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 conclusion drawn is (a) hair contains amino acids.

Step 1: Understanding the Reaction

When human hair is heated with soda-lime, it decomposes. Soda-lime is a chemical mixture that can promote decomposition reactions by providing a strong base and a heating source.

Step 2: Analyzing the Presence of Nitrogen Compounds

Human hair is primarily composed of keratin, which is a protein made of amino acids. Amino acids contain nitrogen in the form of amine groups (-NH2). When heated strongly, these nitrogen-containing compounds can decompose and release nitrogen-based gases.

Step 3: Identifying Released Gases

Upon heating, the amine groups in amino acids can be released as ammonia gas (NH3). The characteristic smell of ammonia is detected, indicating the presence of nitrogen as part of the protein's amino acid structure.

Step 4: Conclusion from Reaction Observation

The detection of ammonia gas upon the heating process indicates that the source of ammonia is likely from the decomposition of amino acids present in the hair, not directly from ammonia or ammonium salts.

Key Concepts

Soda-lime reaction

Soda-lime is a common chemical mixture that comprises calcium oxide (quicklime) and sodium hydroxide (lye). This combination serves as a strong base and is often used to facilitate decomposition reactions by providing both heat and an alkaline environment.
When you heat human hair with soda-lime, it starts a chemical reaction that breaks down the complex compounds in the hair. This process is called a soda-lime reaction.

• Heat and Base Effect: The heat helps speed up the decomposition, while the strong base breaks down acidic components in the hair.
• Use in Experiments: Soda-lime is particularly useful in laboratory settings to identify elements and compounds within substances by observing the gases released.

In the context of hair analysis, the soda-lime reaction helps to release and identify gases like ammonia, which is derived from amino acids.

Amino acids

Amino acids are the fundamental building blocks of proteins. They are made up of carbon, hydrogen, oxygen, and nitrogen. The presence of nitrogen is particularly important because it forms part of the amine group (-NH2) in amino acids.
A prose explanation of amino acids includes:

• Structure: Each amino acid has a central carbon attached to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable "R" group or side chain.
• Function: They are essential for various biological processes, including building proteins like keratin in hair.
• Nitrogen Content: Due to the presence of the amine group, amino acids release nitrogen-containing gases when decomposed, like ammonia.

Recognizing the significance of amino acids helps to understand why ammonia gas can be released when hair decomposes under heat with soda-lime.

Keratin decomposition

Keratin is a tough, fibrous protein that forms the structure of hair, nails, and the outer layer of skin. In human hair, keratin provides strength and resilience, thanks to its complex amino acid composition.
Upon heating:

• Decomposition Process: When heated, keratin breaks down rapidly because the heat disrupts the chemical bonds between its amino acids.
• Role of Amino Acids: The breakdown involves the destabilization of the nitrogen-containing amine groups, which then release ammonia gas.
• Observable Outcome: The smell of ammonia during decomposition indicates that the protein matrix has broken down, proving protein presence.

This understanding of keratin decomposition is crucial to conclude that the release of ammonia signals the breakdown of amino acids within keratin, not the presence of ammonia itself in hair.

Ammonia release

The release of ammonia gas is a key indicator in decomposition reactions involving substances that contain nitrogen, like amino acids in proteins. Ammonia (NH3) is characterized by its distinctive smell, which is easily detected, even in small quantities.
During the process:

• Source of Ammonia: The smell of ammonia suggests it is released from the decomposition of nitrogen in amino acids rather than being originally present in the material.
• Detection: The presence of ammonia is used as an identifier for nitrogenous compounds like amino acids in laboratory experiments.
• Applications: Understanding ammonia release can help in forensic analysis, waste treatment, and even agriculture.

This detailed view of how ammonia release occurs in decomposition helps clarify why it is related to the breakdown of amino acids, rather than ammonia being part of the substance initially.