Introduction:
Starch paper is a commonly used test to detect the presence of certain chemicals in a substance. It is a simple and effective method that relies on the ability of starch to form a dark blue complex with iodine. In this case, starch paper is used to test for the presence of iodine.

Explanation:
Iodine is a nonmetallic element that exists as a diatomic molecule (I2). It is a dark purple solid that can sublimate into a violet gas when heated. Iodine is commonly used in various chemical reactions and is an important reagent in many laboratory procedures.

Starch and Iodine reaction:
When iodine comes into contact with starch, a dark blue complex is formed. This reaction is used as a test for the presence of iodine in a substance. Starch paper is prepared by impregnating filter paper with starch solution and then drying it. This paper can then be used to test for the presence of iodine.

Testing for iodine using starch paper:
To test for the presence of iodine using starch paper, the substance being tested is usually dissolved in water or another suitable solvent. A small piece of the starch paper is then brought into contact with the solution. If iodine is present, it will react with the starch in the paper, forming a dark blue or black color.

Why is starch paper used specifically for iodine?
Starch paper is used specifically for testing iodine because of the strong complex that forms between iodine and starch. This complex is highly visible and distinctive, making it easy to detect the presence of iodine. Other substances may not form such a visible complex with iodine, so starch paper is not suitable for testing their presence.

Conclusion:
Starch paper is a useful tool in chemistry for testing the presence of iodine. It is based on the reaction between iodine and starch, which forms a dark blue complex. By using starch paper, we can easily determine whether iodine is present in a given substance.