Concentrated nitric acid upon long standing turns yellow brown due to formation of

Concentrated nitric acid turns yellow-brown upon long standing due to the formation of nitrogen dioxide (NO2). Here's the step-by-step process:

1. Nitric acid (HNO3) is a strong acid and a powerful oxidizing agent, meaning it has the ability to gain electrons in a chemical reaction.

2. When concentrated nitric acid is left standing, it decomposes slowly.

3. During this decomposition, nitric acid loses an oxygen atom and forms nitrogen dioxide (NO2).

4. Nitrogen dioxide is a brown gas, which gives the nitric acid a yellow-brown color.

5. The chemical equation for this reaction is: 4HNO3 → 2H2O + 4NO2 + O2

So, the yellow-brown color of concentrated nitric acid upon long standing is due to the formation of nitrogen dioxide.