When copper is added to concentrated $H_2SO_4$,$CuSO_4$ is produced along with another sulphur-containing compound $X$. The compound $X$ is

Thiosulphate reacts differently with iodine and bromine in the reactions given below:
$2 S_2 O_3^{2-} + I_2 \rightarrow S_4 O_6^{2-} + 2 I^{-}$
$S_2 O_3^{2-} + 4 Br_2 + 5 H_2 O \rightarrow 2 SO_4^{2-} + 8 Br^{-} + 10 H^{+}$
Which of the following statements justifies the above dual behaviour of thiosulphate?

To an acidified dichromate solution,a pinch of $Na_2O_2$ is added and shaken. What is observed?

$AgCl$ when heated with $Na_2CO_3$ gives:

When a manganous salt is fused with a mixture of $KNO_3$ and solid $NaOH$, the oxidation number of $Mn$ changes from $+2$ to:

In which of the following conversions does a blue colour appear?