Animated Solution for Chemistry - Organic Chemistry: Biuret test is not given by

The Biuret test is a fundamental biochemical assay used to detect the presence of peptide bonds in a given sample. A peptide bond is a specific type of amide linkage, chemically represented as $-C(=O)-NH-$.

When a sample containing these bonds is treated with the Biuret reagent (which contains copper(II) sulfate in an alkaline solution), the $C{u}^{2+}$ ions form a coordination complex with the unshared electron pairs of the peptide nitrogen atoms. This complexation results in a distinct color change from blue to a deep violet or purple, confirming a positive test.

Proteins and polypeptides are essentially long, complex polymers made up of amino acid monomers. These amino acids are stitched together end-to-end by peptide bonds.

Because proteins and polypeptides are packed with these $-C(=O)-NH-$ linkages, they readily complex with the copper ions in the reagent. Therefore, they will naturally and strongly give a positive Biuret test.

You might wonder, why is urea on the list? Urea has the chemical formula $N{H}_{2}CON{H}_2$. At first glance, it does not possess a true peptide bond connecting two amino acids.

However, there is a catch here. When urea is heated, two molecules of urea condense, releasing a molecule of ammonia, to form a new compound called biuret ($N{H}_{2}CONHCON{H}_2$). If you look closely at the structure of biuret, it contains the exact same $-C(=O)-NH-$ functional group found in proteins! Because of this structural mimicry, urea (upon heating) will also give a positive result in the Biuret test.

Finally, let's examine carbohydrates. By definition, carbohydrates are polyhydroxy aldehydes or ketones, composed entirely of carbon, hydrogen, and oxygen.

They completely lack nitrogen atoms, which means it is chemically impossible for them to form an amide or peptide linkage. Without the $-C(=O)-NH-$ group, there are no nitrogen lone pairs to coordinate with the copper ions. Consequently, the solution remains blue.

Therefore, carbohydrates will never give a positive Biuret test. This makes option (a) the correct answer.