$\left[X\right]+H_{2}S{O}_4\to \left[Y\right]$ , Y is a colorless gas with irritating smell.
$\left[Y\right]+K_{2}C{r}_2{O}_7+H_{2}S{O}_4\to$Green solution ,with no coloured precipitate formation

  X and Y are
 

Sulphite gives $S{O}_2$  with $H_{2}S{O}_4$  which turns acidified $K_{2}C{r}_2{O}_7$  green according to following reaction :
$K_{2}C{r}_2{O}_7+H_{2}S{O}_4+3S{O}_2\to K_{2}S{O}_4+C{r}_2{\left(S{O}_4\right)}_3\left(green\right)+H_{2}O$$H_{2}S$ also gives green coloured solution with acidified $K_{2}C{r}_2{O}_7$  solution but a slightly coloured precipitate of sulphur is also obtained.  The $H_{2}S$  gas has rotten egg smell.