A, B and C are three biomolecules. The results of the tests performed on them are given below:
A: Mo

Q: Why doesn't Alanine give a positive Biuret test? Isn't it related to proteins?

While Alanine is an amino acid and a building block of proteins, the Biuret test specifically detects the presence of *peptide bonds* ($-CO-NH-$). A single amino acid molecule does not contain any peptide bonds, so it will not react with the Biuret reagent.

Q: What is the exact difference between Barfoed's test and Benedict's test?

Both tests use copper(II) ions to detect reducing sugars, but the pH makes the difference. Benedict's reagent is strongly alkaline and reacts with all reducing sugars (both mono- and disaccharides). Barfoed's reagent is slightly acidic, making it a weaker oxidizing agent. It only reacts quickly with the more reactive monosaccharides, allowing us to distinguish them from reducing disaccharides.

Q: Would sucrose give a positive Molisch test?

Yes! Sucrose is a carbohydrate (a disaccharide), and Molisch's test is a general test for almost all carbohydrates. It would yield the characteristic purple ring.

Q: Would sucrose give a positive Barfoed test?

No. Sucrose is a disaccharide, and Barfoed's test is only positive for monosaccharides. Furthermore, sucrose is a non-reducing sugar, so it wouldn't even react with stronger reagents like Benedict's or Fehling's.

Q: What exactly causes the color change in the Biuret test?

In an alkaline solution, the nitrogen atoms in the peptide bonds donate lone pairs of electrons to the copper(II) ions ($Cu^{2+}$) from the reagent. This forms a stable, violet-colored coordination complex. You need at least two peptide bonds in the molecule for this complex to form effectively.

Question

A, B and C are three biomolecules. The results of the tests performed on them are given below:

$\begin{array}{llll}
\hline & 	ext{Molisch's test} & 	ext{Barfoed test} & 	ext{Biuret test} \
\hline 	ext{A} & 	ext{Positive} & 	ext{Negative} & 	ext{Negative} \
	ext{B} & 	ext{Positive} & 	ext{Positive} & 	ext{Negative} \
	ext{C} & 	ext{Negative} & 	ext{Negative} & 	ext{Positive} \
\hline
\end{array}$

A, B and C are respectively:

Options:

(A) A = Lactose, B = Glucose, C = Alanine

(B) A = Glucose, B = Fructose, C = Albumin

(C) A = Lactose, B = Glucose, C = Albumin

(D) A = Lactose, B = Fructose, C = Alanine

Sigma Percentile · Accepted Answer

### The Lab Setup: Three Unknowns

Imagine you are standing in a chemistry laboratory, wearing your safety goggles, and you are handed three test tubes labeled A, B, and C. Your mission is to identify the biomolecules inside them using a series of classic chemical tests: Molisch's test, Barfoed's test, and the Biuret test.

These tests are the fundamental tools of a biochemist. They act like chemical interrogators, asking the molecules specific questions about their structure. Let's break down the results and decode the identities of A, B, and C.

### Molisch's Test: The Carbohydrate Gateway

The first test performed is **Molisch's test**. This is a general test for all carbohydrates (with the exception of some very small ones like trioses and tetroses). When a carbohydrate is treated with Molisch's reagent ($\alpha$-naphthol) and concentrated sulfuric acid, a beautiful purple ring forms at the junction of the liquids.

Looking at our data, both **A** and **B** give a positive Molisch's test. This immediately tells us that A and B belong to the carbohydrate family. On the other hand, **C** gives a negative result, meaning it is definitely not a carbohydrate.

### Barfoed's Test: The Monosaccharide Detector

Now that we know A and B are carbohydrates, we need to classify them further. Are they simple single-unit sugars (monosaccharides) or more complex multi-unit sugars (disaccharides or polysaccharides)? Enter **Barfoed's test**.

Barfoed's reagent contains copper(II) acetate in a slightly acidic medium. Because it is acidic, it is a weaker oxidizing agent than Benedict's or Fehling's reagents. It can only oxidize the highly reactive **monosaccharides** within a short time (usually 1-2 minutes), producing a brick-red precipitate of copper(I) oxide ($Cu_2O$). Disaccharides take much longer to react.

Our table shows that **B** gives a positive Barfoed's test, confirming it is a **monosaccharide** (like Glucose or Fructose). Sample **A** gives a negative Barfoed's test, indicating it is a more complex sugar, such as a **disaccharide** (like Lactose or Sucrose).

### Biuret Test: The Protein Identifier

Finally, we turn our attention to sample **C**. It failed the carbohydrate test but gave a positive result for the **Biuret test**.

The Biuret test is specifically designed to detect the presence of **peptide bonds** ($-CO-NH-$ linkages). When a compound containing two or more peptide bonds is treated with alkaline copper sulfate, it forms a violet-colored coordination complex with the copper ions.

Since C is positive for the Biuret test, it must be a **protein** (like Albumin).

*A common trap:* You might wonder why an amino acid like Alanine wouldn't give a positive Biuret test. While Alanine is the building block of proteins, a single amino acid molecule does not contain any peptide bonds. Therefore, Alanine will give a negative Biuret test.

### The Final Verdict

Let's summarize our findings:
*   **A** is a carbohydrate but not a monosaccharide $\Rightarrow$ **Disaccharide (e.g., Lactose)**
*   **B** is a carbohydrate and a monosaccharide $\Rightarrow$ **Monosaccharide (e.g., Glucose)**
*   **C** is not a carbohydrate but contains peptide bonds $\Rightarrow$ **Protein (e.g., Albumin)**

Matching this with our options, we find that **Option (c)** perfectly aligns with our chemical detective work: A = Lactose, B = Glucose, C = Albumin.

Select Answer:

Visualized Solution

\text{Biochemical Tests Overview}

\text{Molisch's Test}

\text{Barfoed's Test}

\text{Biuret Test}

\text{Conclusion}

\text{Further Exploration}

The Sigma Insight: Biomolecules

The Lab Setup

Molisch's Test

Barfoed's Test

Biuret Test

The Final Verdict

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