Which of the following gases present in the air blackens the surface painted with white lead?

Nitrogen gas is absorbed by which of the following substances at high temperature to form a cyanamide derivative?

$A$ crystalline solid $X$ reacts with dil. $HCl$ to liberate a gas $Y$. $Y$ decolourises acidified $KMnO_4$. When a gas $Z$ is slowly passed into an aqueous solution of $Y$,colloidal sulphur is obtained. $X$ and $Z$ could be,respectively

$SiO_{2}$ is attacked by which of the following?

$A$ red solid is insoluble in water. However,it becomes soluble if some $KI$ is added to water. Heating the red solid in a test tube results in the liberation of violet-coloured fumes and droplets of a metal appear on the cooler parts of the test tube. The red solid is:

It is observed that $Pb^{2+}$ is more stable than $Pb^{4+}$, while $Sn^{2+}$ is less stable than $Sn^{4+}$. Consider the following reactions:
$PbO_{2} + Pb \rightarrow 2PbO; \Delta_{r}G^{\circ} (1)$
$SnO_{2} + Sn \rightarrow 2SnO; \Delta_{r}G^{\circ} (2)$
Identify the correct set from the following.