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: Understanding the Reaction

When human hair is heated strongly with soda-lime, the heat causes the decomposition of certain compounds present in the hair. Soda-lime acts as a catalyst in this process, leading to the release of gases that can include ammonia.

Step 2: Identifying Key Components in Hair

Human hair is primarily composed of a protein called keratin, which is made up of amino acids. These amino acids contain nitrogen atoms, which are crucial for the production of ammonia.

Step 3: Linking Ammonia Production to Amino Acids

During the heating process, the amino acids in keratin decompose, and their nitrogen atoms form ammonia (NH3) gas. The smell of ammonia implies that these nitrogen-containing compounds are present in the hair.

Step 4: Evaluating the Options

Option (a) states that hair contains amino acids, which is a known fact due to the protein content. Option (b) suggests the presence of ammonium salts, though less likely than amino acids. Option (c) implies ammonia is directly present, which is inaccurate because ammonia is formed during heating.

Step 5: Drawing the Conclusion

The smell of ammonia upon heating is most directly linked to the decomposition of amino acids. Thus, the correct conclusion from the given options is (a) hair contains amino acids.

Key Concepts

Keratin Protein

Human hair's primary component is a tough and fibrous protein called keratin. This protein is not unique to hair; it's also found in nails and the outer layer of the skin. Keratin is made up of smaller units called amino acids, which are essentially the building blocks of proteins.

• Keratin is rich in the amino acid cysteine, which contains sulfur. These sulfur atoms form disulfide bridges, giving keratin its strength and resilience.
• Keratin's structure is what contributes to the durability and elasticity of hair, helping it withstand wear and damage.

Because keratin contains amino acids, it means these amino acids are inherently present in the hair. This is important because these amino acids can break down under certain conditions, releasing other compounds.

Ammonia Production

Ammonia, a compound with the formula NH3, is often noticed by its distinctive and sharp smell. The production of ammonia from hair primarily involves the breakdown of amino acids within keratin.

• Amino acids contain nitrogen atoms, which are released when these molecules are degraded through a process known as decomposition.
• When human hair is heated, these nitrogen atoms can combine with hydrogen to form ammonia gas.

This chemical process is why the smell of ammonia can be detected when hair is subjected to certain conditions. This does not mean that ammonia is naturally found in hair but rather that it is produced as a result of heating and decomposition of amino acids within keratin.

Soda-Lime Reaction

The reaction of soda-lime with hair involves a chemical decomposition process. Soda-lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO), commonly used to absorb carbon dioxide, among other applications.

• When applying heat with soda-lime to hair, the substance acts as a catalyst to enhance the breakdown of compounds in the hair.
• Soda-lime helps in accelerating the decomposition of amino acids, leading to the quick release of gases like ammonia.

This reaction is key in experiments to determine the composition of hair or alter its physical properties, demonstrating the intricate interactions between hair's protein content and chemical agents.

Decomposition of Amino Acids

The decomposition of amino acids is a critical chemical process involved in the breakdown of organic matter. Amino acids, which build proteins like keratin, can break down through thermal or chemical processes.

• When heated, especially in the presence of catalysts like soda-lime, amino acids decompose, releasing their nitrogen content as ammonia.
• This reaction showcases the ability of amino acids to change under heat, demonstrating their reactive nature.

Understanding this process helps in explaining why proteins like keratin, when heated, can produce ammonia, providing insights into both protein chemistry and reactions that produce nitrogen-based compounds.