Hydrocarbons

A hydrocarbon is defined as a compound containing only carbon and hydrogen atoms. Examples include methane, ethene, and benzene. Carbonates contain the carbonate ion and oxygen, alcohols contain an –OH group and therefore oxygen, and esters contain oxygen in their functional group. Since only carbon and hydrogen are allowed in the definition, option B is the only correct answer.

Methane has the formula CH₄ and contains one carbon atom bonded to four hydrogen atoms. It has no carbon–carbon bond, and certainly no carbon–carbon double or triple bond. In the school-level classification, hydrocarbons lacking multiple carbon–carbon bonds are saturated; methane is the simplest member of the alkane family. Aromatic and cyclic describe different structural categories and do not change this classification. Hence, option A is correct.

The governing concept is organic nomenclature based on the type of carbon–carbon bonding. Alkanes are saturated hydrocarbons containing only single carbon–carbon bonds, and their names generally end in “-ane”; examples include methane, ethane and propane. Alkenes contain a double bond and usually end in “-ene”, while alkynes contain a triple bond and end in “-yne”. Alcohols are identified by the suffix “-ol”, so option A is correct.

The governing concept is the relationship between a hydrocarbon’s bonding and its IUPAC name. Alkynes contain at least one carbon–carbon triple bond, written as C≡C, so their names generally end in “-yne”; ethyne is a common example. Alkanes have only single bonds and use “-ane”, whereas alkenes contain a double bond and use “-ene”. Aldehydes have the functional-group suffix “-al”. Therefore, option C is correct.

The governing classification is based on carbon-carbon bonds. Saturated hydrocarbons contain only single C–C bonds, so the carbon framework holds the maximum possible number of hydrogen atoms; alkanes are examples. Unsaturated hydrocarbons contain at least one C=C double bond or C≡C triple bond, as in alkenes and alkynes. Therefore option A is correct. Options B and C wrongly deny the presence of carbon or hydrogen, and D ignores the defining bond difference.

Ethene has the molecular formula C₂H₄ and structural formula CH₂=CH₂. The equals sign represents a carbon-carbon double bond, making ethene an alkene and an unsaturated hydrocarbon. Ethane and propane are alkanes with only single carbon-carbon bonds, while methane contains only one carbon atom and cannot have a carbon-carbon bond. Therefore option A is the only correct answer.

Ethyne has the molecular formula C₂H₂ and structural formula HC≡CH. The symbol ≡ shows that the two carbon atoms share a triple bond, so ethyne is an alkyne and an unsaturated hydrocarbon. Ethane contains only a single C–C bond, ethene contains a double C=C bond, and methane has just one carbon atom. Hence option A is correct and the other choices represent different bonding situations.

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.

The governing concept is the overall reaction of photosynthesis. Green plants use light energy to convert carbon dioxide and water into carbohydrate, and oxygen is formed as a by-product, mainly from the splitting of water. It diffuses out through stomata, so option A is correct. Carbon dioxide is a reactant, while nitrogen and ammonia are not the normal gaseous product of photosynthesis.

Methane is the simplest alkane and therefore the simplest saturated hydrocarbon. It has one carbon atom covalently bonded to four hydrogen atoms, giving the formula CH₄. This also follows from the alkane formula CₙH₂ₙ₊₂: when n = 1, the result is CH₄. C₂H₆ is ethane, CO₂ is carbon dioxide, and CH₃OH is methanol.

Unsaturated hydrocarbons have at least one carbon-carbon double bond, \(C=C\), or triple bond, \(C\equiv C\). Those with a double bond are alkenes, and those with a triple bond are alkynes. A hydrocarbon with only carbon-carbon single bonds is saturated, so option B is incorrect. Exam tip: Identify an unsaturated hydrocarbon by looking for \(C=C\) or \(C\equiv C\) in its structure.