Answer:
Sodium extract is boiled with conc. \[HN{{O}_{3}}\]to
decompose \[NaCN\]and \[N{{a}_{2}}S\] (if N and S are present).
\[NaCN+HN{{O}_{3}}\to
NaN{{O}_{3}}+HCN\uparrow \]
\[N{{a}_{2}}S+2HN{{O}_{3}}\to
2NaN{{O}_{3}}+{{H}_{2}}S\uparrow \]
Otherwise
these will react with \[AgN{{O}_{3}}\] solution to give precipitates and
interfere with the test.
\[NaCN+AgN{{O}_{3}}\to
\underset{\begin{smallmatrix}
Silver\,cyanide
\\
(white\,ppt.)
\end{smallmatrix}}{\mathop{AgCN}}\,+NaN{{O}_{3}}\]
\[N{{a}_{2}}S+2AgN{{O}_{3}}\to
\underset{\begin{smallmatrix}
Silver\,sulphide
\\
(Black\,ppt.)
\end{smallmatrix}}{\mathop{A{{g}_{2}}S}}\,+2NaN{{O}_{3}}\]
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