A leaf was plucked from a plant on a sunny day and kept for 2 minutes in boiling water. It was subsequently immersed in boiling alcohol and treated with iodine solution. What will be the final colour of the leaf after the test?

The experiment described is the standard iodine test for starch in leaves. Plucking a leaf on a sunny day ensures that photosynthesis has occurred, leading to the production and storage of starch [1]. Boiling the leaf in water for 2 minutes kills the cells and stops all chemical reactions. Subsequent immersion in boiling alcohol (ethanol) is a critical step that dissolves and removes the green chlorophyll pigment, decolourising the leaf to make color changes visible [1]. When the decolourised leaf is treated with iodine solution, the iodine reacts with the stored starch. This chemical reaction results in a characteristic change from the iodine's brownish-yellow color to a distinct blue-black or blue coloration [2]. Therefore, the final color of the leaf after the test will be blue, indicating the presence of starch synthesized during the sunny day.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 10: Life Processes in Plants > 10.2.2 Role of air in the preparation of food > p. 144
2. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 9: Life Processes in Animals > Table 9.1: Action of saliva on starch > p. 124