When iodine solution comes into contact with bread, the iodine molecules react with the starch molecules in the bread. This reaction produces a blue-black colour. The colour change is a result of the formation of a new substance, iodine-starch complex. This complex is different from both iodine and starch in its properties.
Thus, the reaction between iodine solution and bread is a chemical change because it results in the formation of a new substance with different properties.
