Question

How will you show that nitric acid contains Hydrogen, Nitrogen and Oxygen?

[Hint $\mathrm{Mg}+{\mathrm{HNO}}_3\to \mathrm{show}\mathrm{Hydrogen}$

$4{\mathrm{HNO}}_3\stackrel{∆}{\to }$ $4{\mathrm{NO}}_2+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$, show oxygen

brown ${\mathrm{NO}}_2$fumes show Nitrogen.

Answer 1

properties of Nitric acid:

Action on metals: Dilute Nitric acid reacts with metals like Magnesium , Manganese and give respective nitrates and Hydrogen gas.

$$\begin{gathered} \mathrm{Mg}\left(\mathrm{s}\right)+2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to \mathrm{Mg}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right) \\ \mathrm{Magnesium}\mathrm{Nitric}\mathrm{acid}\mathrm{Magnesium}\mathrm{nitrate}\mathrm{Hydrogen}\mathrm{gas} \end{gathered}$$

So evolution of Hydrogen gas when Nitric acid reacts with Magnesium metal indicates the presence of Hydrogen in Nitric acid.

Test for Nitric acid and Nitrates: Concentrated Nitric acid gives brown fumes of Nitrogen dioxide(${\mathrm{NO}}_2$) , steam and colorless Oxygen gas.

$4{\mathrm{HNO}}_3\left(\mathrm{l}\right)\stackrel{∆}{\to }$ $2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+4{\mathrm{NO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)$

Nitric acid water vapor Brown gas colorless gas.

Test for Oxygen: Colorless Oxygen gas rekindles a glowing splinter which confirms the presence of Oxygen in Nitric acid.

The evolution of brown fumes of Nitogen dioxide(${\mathrm{NO}}_2$) indicates the presence of Nitrogen in Nitric acid.

So reaction of Nitric acid with metals indicate the presence of Hydrogen and decomposition of Nitric acid gives test for the presence of Oxygen and Nitrogen in Nitric acid.