The term chemical is a broadly used, but relatively vague term. People often misuse the word chemical to mean dangerous human-made substances; this misuse of the word implies that chemicals are only substances that were created in a laboratory. However, chemicals refer to a far more general substance, and most occur naturally, and many (made in laboratories or not) are harmless. Chemicals make up the food people eat, and people themselves. A chemical is some type of matter that has a constant composition (in terms of molecules or atoms). It also has physical properties such as a density, refractive index, conductivity, melting point, and boiling point that can be used to characterize this substance.
Essentially, anything that is made up of matter is made up of chemicals and thus nearly everything a person uses throughout the day is composed of chemicals - whether it is natural or man-made. Chemicals are essential to daily life, and they combine in ways that provide us with commonly used substances from water to gasoline.
As well, chemicals can exist as solids, liquids, gases, or plasma and change between these states with changes in temperature and pressure. As well, chemicals can take part in chemical reactions which change one chemical substance into another. When chemicals are changed and combined in these ways, they must be given a name and chemical formula. The chemical formula provides a list of atoms that make up one molecule of the new chemical. As well, some are provided more specific common names.