Sugar (like sucrose) doesn’t turn blue-black with iodine because iodine reacts specifically with starch, not with simple sugars like sugar. Iodine forms a blue-black complex with the helical structure of amylose, a type of starch. Sugars lack this structural feature, so no color change occurs.
Here’s a more detailed explanation:
Starch and Iodine:
Starch is a complex carbohydrate made of many glucose molecules linked together. The helical structure of starch (specifically amylose) allows iodine molecules to fit inside, forming a blue-black complex.
Simple Sugars (like Sugar):
Simple sugars like sucrose, glucose, and fructose are not made of long chains like starch. They don’t have the same helical structure to trap iodine, so no color change occurs.
Iodine Test:
The iodine test is used to detect the presence of starch, not simple sugars.
In summary:
The iodine test relies on a specific interaction between iodine and the unique structure of starch, not on a general reaction with carbohydrates.