When ethyne is treated with H2SO4 in presence of HgSO4, then addition of water to ethyne takes place. This reaction is generally known as hydration reaction.
The water molecule under the mentioned conditions add to the tripple bond in order to form an enol intermediate. This intermediates is shot lived which readily undergoes tautomerisation to form acetaldehyde i.e. ethanal.
The net equation for the above reaction is as following
C2H2 -----------> CH3CHO