Chemicals are substances used in the manufacture and processing of materials, products, and food products. They can be natural or synthetic, and vary in composition, structure, toxicity, and reactivity. Common classes of chemicals include acids, bases, solvents, ligands, catalysts, and surfactants. Chemical usage is widespread in manufacturing, energy production, resource extraction, and food production, and can also be used as medicines and for other purposes. Many of these chemicals pose health and environmental risks and need to be monitored and regulated to ensure safe use.