$ZnO \cdot CoO$ is called

The colour of the bead in the borax bead test is mainly due to the formation of:

To support the tungsten filament in an electric bulb,the metal used is:

Fill in the blanks:
$(1)$ In the dichromate ion,the $O-Cr-O$ bond angle is ....... $^o$.
$(2)$ The reaction $3Sn^{2+} \to 3Sn^{4+} + 6e^-$ occurs via .........,in which $Sn^{2+}$ is ......... .
$(3)$ The symbols for stannous and stannic are ......... and ......... respectively.
$(4)$ $V_2O_4$ dissolves in acidic compounds to give ......... salts.

Provide the structures for:
$1.$ $MnO_4^{-}$
$2.$ $MnO_4^{2-}$

How is sodium chromate converted into sodium dichromate in the manufacture of potassium dichromate from chromite ore?