Corundum is

Match the following:

List-$I$	List-$II$
$A$. $Co^{2+}$	$I$. Yellow colour
$B$. $Fe^{2+}$	$II$. Dark green colour
$C$. $Ni^{2+}$	$III$. Blue colour
$D$. $Cu^{2+}$	$IV$. Pale green colour
	$V$. Pink colour

The correct answer is

On heating potassium permanganate,one of the following compounds is not obtained:

The number of terminal oxygen atoms present in the product $B$ obtained from the following reaction is $....$
$4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$
$2Na_2CrO_4 + 2H^{+} \rightarrow B + H_2O + 2Na^{+}$

Which of the following elements is doped into a fiber amplifier in an optical fiber communication system?

Which of the following statements is true regarding the structure of $Cr_2O_7^{2-}$?