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

The iodine test is shown by (a) starch.

Step 1: Understanding the Iodine Test

The iodine test is a chemical reaction used to test for the presence of starch. In this test, iodine solution, which is typically made by dissolving iodine in an aqueous solution of potassium iodide, is added to the test substance. If starch is present, the solution will turn a deep blue-black color.

Step 2: Analyzing Each Option

Let's analyze each option: (a) Starch is a polysaccharide, and it will turn blue-black with the iodine test. (b) Glycogen is a branched polysaccharide and might show a slight reaction but is not a standard test for glycogen. (c) Polypeptides are chains of amino acids, which do not react with iodine to produce a color change. (d) Glucose is a monosaccharide and does not react with iodine to show any color change.

Step 3: Identifying the Correct Answer

Based on the iodine test's reaction, starch is the substance that shows a significant color change (blue-black) with iodine. This is because of the particular structure of starch, which traps iodine molecules causing the visible color change.

Key Concepts

Starch

Starch is a well-known carbohydrate and serves as a major energy storage molecule in plants. It is a polysaccharide composed of a large number of glucose units linked together. The iodine test is particularly associated with starch due to its unique helical structure. When iodine is introduced to starch, the iodine molecule fits neatly into the helical structure, leading to a characteristic blue-black coloration. This reaction is highly specific to starch, making it a reliable test for detecting its presence in various substances.

Glycogen

Glycogen is often referred to as the animal equivalent of starch. It is a highly branched polysaccharide primarily stored in the liver and muscles of animals. Glycogen serves as a quick-release form of energy when it is broken down into glucose. Despite being similar to starch, glycogen does not interact with iodine in the same distinctive way. It may produce a slight reddish-brown color, but this change is less noticeable and is not a standard test for glycogen. Thus, for iodine testing purposes, glycogen yields minimal reaction compared to starch.

Polypeptides

Polypeptides are chains of amino acids linked by peptide bonds and are the building blocks of proteins. Unlike carbohydrates like starch and glycogen, polypeptides do not contain glucose units or a structure that interacts with iodine. As a result, when polypeptides are subjected to the iodine test, no color change occurs. Polypeptides are best analyzed with different types of tests, such as the Biuret test, which detects peptide bonds by changing color in the presence of proteins.

Glucose

Glucose is a simple sugar, classified as a monosaccharide, and is a vital energy source for cells. Being a basic building block of carbohydrates, glucose itself does not form the complex structures necessary for the iodine test reaction. Hence, it will not result in any color change when iodine is added. Glucose can be identified through other biochemical methods, such as the Benedict's test, where a color change occurs in the presence of reducing sugars, providing an alternative way to detect glucose.