Final answer:
Not all characteristics apply to alkynes; they do not contain only single bonds nor do they contain only one σ bond. Alkynes are identified by their triple bonds, which consist of one σ bond and two π bonds, and noncyclic alkynes have the molecular formula CnH2n-2.
Step-by-step explanation:
Alkynes are a family of hydrocarbons that contain at least one carbon-carbon triple bond. This triple bond consists of one σ (sigma) bond and two π (pi) bonds. Based on the options provided, the characteristics that are not true for alkynes include:
They contain only single bonds. Alkynes have triple bonds between carbon atoms.
They contain only one σ bond. Alkynes actually have one σ bond and two π bonds in their triple bond.
Noncyclic alkynes do indeed have the molecular formula CnH2n-2 and some carbon chains can be linear and rod-like in shape.