Chemistry

Chemistry examines matter at the level of elements, compounds and mixtures: what substances contain, how their particles are arranged, which properties follow from that structure, and how substances interact. Atoms are the basic units of chemical elements; atoms can join into molecules or extended structures. Their rearrangement in a chemical reaction produces different substances. A physical change, such as water freezing, changes a material's state without changing its chemical identity, whereas a chemical reaction changes the substances present. Chemists also track the energy absorbed or released as transformations occur.

Chemistry combines observation and experiment with quantitative measurement. Researchers may analyse a sample to identify its components and their amounts, investigate a material's properties, or synthesize a substance under controlled conditions. Because particles such as atoms and molecules are generally too small to observe directly, chemists use models to connect measurable, large-scale behaviour with proposed microscopic structure. Chemical symbols, formulae and equations provide a compact language for representing elements, substances and reactions. Experimental results test these representations and can require a theory or model to be revised.

Chemistry is divided into overlapping specialties rather than a single exhaustive list. Organic chemistry studies carbon-containing compounds; inorganic chemistry focuses on many other compounds and materials, including metals and minerals. Analytical chemistry develops ways to determine what a sample contains and in what quantities. Physical chemistry applies physical principles and mathematical methods to molecular behaviour, reaction rates and energy changes. Biochemistry investigates substances and reactions in living systems. Materials, environmental, medicinal and industrial chemistry address particular materials, applications or settings. Chemical engineering works closely with chemistry to design and scale processes for making products.

People used chemical processes long before chemistry became a formal science: cooking, fermentation, pottery, dyeing and metalworking all involve transforming materials. For centuries, practical work coexisted with speculative accounts of what matter was made of. In the eighteenth century, systematic experiments and careful weighing helped establish a more quantitative discipline. Antoine-Laurent Lavoisier's work challenged phlogiston explanations of combustion and helped popularize conservation of mass: in ordinary chemical transformations, matter is rearranged rather than created or destroyed. This approach made precise measurement and reproducible analysis central to chemical research.

Chemical knowledge supports the analysis of food, water and other materials; the development of medicines, fuels, fertilizers and polymers; and explanations of processes in living organisms. These uses can improve health and everyday life, but chemical production and disposal can also create hazardous substances and pollution. Green chemistry responds by designing chemical products and processes to prevent or reduce hazardous substances at their source, considering stages from design and manufacture through use and disposal. This differs from remediation, which treats waste or cleans up releases after hazardous substances have been generated.