Final answer:
Mixing iodine with water results in a brown solution, due to the formation of triiodide ions with iodide. When iodine is mixed with starch, a deep blue color is produced, indicating the presence of starch through the starch-iodine test.
Step-by-step explanation:
When iodine and water are mixed, iodine dissolves only slightly, forming a brown solution. This is because iodine molecules have empty valence d orbitals and can act as weak Lewis acids towards the iodide ion. In the presence of iodide ions in the solution, the dissolved iodine can form a triiodide ion. When iodine and starch are mixed, a characteristic deep blue color appears. This reaction is due to the formation of an amylose-iodine complex, with the iodine molecule fitting into the coiled structure of amylose, a component of starch.
The color change upon adding iodine to starch is a highly sensitive method for testing the presence of starch in a solution. This starch-iodine test is commonplace in chemistry labs to detect starch, since even minute amounts can be identified by the appearance of the blue-violet color.