Proteins when heated with conc. `HNO_3` give a yellow colour. This is

Options

A. oxidising test
B. xanthoprotein test
C. Hoppe's test
D. acid base test

Explanation

To solve the question regarding the reaction of proteins when heated with concentrated nitric acid, we can follow these steps:

### Step 1: Understand the Reaction
When proteins are heated with concentrated nitric acid, they undergo a chemical reaction. This reaction is significant because it helps identify certain components of the protein.

### Step 2: Identify the Color Change
The key observation in this reaction is the development of a yellow color. This color change is crucial for identifying the type of test being performed.

### Step 3: Recognize the Test
The yellow color produced when proteins react with concentrated nitric acid is indicative of the xanthoproteic test. This test specifically detects the presence of aromatic amino acids, such as tyrosine and tryptophan, in proteins.

### Step 4: Conclusion
Based on the observations and the nature of the reaction, we conclude that the correct answer to the question is the xanthoproteic test.

### Final Answer
Proteins when heated with concentrated `HNO_3` give a yellow color. This is the **xanthoproteic test**.

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