Carbon reacts with hot concentrated nitric acid then conc. nitric acid oxidises non-metal carbon to their corresponding oxy acid and itself gets reduced to Brown coloured gas nitrogen dioxide. $\mathrm{C}+4\mathrm{HNO}_{3\text{conc.}}\rightarrow\mathrm{CO}_2+2\mathrm{H}_2\mathrm{O}+4\mathrm{NO}_2$