Q.What happens when CH≡CH reacts with dil. H2SO4 in presence of HgSO4 ? What is the role of HgSO4 in the reaction?

$HgSO_4$-catalysed hydration of acetylene with dilute $H_2SO_4$ gives acetaldehyde via a transient vinyl alcohol (enol) intermediate that tautomerises.

Acetylene (ethyne, $CH\equiv CH$) reacts with water in the presence of dilute sulphuric acid, using mercuric sulphate ($HgSO_4$) as a catalyst, in an addition reaction following Markovnikov's rule (the terminal, unsubstituted alkyne here is symmetrical, so a single unambiguous product is obtained):

$CH\equiv CH + H_2O \xrightarrow[H_2SO_4]{HgSO_4} [CH_2=CH-OH] \xrightarrow{\text{tautomerises}} CH_3-CHO$

The initial product of addition of water across the triple bond is vinyl alcohol ($CH_2=CH-OH$), an unstable enol which cannot be isolated because it rapidly tautomerises (via a 1,3-proton shift, keto-enol tautomerism) to the thermodynamically far more stable carbonyl (keto) form — acetaldehyde, $CH_3CHO$.

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