Chemical

A chemical is any substance with a definite composition that can be described by a chemical formula or empirical formula. In practical use, the word often refers to substances produced or studied in chemistry, including elements, compounds, and mixtures. Chemicals can exist as solids, liquids, or gases and may be natural or synthetic.

Chemicals are classified as pure substances or mixtures. Pure substances include elements (such as oxygen) and compounds (such as water) with fixed compositions. Mixtures contain two or more substances that can be separated by physical means. Substances are also categorized as organic (carbon-containing) or inorganic.

Chemical properties describe how a substance can change its identity through a chemical reaction, including reactivity, acidity, and oxidation state. Physical properties describe attributes that can be observed without changing composition, such as color, phase, and melting point. Chemical reactions rearrange atoms to form new substances, often described by balanced equations.

Chemicals are central to industry, medicine, and daily life. They serve as fuels, solvents, polymers, pharmaceuticals, dyes, and catalysts. Handling and use follow safety data sheets, regulatory standards, and risk assessments to manage hazards such as toxicity, flammability, and environmental impact.

The modern science of chemistry developed from alchemy, becoming systematic in the 18th century with atomic theory and chemical nomenclature. Today chemistry encompasses organic, inorganic, analytical, physical, and biological chemistry, and intersects with fields such as materials science, biology, and environmental science.