Which of the following does not give oxygen on heating?

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

Provide the structures of the following ions:
$1.$ $CrO_4^{2-}$
$2.$ $Cr_2O_7^{2-}$

The transition metal ions responsible for colour in ruby and emerald are,respectively:

One of the important uses of ferrous sulphate is in the

$X$ is an oxoanion of the lightest element of group $7$ in the periodic table. The metal is in $+6$ oxidation state in $X$. The color of the potassium salt of $X$ is