Question: why does iodine turn black when starch is present?

Kate Dobson answered on 17 Nov 2014:

I remember doing this experiment in science when i was at school, but I must admit biology was never my strongest subject and I’ve had to look up the answer….

Apparently the long starch molecules have coil and ball like structures, and when you add the iodine, the iodine atoms in the solution moves into the middle of the coils and balls. This changes the part of the spectrum absorbed and reflecetd by the starch molecule, and so changes the colour. I guess in a similar way to the way magnesium ion in the middle of a chlorphyll molecule makes it green.