Question

Please help :3 .... Explain why a negative iodine test for starch would indicate a positive result for the enzymatic degradation of starch by amylase.

Answer 1

As the starch is digested by amylase the products will be reducing sugars and a -ve iodine test for starch will indicate the starch has been broken down by the amylase.

Answer 2

The chemical idea prospective can be summarized in attachment 1.
As we can see iodine can place it self inside the amylose ring getting a number of excited states available making it colored (purple close to something reminding of black - remember relation between energy and wavelength and frequency)
Now, the only reason it become colored is provided amylose is there, so if we add amylase we hydrolyze the α(1,4)-glykosidbounds (After J. Bremer), see attachment two.
So when the amylose is being broken down iodide can no longer place it self in the spiral making the solution uncolored.