Diamond and emerald are forms of which of the following respectively?

Which of the following species is not stable?

Which of the following species is not stable?

Bone black is a form of which element?

Silica is soluble in which of the following?

Rationalise the given statements and give chemical reactions:
$1$. Lead $(II)$ chloride reacts with $Cl_2$ to give $PbCl_4$.
$2$. Lead $(IV)$ chloride is highly unstable towards heat.
$3$. Lead is known not to form an iodide,$PbI_4$.