Question

Skin becomes yellow in conc. ${\text{HN}}{{\text{O}}_{\text{3}}}$ because:
A) The proteins are converted into xanthoproteins.
B) ${\text{HN}}{{\text{O}}_{\text{3}}}$ acts as dehydrating agent
C) Nitrocellulose is formed
D) ${\text{HN}}{{\text{O}}_{\text{3}}}$ acts as an oxidising agent.

Answer 1

We know that ${\text{HN}}{{\text{O}}_{\text{3}}}$ is nitric acid. Pure nitric acid is colourless but older samples turn yellow in colour. This is because the acid decomposes into oxides of nitrogen and water.

Complete step by step answer:
We know that ${\text{HN}}{{\text{O}}_{\text{3}}}$ is nitric acid. Nitric acid is also known as aqua fortis or spirit of niter. Nitric acid is a mineral acid and it is highly corrosive in nature.
Nitric acid generally reacts with proteins and forms yellow coloured nitrated products. This reaction is known as xanthoproteic reaction.
Skin contains proteins that contain aromatic amino acids. When skin comes in contact with concentrated nitric acid the proteins are converted into xanthoproteins. The xanthoproteins are yellow in colour. And thus, the skin becomes yellow.
Thus, skin becomes yellow in conc. ${\text{HN}}{{\text{O}}_{\text{3}}}$ because the proteins are converted into xanthoproteins.
Thus, the correct option is (A).

Additional Information: Xanthoproteic reaction is used to detect the presence of proteins soluble in a solution by using concentrated nitric acid. Xanthoproteic reaction gives positive results in case of proteins containing aromatic amino acids. The positive test is proofed by neutralizing with a base. The base then turns the solution dark yellow or orange in colour. Xanthoproteic reaction is a qualitative test used to detect the presence or absence of proteins.

Note: Skin becomes yellow in concentrated nitric acid due to the formation of xanthoproteic acid. Xanthoproteic acid is produced by the nitration of aromatic amino acids. Examples of aromatic amino acids are tyrosine and tryptophan.