Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Iodine test is shown by (a) starch (b) glycogen (c) polypeptides (d) glucose

Step-by-Step Solution

Answer

Starch shows a positive iodine test.

Step 1: Understand the Iodine Test

The iodine test is a chemical test used to detect the presence of starch. When iodine solution is added to a substance containing starch, it turns a dark blue or black color. This is due to the formation of a starch-iodine complex.

Step 2: Evaluate Each Option

Consider each substance listed in the options to see if it typically reacts with iodine. - (a) Starch is a polysaccharide and does react with iodine, resulting in a blue-black color. - (b) Glycogen is also a polysaccharide similar to starch, but its reaction with iodine is less intense and results in a brownish-red color. - (c) Polypeptides, which are chains of amino acids, do not react with iodine. - (d) Glucose is a monosaccharide and does not react with iodine to show the blue-black color.

Step 3: Identify the Correct Option

Based on the iodine test properties and the evaluation of each option, starch is the only option that reacts with iodine to give a distinct blue-black color. Thus, starch shows a positive reaction in the iodine test.

Key Concepts

Starch Detection

The iodine test is an essential primary method to detect the presence of starch in a sample. In this test, iodine solution is added to the substance being tested. If starch is present, the color of the sample will change to a dark blue or black.
The color change provides a clear visual indicator, which is useful in various fields, from biology to food quality testing.
During the test:

• A starch-iodine complex is formed, leading to the dark coloration.
• This indicates the presence and concentration of starch in the sample.

Because of its simplicity, the iodine test is popular in educational settings to illustrate principles of detection and analysis.

Chemical Reactions

Chemical reactions involve changes in the structure of molecules, forming new substances. In the context of the iodine test, a chemical reaction occurs between the iodine and the starch molecule.
The reaction between iodine and starch is not a chemical bond formation. Instead, it is a physical change that results in a visible color change.
This can be summarized as:

• Iodine molecules fit inside the helical structure of amylose, a component of starch.
• This interaction reflects light differently, leading to the blue-black color.

Understanding chemical reactions like this helps us make connections between molecular structures and observable physical changes.

Polysaccharides

Polysaccharides are long chains of monosaccharide molecules. They are carbohydrates that include starch, glycogen, and cellulose. These large molecules serve as energy storage or structural components in plants and animals.
For the iodine test:

• Starch, a plant-based polysaccharide, shows a strong reaction with iodine.
• Glycogen, an animal-based polysaccharide, shows a weaker reaction, indicating molecular differences.

These reactions highlight the diversity and complexity of polysaccharides, illustrating how slight changes in structure can influence chemical behavior.

Complex Formation

When iodine is added to starch, it results in a transient complex formation, crucial for the color change observed in the iodine test. This happens because iodine gets trapped in the helical structure of amylose within the starch.
This formation is not due to a chemical bond, but rather:

• A physical interaction that causes light absorption changes, producing the dark blue-black color.
• The complex is temporary and reversible, reverting back when iodine is washed out.

This concept is important for comprehending how molecules can interact in multiple ways, leading to different visible outcomes without permanent changes to the molecules themselves.