Green plants use energy from the Sun to change carbon dioxide and water into food and oxygen. What clue tells you that this process is a chemical change?

Chemical change is a change in which there is rearrangement of atoms and thus new substance is formed. There may or may not be a change in physical state.

Photosynthesis is a phenomenon in which green plants containing chlorophyll use sunlight as a source of energy to convert carbon dioxide and water to form glucose and oxygen.  The energy from light causes a chemical reaction that breaks down the molecules of carbon dioxide and water and reorganizes them to make the sugar and oxygen gas.

compost can be added to topsoil as a:

pesticide