Unsaturated compounds like ethyne readily undergo addition reactions where a molecule adds across the double or triple bond. 

Ethyne reacts with HI in a two-step process, with the first step forming an intermediate with a carbon-carbon double bond, and the second step forming the final product, 1,1-diiodoethane. 

 First step :  H-C≡C-H  +  HI  →  H\(_2\)C=CHI 

 Second step: H\(_2\)C=CHI  +  HI → H\(_3\)C-CHI\(_2\)