A Level Biology Practice Exam

Potassium iodide solution is specifically used in the iodine test for starch because it reacts with starch to produce a characteristic blue-black color. This reaction occurs because the iodine molecules fit into the helical structure of the amylose component of starch, forming a starch-iodine complex. This color change serves as a clear indicator of the presence of starch in the sample being tested. The test is highly selective for starch, making potassium iodide an essential reagent for this purpose.

In contrast, the other tests mentioned do not utilize potassium iodide. For example, the Biuret test uses copper(II) sulfate and sodium hydroxide to identify proteins, while Benedict's test employs copper(II) ions to detect reducing sugars. The Dische test for nucleic acids relies on the use of other reagents to identify DNA by forming a colored complex with deoxyribose. Therefore, potassium iodide's role in the iodine test is crucial for determining the presence of starch, distinguishing it from these other biochemical tests.