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.

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.