Copper (II) Sulphate is benedict reagent is a reducing species. The blue copper (II) ions from copper (II) sulphate are reduced by aldehyde groups in reducing sugars to form red copper (I) ions precipitate. This is why colour changes can be seen in this test.
A positive result is seen when there is a colour change and formation of precipitate in 1% solution of fructose, glucose and lactose. This shows that, glucose, fructose and lactose are reducing sugars and they react with benedict reagent upon heating. However, negative result were obtained when 1% solution of sucrose, glycogen and starch reacted with benedict reagent. This is because, a free ketone or aldehyde group, therefore contains an acetal instead of a hemiacetal2.
Starch gives positive result in iodine test. The colour of starch changed from yellow solution to dark blue solution. The dark blue solution is obtained due to the polyiodide complex formed. The iodine molecules will slip into the alpha-helical structure of amylase, which aids the solution to form dark blue colour.