Concept 2.4 Chemical reactions make and break chemical bonds In chemical reactions, chemical bonds are broken and reformed, leading to new arrangements of atoms. The starting molecules in the process are called reactants, and the final molecules are called products. In a chemical reaction, all of the atoms in the reactants must be present in the products. The reactions must be “balanced”. Matter is conserved in a chemical reaction. Chemical reactions rearrange matter; they do not create or destroy matter. For example, we can recombine the covalent bonds of H 2 and O 2 to form the new bonds of H 2 O. In this reaction, two molecules of H 2 combine with one molecule of O 2 to form two molecules of H 2 O. Photosynthesis is an important chemical reaction. Humans and other animals ultimately depend on photosynthesis for food and oxygen.
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