State your observation when:

(i) nitric acid is added to saw dust

(ii) conc. nitric acid is heated (a) in absence of copper (b) in presence of copper.

(i) Saw dust bursts into flames when hot conc. nitric acid is poured on it. The reason is that Nitric acid is a strong oxidising agent. It decomposes to give nascent oxygen which oxidises organic compounds to carbon dioxide and water.

Saw dust [C₆H₁₀O₅]_n + Nitric acid [hot conc.] ⟶ carbon dioxide + water + nitrogen dioxide.

(ii) (a) When conc. nitric acid is heated in absence of copper, reddish brown fumes of nitrogen dioxide are evolved.

$4\text{HNO}$3 \xrightarrow{\Delta} \text{4NO}2 + \text{2H}\text{2}\text{O} + \text{O}2

(ii) (b) Cu reacts with conc. nitric acid liberating reddish brown fumes of nitrogen dioxide along with blue coloured solution of copper nitrate.

Cu + 4HNO₃ [conc.] ⟶ Cu(NO₃)₂ + 2H₂O + 2NO₂