為何會有starch solution??

A 0.3% w/w solution is the standard concentration for a dilute starch indicator solution. It is made by adding 4 grams of soluble starch to 1 litre of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35°C).

Although starch itself is insoluble in water, the very small amount of starch can be added in water and very small amount of starch is the ppt in the solution. (The starch is more soluble in hot water.) The starch ppt in the solution can form complex with iodine.