Acetylene is a terminal alkyne and carbon in acetylene (or in general in a alkyne) is sp hybridised. As in sp hybridisation s character is 50%(as compared to 25% in sp3 and 33.3% in sp2) and we know that the more is s character the more is the penetration power and hence more is the electroNegativity. Since in any TERMINAL ALKYNE LIKE ACETYLENE carbon is bonded only to a hydrogen which is fairly less electronegtive than carbon so the sp character of carbon allows it to hog or take the electrons of one hydrogen quite easily and in this process itself becomes negatively charged and therefore h+ is released. This makes the acetylene acidic (acidic means to give h+) Sodium +acetylene = sodium acetalinide (na c triple bond c na)+ h2 (sodium reacts only with water and acids to give hydrogen and both are acidic which again shows that acetylene is acidic in nature) Acetylene +tollens reagent = silver mirror of Ag (s)[gives positive tollens test which shows its acidic character]
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Acetylene shows acidic character because carbon is sp hybridized containing 50% s-character which makes -C-H bond highly acidic in nature. The acidic character of -C-H can be easily justified when heated with sodium metal which displaces hydrogen from acetylene forming sodium acetylide.
HC=CH+Na->HC= CNa = 1/2 H2
C2 H2 +Bu Li -> Li C2 H + Bu H
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