Which one of the following statements regarding starch and cellulose is not correct ?

The incorrect statement is option 3. Starch gives a characteristic blue–black colour with iodine, as shown by classroom tests (boiled rice turning blue–black) indicating presence of starch [1]. Both starch and cellulose are plant-derived polysaccharides built from glucose units and thus are polymers of glucose. However, the iodine test specifically detects the helical amylose component of starch which forms a blue–black charge-transfer complex with iodine, whereas cellulose does not produce this blue–black coloration and remains essentially colorless in the iodine test. Therefore it is false that both starch and cellulose give colour with iodine.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 9: Life Processes in Animals > Table 9.1: Action of saliva on starch > p. 124