For a free trial of World Book Online, click here . Chemistry is the scientific study of substances. They study how those substances behave under different conditions. They attempt to explain the behavior of a substance in terms of the substance's structure and composition.
Chemists also seek to understand chemical changes.
Chemical changes involve alterations in a substance's chemical makeup. The combination of iron with oxygen from the air to form rust is a chemical change. Substances may also go through physical change without altering their chemical makeup. Water changes physically but not chemically when it freezes. Chemists have learned much about the chemical substances and processes that occur in nature.