Acidity of C–H bonds is determined by the s-character of the carbon orbital involved in the bond. Higher s-character means the electrons are held closer to the nucleus, stabilising the conjugate base (carbanion) and thus increasing acidity. sp carbons in alkynes have 50% s-character (most electronegative), sp² carbons in alkenes have 33% s-character, and sp³ carbons in alkanes have 25% s-character. Therefore the order of acidity is: Alkyne (pKa ≈ 25) > Alkene (pKa ≈ 44) > Alkane (pKa ≈ 50). Terminal alkynes (R–C≡C–H) are weakly acidic enough to react with strong bases like NaNH₂ to form acetylide ions.