Plants make food through photosynthesis, a chemical reaction. What are the starting substances of the reaction?

The starting substances for photosynthesis is sunlight, water and carbon dioxide.

Explanation: Photosynthesis is a process by which green plants manufacture their own food using energy from the sun, carbon dioxide and water. Photosynthesis takes place in the Chloroplast. A Chloroplast is an organelle found in plant cell which houses the chlorophyll. Chlorophyll is a green pigment found in plant leaves which plants use to trap energy from the sun and use it to drive to production of glucose in the presence of water and carbon dioxide.

Plants are called producers or autotrophs because they produce their own food.

The balanced chemical equation for photosynthesis is: 6 H2O + 6 CO2 + sunlight --> C6H12O6 + 6 O2