Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

When pieces of human hair are heated strongly with soda-lime, smell of ammonia can be detected. Which one of the following conclusion can be drawn from this observation? (a) hair contains amino acids (b) ammonium salt is present in hair (c) ammonia is present in the human hair (d) none of the above

Step-by-Step Solution

Answer

The correct conclusion is (a) hair contains amino acids.

Step 1: Understand the Reaction

When human hair is heated with soda-lime, a chemical reaction occurs that releases specific gases. Soda-lime acts as a base and a source of sodium ions.

Step 2: Analyze Ammonia Formation

The formation of the smell of ammonia indicates that nitrogen-containing compounds break down to form ammonia. This is typically an indication of the presence of amino groups in the original material.

Step 3: Identify Amino Acid Components

Human hair is primarily made up of a protein called keratin. Proteins are made up of amino acids, which contain amino groups 8-CO-NH29. Upon heating, the amino acids decompose, releasing ammonia.

Step 4: Evaluate Option (a)

Option (a) states that hair contains amino acids. This aligns with the protein structure of hair and the process of ammonia release during heating.

Step 5: Evaluate Other Options

Option (b) suggests that ammonium salt is present, which is less likely since ammonium salts typically decompose to release ammonia without requiring the breakdown of organic structures like hair. Option (c) implies ammonia is directly present in hair, which is incorrect as it is formed by decomposition.

Step 6: Draw a Conclusion

Based on the analysis, the release of ammonia due to heating indicates the presence of amino acids in hair, which supports option (a).

Key Concepts

Chemical Reaction with Soda-Lime

When human hair is exposed to heat in the presence of soda-lime, a specific chemical reaction occurs. Soda-lime, a mixture commonly containing sodium hydroxide and either calcium oxide or lime, acts as a strong base in this scenario. It is frequently used in various chemical applications due to its ability to absorb moisture and carbon dioxide, but here its role is as a reactive agent.

In the context of hair, soda-lime facilitates the breakdown of more complex molecules within the hair structure. When heated, soda-lime reacts with the organic material in hair, initiating a breakdown process. This reaction typically results in the formation of simpler compounds, one of which is ammonia. The detection of ammonia through its distinctive smell serves as evidence of this chemical transformation.

Decomposition of Amino Acids

Amino acids, the building blocks of proteins, play a crucial role in hair structure. When hair is heated with soda-lime, it undergoes a process called decomposition. This process breaks down the amino acids present in the hair.

Amino acids contain nitrogen in the form of amino groups \(NH_2\). When heated, these amino groups decompose, and the nitrogen atoms are released in the form of ammonia gas \(NH_3\). This transformation is what causes the noticeable smell of ammonia during the heating process.

This occurrence confirms that amino acids were present in the original material, as hair consists largely of the protein keratin, which is made up of long chains of these amino acids. The decomposition and release of ammonia serve as an indirect indication of the amino acids being broken down.

Presence of Keratin in Hair

Keratin is a tough, fibrous protein that forms the structural foundation of human hair. It provides strength, resilience, and elasticity.

Keratin is composed of long chains of amino acids, linking together like beads on a string to form complex protein structures. Each of these amino acids includes an amino group which is crucial to the keratin's function and stability.

When hair is heated in the presence of soda-lime, it’s the keratin that's primarily affected. The heat breaks down the protein, splitting the amino acids and resulting in the release of ammonia gas.

Understanding the presence of keratin and its composition is vital, as it explains why ammonia is released when hair is heated. Without keratin, the results observed during the heating process with soda-lime would be fundamentally different.