Chemical composition refers to the specific arrangement and types of atoms that make up a substance. It is often expressed in terms of a chemical formula, which indicates the elements present and their proportions. For example, the chemical composition of water is represented as H2O, meaning it 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 each other. For instance, the chemical composition of carbon dioxide is CO2, which plays a crucial role in processes like photosynthesis and climate change.