Terminal alkynes, such as 1-butyne, possess weakly acidic hydrogen atoms attached to the $sp$ hybridised carbon atoms. When treated with a strong base like sodamide ($\text{NaNH}_2$), they undergo an acid-base reaction to form a sodium acetylide salt and liberate ammonia gas . Internal alkynes, such as 2-butyne, lack these acidic hydrogen atoms and therefore do not react with sodamide . Thus, $\text{NaNH}_2$ serves as a distinguishing reagent between terminal and internal alkynes.