When iodine comes into contact with starch, which of the following observations is made?

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

Answer: (D)

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, resulting in a color change.
3. Recall that the characteristic observation when iodine contacts starch is the appearance of a deep blue-black color, indicating the formation of this complex.
4. Note that the other options (colorless solution, white gas release, red precipitate) do not correspond to the known iodine-starch reaction.
5. Conclude that the deep blue-black color is a qualitative test commonly used to detect the presence of starch in a sample.