A chemical is any mix of raw elements: hydrogen + oxygen produce the chemical water. Water is created by a change in the chemistry of hydrogen and oxygen atoms — a chemical change.

Be cautious of the negative reaction people have to chemical substances. Chemicals often bring to mind deadly substances, such as asbestos or deadly chemical weapons. But many chemicals are part of our everyday life. Our lives would be drastically different without chemical compounds — think of the infection that would result if we didn't have the chemical hydrogen peroxide for cleaning out cuts.
He felt comfortable and safe in here, surrounded by the sloshing noises of virus-killing liquids and the hiss of air and the ruffling sensation across his back as the chemicals played over his suit.
The situation is made even more complicated by the fact that one chemical may act on another to alter its effect.
Chemical element, any substance that cannot be decomposed into simpler substances by ordinary chemical processes. Elements are the fundamental materials of which all matter is composed.

This article considers the origin of the elements and their abundances throughout the universe. The geochemical distribution of these elementary substances in the Earth’s crust and interior is treated in some detail, as is their occurrence in the hydrosphere and atmosphere. The article also discusses the periodic law and the tabular arrangement of the elements based on it. For detailed information about the compounds of the elements, see chemical compound.
In the latter part of the Middle Ages, as alchemists became more sophisticated in their knowledge of chemical processes, the Greek concepts of the composition of matter became less satisfactory. Additional elemental qualities were introduced to accommodate newly discovered chemical transformations. Thus, sulfur came to represent the quality of combustibility, mercury that of volatility or fluidity, and salt that of fixity in fire (or incombustibility). These three alchemical elements, or principles, also represented abstractions of properties reflecting the nature of matter, not physical substances.
Two types of experimental evidence gave support to the atomic hypothesis: first, the detailed behaviour of gaseous substances and, second, the quantitative weight relationships observed with a variety of chemical reactions. The English chemist John Dalton was the first to explain the empirically derived laws of chemical combination by postulating the existence of atoms with unique sets of properties. At the time, chemical combining power (valence) and relative atomic weights were the properties of most interest. Subsequently numerous independent experimental verifications of the atomic hypothesis were carried out, and today it is universally accepted. Indeed, in 1969 individual uranium and thorium atoms were actually observed by means of an electron microscope.