Identification of unknowns with varying solubilities and states of matter
The solutions which you have already found look good so far. For solid A, we need to find a compound which thermally decomposes into $\ce{N2}$ and an element which is only soluble in oxidizing acids like $\ce{HNO3}$, and whose chloride is insoluble in water or acids. Silver azide could possibly be a good fit, as it can be cleaved into $\ce{Ag}$ and $\ce{N2}$ by heating, and silver chloride has a very low solubility in water. Care would have to be taken with the temperature when actually performing the decomposition of $\ce{AgN3}$, as it is explosive.

$$\ce{2AgN3 -&gt; 3N2\uparrow + 2Ag}$$

Dissolving silver (solid B) in nitric acid gives a silver nitrate solution D and $\ce{NO}$ as colorless gas E.

$$\ce{3Ag + 4HNO3 -&gt; 3AgNO3 + 2H2O + NO\uparrow}$$

$\ce{NO}$ turns red on contact with air when it is oxidized to nitrogen dioxide by atmospheric oxygen. 

$$\ce{2NO + O2 &lt;=&gt; 2NO2}$$

From solution D, insoluble $\ce{AgCl}$ (F) precipitates when $\ce{HCl}$ is added.

$$\ce{Ag+ + Cl- -&gt; AgCl\downarrow}$$