The copper $(II)$ halide which does not exist is:

The nature of oxides $V_{2}O_{3}$ and $CrO$ is indexed as '$X$' and '$Y$' type respectively. The correct set of $X$ and $Y$ is:

Iron becomes passive by ..................... due to formation of .....................

Iron is rendered passive by the action of

Ferrous sulphate on heating gives

The highest oxidation state of manganese in fluoride is $+4$ $(MnF_4)$,but the highest oxidation state in oxides is $+7$ $(Mn_2O_7)$ because __