Question

Proteins when heated with concentrated nitric acid give a yellow colour. This is
A. oxidising test
B. xanthoproteic test
C. Hoppe's test
D. acid base test

Answer 1

Hint: Concentrated nitric acid gives yellow colour when heated with proteins.
This happens due to the nitration of specific amino acids.

Complete step-by-step answer:
Nitric acid is a strong mineral acid and highly corrosive as well.
It is an inorganic compound.
This acid in reaction with proteins forms certain products which are yellow.
This reaction is the xanthoproteic reaction.
Human skin comprises proteins that involve aromatic amino acids.
When our skin gets in touch with concentrated nitric acid these proteins change into xanthoproteic acids. These compounds are yellow.
The formation of xanthoproteic acid is due to the nitration of specific amino acids.
For example:- tyrosine, tryptophan, etc.
As a result, the skin turns yellow.
Thus, proteins when heated with concentrated nitric acid give yellow colour and this reaction is the xanthoproteic test.

So, option B is correct.

Additional Information: Xanthoproteic reaction is utilised to observe the existence of proteins in a solution with the help of concentrated nitric acid.
This reaction provides favourable results if the protein comprises aromatic amino acids.
This reaction is a qualitative test utilised for the detection of the existence or lack of proteins in a given solution.

Note: When a person drops nitric acid on their skin or nails, it becomes yellow after some duration which demonstrates the presence of protein.
The fingernails exhibit a bright yellow colour as these contain keratin, a protein.