Terminal alkynes `(R-C equiv C-H)` are weakly acidic in nature because of `sp`-hybridised state of the triple bonded carbon atom. Due to substantial s-character `(50 %)`, the triple bonded carbon is electronegative in nature and this enables the release of `H^(+)` ion form `equiv C-H` bond. However, ethyne is a stronger acid than propyne. This is attributed to the electron releasing `+ I` effect of the methyl group which tends to increase the electron density on the triple bonded carbon atom is propyne. the release of `H^(+)` ion from propyne is more difficult than from ethyne or it is a weaker acid than ethyne.
