2. What observable change occurs when iodine comes into contact with starch?

• A. The solution turns colorless.
• B. A white gas is released.
• C. A blue-black color appears.
• D. A red precipitate forms.

Answer: (C)

Verified step by step guidance

1. Understand the chemical interaction between iodine and starch: iodine molecules fit inside the helical structure of amylose, a component of starch.
2. Recognize that this interaction causes a charge-transfer complex, which changes the way light is absorbed and reflected, producing a characteristic color.
3. Recall that the observable change when iodine contacts starch is the appearance of a blue-black color, which is a classic qualitative test for the presence of starch.
4. Note that other options such as the solution turning colorless, release of a white gas, or formation of a red precipitate do not correspond to the iodine-starch reaction.
5. Conclude that the blue-black color is the key indicator used in laboratories to confirm starch presence when iodine is added.