Hydrocarbons

Bromine water is a common test for carbon-carbon unsaturation. Ethene contains a C=C double bond, which reacts readily with bromine in an addition reaction; bromine is consumed and its orange-brown colour disappears. Therefore option A is correct. Ethane and methane are saturated hydrocarbons and do not rapidly decolourise bromine water under ordinary test conditions. Sodium chloride is ionic and does not provide an unsaturated bond.

Soot consists mainly of tiny unburnt carbon particles. Unsaturated hydrocarbons contain carbon-carbon double or triple bonds and often have a relatively higher carbon proportion than saturated compounds with comparable hydrogen content. If oxygen is insufficient or mixing is poor, combustion is incomplete; carbon particles then remain incandescent and produce a yellow, smoky, sooty flame. The other options contradict the composition of hydrocarbons or combustion chemistry.

Methane undergoes complete oxidation only when enough oxygen is available. Its balanced combustion equation is CH₄ + 2O₂ → CO₂ + 2H₂O, showing that two oxygen molecules are needed for each methane molecule. With an oxygen shortage, incomplete combustion may produce carbon monoxide and soot instead of fully oxidising carbon to carbon dioxide. Therefore option A states the purpose of sufficient oxygen correctly.

Butane has the molecular formula C₄H₁₀. Its four carbon atoms can form a continuous straight chain in n-butane or a three-carbon chain with a methyl branch in 2-methylpropane (isobutane). Both structures contain four carbon and ten hydrogen atoms, but their connectivity differs, so they are structural isomers. The other options incorrectly describe its composition or bonding.

The governing concept is saturation and the reactivity of multiple bonds. Ethane, C2H6, is a saturated hydrocarbon containing only strong carbon–carbon single bonds. Ethene, C2H4, is unsaturated and contains a carbon–carbon double bond made of one sigma and one weaker pi bond. The pi bond can break during addition reactions, making ethene more reactive in this context. Thus option A is correct; ethane has no triple bond and both compounds are not completely inert.

The governing concept is that addition reactions occur readily at unsaturated carbon–carbon bonds. Saturated hydrocarbons, such as alkanes, contain only carbon–carbon and carbon–hydrogen single bonds and already have the maximum possible number of hydrogen atoms. Since there is no carbon–carbon double or triple bond whose pi bond can open to accept new atoms, addition is generally not their characteristic reaction. Therefore option A is correct.

Cyclohexane has the molecular formula C₆H₁₂. The six carbon atoms join end-to-end to form a closed ring, which explains the word cyclic. Every carbon–carbon connection in the ring is a single bond, and the remaining valencies are satisfied by hydrogen; therefore it is saturated. It is a hydrocarbon because it contains only carbon and hydrogen. Option A states all three defining features.