The rice water turns blue-black on addition of a few drops of iodine solution. This indicates the presence of :

Question

Accepted Answer

The rice water turns blue-black on the addition of a few drops of iodine solution, indicating the presence of starch. This reaction is a well-known chemical test for starch, known as the iodine test. Here's a detailed explanation supporting why Option 4 is correct and why the other options are incorrect:

1. **Explanation for Option 4 (Starch):**
   - **Iodine Test for Starch:** When iodine solution is added to a substance containing starch, it reacts with the amylose component of starch to form a blue-black complex. This color change is a classic indicator of the presence of starch and is widely used in both educational and laboratory settings to test for starch in various substances.
   - **Chemical Basis:** The iodine molecules fit inside the helical structure of amylose, a component of starch, causing the color change. This specific interaction does not occur with other macromolecules like proteins or fats, making it a reliable test for starch.

2. **Why Option 1 (Fats) is Incorrect:**
   - **No Reaction with Iodine:** Fats do not react with iodine to produce a blue-black color. Instead, fats are typically tested using the Sudan III stain or by observing their solubility in non-polar solvents. The iodine test is not applicable for detecting fats.

3. **Why Option 2 (Cholesterol) is Incorrect:**
   - **No Reaction with Iodine:** Cholesterol, a type of lipid, does not react with iodine to produce a blue-black color. Cholesterol is usually detected using specific biochemical assays or colorimetric tests, but not with iodine.

4. **Why Option 3 (Complex Proteins) is Incorrect:**
   - **No Reaction with Iodine:** Proteins do not react with iodine to produce a blue-black color. Proteins are typically tested using the Biuret test, which involves a color change to violet in the presence of peptide bonds, not with iodine.

In conclusion, the blue-black color change upon adding iodine solution to rice water is a specific and well-documented reaction indicating the presence of starch, thereby confirming Option 4 as the correct answer.