Question

Write the balanced equation for nitric acid turning yellow for long standing

Answer 1

ANSWER

Nitric acid, upon exposure to light and heat, will spontaneously decompose to nitrogen dioxide, water, and oxygen, creating the brownish-yellow color. It's not actually completely decomposing unless you leave it for a long time (in which case it will eventually turn red), but there's enough dissolved nitrogen dioxide, which is what imparts the color, to give it a distinct hue.

The chemical equation is:

4HNO3→2H2O+4NO2+O24HNO3→2H2O+4NO2+O2

Most of the gas that escapes from nitric acid that's been exposed to air is nitrogen dioxide, hence the names "red fuming" nitric acid to describe nitric acid at high concentration that has been exposed to light.