Among the following oxides of $3d$ elements, the number of mixed oxides are . . . . . .
$Ti_{2}O_{3}, V_{2}O_{4}, Cr_{2}O_{3}, Mn_{3}O_{4}, Fe_{3}O_{4}, Fe_{2}O_{3}, Co_{3}O_{4}$

When dilute $H_2SO_4$ is added to an aqueous solution of potassium chromate,the yellow color of the solution turns to orange. What does this indicate?

$Cu^{2+}$ aqueous solution has . . . . . . colour.

$Fe_2Cr_2O_4 + Na_2CO_3 + O_2$ $\xrightarrow{\text{Fusion}} [X]$ $\xrightarrow[H_2O]{H^{+}} [Y]$ $\xrightarrow{H_2O_2/H^{+}} [Z]$
Which of the following statements is true for the compounds $[X]$,$[Y]$,and $[Z]$?

Superconductors are derived from compounds of

Which oxide of transition metals exhibits electrical conductivity as high as that of metals?