Although the acidity is very, very weak, primary alkynes, unlike alkanes and alkenes, are weakly acidic? Why are primary alkynes weakly acidic?

The C-H bonding electrons in acetylene are contained in an sp hybrid orbital. In the conjugate base of acetylene, acetylide carbanion, this sp hybrid orbital holds an unshared electron pair. The sp hybrid orbital holding this unshared electron pair has greater s character compared to the sp2 or sp3 orbitals holding an electron pair in analogous vinylic or aliphatic carbanions. This allows the unshared electron pair to be held at lower energy, which allows acetylene to have measurable, though still very weak, acidity.

With more s character, the acetylide carbanion holds the negative charge closer in to the carbon nucleus, a decrease in electrostatic potential energy (and by extention enthalpy and free energy), which makes the equilibrium favorable enough to be of practical laboratory benefit. Anions of acetylene and terminal alkynes are useful nucleophiles.