The reagent used for toning in photography is

Oxidation states of the metal in the minerals haematite and magnetite,respectively,are

Choose the correct statements from the following:
$A$. $Mn_2O_7$ is an oil at room temperature.
$B$. $V_2O_4$ reacts with acid to give $VO^{2+}$ ions.
$C$. $CrO$ is a basic oxide.
$D$. $V_2O_5$ does not react with acid.
Choose the correct answer from the options given below:

Superconductors are derived from compounds of

The purple colour of $KMnO_4$ is due to the transition:

Identify $(A)$,$(B)$ and $(C)$ in the given reaction sequence: