Why does iodine change its colour?

Answer 1

This phenomena depends upon the what is tested. iodine test usually gives a positive result of blue color for starch. The blue color is caused by the trapping of the iodine inside the structure of the starch (note that starch is a polyssacharide that has a helical structure). It is only with starch that this happens cause starch has the only one with a helical structure.

Now your question is why is it that the color disappears when heated, right?

This is because the heat causes an unwinding of the helical structure of starch releasing the trapped iodine making the solution colorless. However, if cooled, with no heat present, the starch structure becomes helical once more trapping the iodine returning the blue colored solution.

this is just what i learned from my Biochemistry class, hope it helps!!

Answer 2

Iodine changes color because of a chemical reaction with starch. In the absence of starch, iodine appears brownish in color. When it reacts with starch, the iodine-starch complex forms, resulting in a blue-black color change, indicating the presence of starch.

Answer 3

The colour of iodine molecule I2 is due to n-> sigma anti bonding electronic transition. It changes its colour when it is adsorbed on starch or reduced to I-

Answer 4

because of the iodine dissolved in an aqueous solution of potassium iodide then reacts with starch then producing a dark purple-black color.

Answer 5

Iodine changes color when I contact with starch

Answer 6

Blue-Black

Answer 7

Black