Q.$HO-CH_2-CH_2-OH$ on heating with periodic acid gives :

Periodic acid cleaves the C–C bond of the vicinal diol $HO-CH_2-CH_2-OH$, converting each $-CH_2OH$ carbon into $-CHO$, so the product is methanal (HCHO); $HIO_4$ itself is reduced to $HIO_3$.

The general reaction of a 1,2-diol with periodic acid is $R_2C(OH)-C(OH)R_2' + HIO_4 \rightarrow R_2C=O + R_2'C=O + HIO_3 + H_2O$, cleaving the bond between the two hydroxyl-bearing carbons. For ethylene glycol, $HO-CH_2-CH_2-OH$, both carbons are $-CH_2OH$ (each bearing 2 H, 1 OH, and the C–C bond to be cleaved), so cleavage converts each into $H_2C=O$, i.e. methanal. No further oxidation to methanoic acid or $CO_2$ occurs under these specific cleavage conditions, and glyoxal (OHC–CHO) would only …