(a) The chemical substances needed for photosynthesis to take place are water and carbon dioxide.
(b) Photosynthesis is the process of manufacturing food in plants by using water and carbon dioxide in the presence of sunlight.
(c) The food formed by plants is starch. The presence of starch in the leaf can be tested through the iodine test. The iodine turns blue-black on reaction with starch. In this way, the starch test can show that food has been prepared in the leaf.