Starch is a mixture of two molecules: amylose and amylopectin. Usually these are found in a ratio of 30:70 or 20:80, with amylopectin found in larger amounts than amylose.
Amylose is formed when α-glucose monomers are linked together by condensation to form a long, unbranching chain. The amylose chain can be made up of several thousand glucose molecules. The chains are curved and result in a coiled, helical structure (like a spring).
Amylopectin is very similar to amylose but the chains are shorter and side branches are present.
An iodine solution is used to test for starch. A blue-black colour is produced if it comes into contact with starch. The spiral tube that run down the middle of amylose and amylopectin is just the right size for an iodine molecule to fit into. The starch-iodine complex that forms has a dark blue-black colour.