Chemical reactivity of alkynes : C≡C is less reactive than the carbon-carbon double bond towards electrophilic addition reaction. This is cause in alkyne carbon has more S-character so more strongly will be the attraction for Π electrons. Alkyne also undergoes nucleophilic addition with electron rich reagents. Ex. Addition of water, carboxylic acid, alcohols, cyanide. Nucleophilic addition can be explained on the basis that alkynes form vinylic carbanion which is more stable than alkyl carbanion formed by alkene
Vinylic carbanion
(more stable) 
(alkyl carbanion)
(less stable)
(i) Acidity of alkynes : Acetylene and other terminal alkynes (1- alkynes) are weakly acidic in character
Ex. 
(Monosodium acetylide)
The acetylenic hydrogen of alkynes can be replaced by copper (I) and silver (I) ions. They react with ammonical solutions of cuprous chloride and silver nitrate to form the corresponding copper and silver alkynides.
Dicopper acetylide (Red ppt)
Disilver acetylide (white ppt)
This reaction can be used to distinguish between 1-alkynes and 2-alkynes. 1-alkynes will provide this test while 2-alkynes, will not produce this test.
Explanation for the acidic character : It explained by sp hybridisation. We know that an electron in s orbital is more tightly held than in a p-orbital. In sp hybridisation s-character is more (50%) as compared to sp2 (33%) or sp3 (25%), due to large s-character the carbon atom is quite electronegative.
(ii) Reaction with formaldehyde
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