The test of ozone $O_3$ can be done by

Caro's acid is

Which element from the following exhibits the highest number of allotropes?

The commercial name of peroxydisulphuric acid is:

Given below are two statements:
Statement $I$: The boiling point of hydrides of Group $16$ elements follows the order
$H_2O > H_2Te > H_2Se > H_2S$.
Statement $II$: On the basis of molecular mass,$H_2O$ is expected to have a lower boiling point than the other members of the group,but due to the presence of extensive $H$-bonding in $H_2O$,it has a higher boiling point.
In the light of the above statements,choose the correct answer from the options given below:

When $H_2SO_4$ reacts with $PCl_5$,the obtained product$(s)$ is/are: