Iodine

[Hix3r]

according to the equation you need equal parts from potassium-iodide and iodine to make it work:

KI(aq) + I2(s) -> KI3(aq)

KI3 dissolves in water into K+ and I3-, which are soluble ions. So for 1 mol/l iodine solution you need 1 mol or 166 g KI per litre(which is 1 dm^3).

[KathChem82]

Iodine is actually solid at room temperature. Its color is purple. KI is what's brown. In solution it produces I- and not I2. I2 is purple, in the solid, liquid, and gaseous states. It changes its color when combined with other chemicals. Iodine is used as a test for the presence of starch. The blue-black color is due to the starch-iodine complex formed. It is highly colored. Maybe the valence bond theory, crystal field theory, and ligand-field theory can help you explain the existence of the color of the complex. To make it simple, the complex absorbed electromagnetic radiation in the visible light region, and emitted the blue-black color.

[KathChem82]

and oh! the intensity of the blue-black color formed when iodine is added to starch is also a measure of how long the starch molecule is. The lighter the color, the shorted the chain of the starch molecule. Maybe some part of it has hydrolyzed to form its monomer, glucose.

[KathChem82]

countygrl, yes it is still indicative of starch. starch alone is white. when dropped with an iodine solution, it turns blue. If the starch chain is longer, then a blue-black color is produced. The light blue color simply tells you that the starch chain is not that long.