Identification of unknowns with varying solubilities and states of matter White solid A water insoluble cleaved hot to solid gas B and C. B is insoluble in water and in $\ce{HCl}$ but soluble to $\ce{HNO3}$ the react gives solution D and E colorless gas which becomes red on contact with air. The solution D gave white precipitate F with $\ce{HCl}$. The gas C is reacted by heating with magnesium G yielding a white solid which reacts with water yielding a white solid and the gas J characteristic suffocating odor. J is a basic , H is soluble in dilute $\ce{H2SO4}$. The gas J oxidized to C while the gas under specific conditions ($\ce{Pt}$ catalyst and $800^\circ\mathrm{C}$) is oxidized to the gas E.


My attempt:
C: $\ce{N2}$,
G: $\ce{Mg3N2}$,
H: $\ce{Mg(OH)2}$,
J: $\ce{NH3}$,
E: $\ce{NO}$
\begin{aligned}\ce{ 
N2 + 3Mg &amp;-&gt; Mg3N2\\
 Mg3N2 + 6H2O &amp;-&gt; 3Mg(OH)2 + 2NH3\\
 Mg(OH)2 + H2SO4 &amp;&lt;=&gt; 2 H2O + MgSO4\\
 4 NH3 + 5 O2 &amp;-&gt;[\ce{Pt~$800^\circ\mathrm{C}$}] 6 H2O + 4 NO\\
 4 NH3 + 3 O2 &amp;-&gt; 6 H2O + 2 N2\\
}\end{aligned} 

I cannot really find anything else.
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.

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