Sugar is an example of carbohydrates. Sugar is tested with iodine solution but it does not change to blue-black colour. What can be a possible reason?
The iodine test specifically detects the presence of starch, not all types of carbohydrates.
Sugar (sucrose) is a carbohydrate but it is not starch. The blue-black colour appears only when iodine reacts with starch.
Since sugar does not contain starch, it does not turn blue-black when tested with iodine solution.
Explanation
The chapter explains in section 3.3.1 that a blue-black colour indicates the presence of starch specifically. The iodine solution test is designed to detect starch, which is one type of carbohydrate. Sugar (common sugar/sucrose) is also a carbohydrate as mentioned in the chapter ('Do you know that common sugar is also a type of carbohydrate?'), but it is not starch. Therefore, the iodine test does not produce a blue-black colour with sugar.
Solution Steps
The iodine test specifically detects starch, not all carbohydrates.
Sugar (sucrose) is a carbohydrate but is not starch.
Blue-black colour appears only when iodine reacts with starch.
Since sugar does not contain starch, it does not produce the blue-black colour.