Chapter 9 An overview of chemical reaction Chemical reactions happen in nature to produce natural substance, and even living organism Chemical reactions have been used by us to produce millions of substance that do not exist in nature Chemical reactions are the central part of the chemical sciencesChapter 9 An overview of chemical reaction Chemical reactions happen in nature to produce natural substance, and even living organism. Chemical reactions have been used by us to produce millions of substance that do not exist in nature. Chemical reactions are the central part of the chemical sciences