Chemical composition refers to the specific arrangement and types of atoms that make up a substance. It is often expressed in terms of the elements present and their proportions, which can be represented by a chemical formula. For example, the chemical composition of water is represented as H2O, indicating that each molecule consists of two hydrogen atoms and one oxygen atom. Understanding chemical composition is essential in fields like chemistry, biology, and materials science. It helps scientists determine the properties and behaviors of substances, such as how they react with one another. For instance, the chemical composition of carbon dioxide is CO2, which plays a crucial role in processes like photosynthesis and climate change.