A) Carbon dioxide and hydrogen
(B) Carbon monoxide and water
(C) Carbon monoxide and hydrogen
(D) Carbon dioxide and water
Answer: (D) Carbon dioxide and water
Explanation: Excess air provides enough oxygen for complete combustion. The carbon atoms in a hydrocarbon form carbon dioxide and the hydrogen atoms form water. This oxidation releases useful energy and explains why hydrocarbons serve as widely used fuels.
