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

(a) starch

Step 1: Understanding the Iodine Test

The iodine test is used to detect the presence of starch. When iodine solution is added to a substance, it will turn blue-black in color if starch is present.

Step 2: Evaluating Each Option

- **Starch:** Starch is a polysaccharide that reacts with iodine, resulting in a blue-black color. - **Glycogen:** Glycogen, although similar to starch, reacts differently and gives a reddish-brown color with iodine. - **Polypeptides:** These are chains of amino acids and do not react with iodine. - **Glucose:** A simple sugar that does not change color with iodine.

Step 3: Identifying the Correct Answer

Starch is the only one among the options that produces a blue-black color with iodine, indicating a positive iodine test.

Key Concepts

Starch detection

Detecting the presence of starch can be quite simple thanks to the iodine test. This chemical method is widely used in laboratories to identify whether a substance contains starch, which is a common carbohydrate found in plants. When performing the iodine test, a few drops of iodine solution are added to the substance. If starch is present, the solution will change to a distinctive blue-black color.
This color change happens because iodine molecules fit neatly into the helical structure of amylose, a component of starch. The result is a complex that absorbs light differently, giving the blue-black appearance.
Students learning this test can easily observe this reaction visually, making it a popular educational tool to demonstrate starch presence.

• Useful in both school experiments and industry applications.
• Simple to carry out with immediate results.

Polysaccharides

Polysaccharides are large carbohydrate molecules composed of many monosaccharide units linked together. Starch, cellulose, and glycogen are all examples of polysaccharides. Starch, in particular, is crucial for energy storage in plants, which is why it's often the target of the iodine test.
Polysaccharides have unique properties due to their size and complexity. Structurally, starch consists of two components: amylose and amylopectin. Amylose is a linear chain, while amylopectin is branched. This structural variability gives starch the ability to interact uniquely with iodine.
Understanding polysaccharides involves knowing:

• How they form long chains through glycosidic bonds.
• Their roles in biological systems, providing energy storage or structural support.
• Why their diverse structures result in different chemical reactions, as seen with the iodine test.

Recognizing these properties helps explain not only the biology of plants but also how we can identify substances like starch using simple tests.

Chemical reaction with iodine

The iodine test relies on a specific chemical reaction that occurs between iodine and the polysaccharide starch. This reaction is not seen with other similar polysaccharides or simpler sugars, which highlights the importance of molecular structure in chemical processes.
When iodine is added to starch, it slips into the coils of the amylose, leading to a color change. This reaction is specific to starch and a few other polysaccharides that have an appropriate structure, like glycogen, though it reacts differently. For instance, glycogen produces a reddish-brown color instead of the blue-black seen with starch.
Other substances like polypeptides or glucose do not provide any color change, demonstrating a negative iodine test. This specificity is valuable:

• It provides a clear indication of starch presence.
• Offers insights into the molecular interactions at play.
• Reinforces the importance of chemical structure in reactions.

Overall, the iodine test represents a simple yet powerful tool to explore chemical interactions and identify specific carbohydrates efficiently.