Question

glucose and starch are both carbohydrates. why does glucose produce a negative result when testing using iodine?

a. iodine only tests for polysaccharides, and glucose is a monosaccharide.
b. the glucose solution is not concentrated enough.
c. the iodine test is not sensitive enough to detect glucose.

Explanation:

Understand the chemical specificity of the test

Using the Iodine Test and Starch Detection knowledge points

• Iodine reagent (\(\text{I}_3^-\) complex) specifically interacts with the helical structure of amylose (a component of starch) to produce a dark blue-black color.
• This physical entrapment of iodine inside the carbohydrate helix requires long, coiled polysaccharide chains.

Analyze the structure of glucose

• Glucose is a simple single-ring sugar, classified structurally as a monosaccharide.
• Because it lacks the long, coiled polymer chains of a polysaccharide, it cannot physically trap iodine molecules to produce a color change.

Evaluate the given options

• A: Correctly states that iodine specifically tests for complex polysaccharides (like starch), whereas glucose is a monosaccharide.
• B: Incorrect because concentration is not the issue; even highly concentrated glucose will not react with iodine.
• C: Incorrect because the test is chemically designed for starch, not a matter of sensitivity threshold for glucose.

Answer:

• A. Iodine only tests for polysaccharides, and glucose is a monosaccharide. (Correct answer)
• B. The glucose solution is not concentrated enough.
• C. The iodine test is not sensitive enough to detect glucose.