Chemistry Regents Practice Test 2025 – The All-in-One Guide to Exam Success!

An unsaturated hydrocarbon is defined by the presence of one or more multiple bonds between carbon atoms, which include double bonds (as in alkenes) or triple bonds (as in alkynes). The presence of these multiple bonds allows the molecule to contain fewer hydrogen atoms than saturated hydrocarbons, which contain only single carbon-carbon bonds. In essence, unsaturation refers to the ability of the molecule to undergo reactions that could add more hydrogen atoms or convert the multiple bonds into single bonds.

This characteristic of having at least one multiple carbon-carbon bond is crucial, as it directly differentiates unsaturated hydrocarbons from saturated ones. Therefore, the correct option accurately defines an essential feature of unsaturated hydrocarbons. Other choices do not align with this definition, as saturated hydrocarbons would only have single bonds, and there are no unique carbon-carbon bonds in the case of no bonds existing at all.