When HNO3 is dropped into the palm and washed with water it turns yellow It’s shows the presence of

NO2

NO

N2O

N2O5

Proteins and amino acids that contain phenyl rings form a yellow colored compound when treated with concentrated nitric acid. The yellowstains on the skin are caused due to the reaction of nitric acid with protein keratin present in the skin. This reaction is called xanthoproteic reaction.

Nitric acid takes yellowish brown colour due to the presence of dissolved nitrogen dioxide. Nitric acid slowly decomposes even at room temperature, specially in the presence of sunlight. Liberated nitrogen dioxide dissolves in the acid and gives a yellowcolour

Therefore NO2 or nitrogen dioxide is responsible for this change in colour