Which of the following statements describes alkenes and alkynes but not alkanes?
a. they are aromatic compounds.
b. they are unsaturated.
c. they are saturated.
d. they are hydrocarbons.

B. they are unsaturated

Explanation:

Alkanes are long chain hydrocarbons with only single bonds, alkenes are hydrocarbons with at least one double bond and alkynes are hydrocarbons with at least one triple bond.

Alkanes, alkenes and alkynes are all hydrocarbons. Therefore statement D. is incorrect

Hydrocarbons are compounds containing carbon and hydrogen only

Neither of the three are aromatic compounds therefore statement A is incorrect

Saturated hydrocarbons are where all the bonds between atoms are single bonds. Unsaturated hydrocarbonds are when at least there's one double or triple bond.

Alkanes are saturated and Alkynes and alkenes are unsaturated.

therefore statement B is correct where alkenes and alkynes are unsaturated but alkanes are not

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chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.